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We define a physically-motivated measure for galactic bar length, called the dynamical length. The dynamical length of the bar corresponds to the radial extent of the orbits that are the backbone supporting the bar feature. We propose a direct observational technique using integral field unit spectroscopy to measure it. Identifying these orbits and using the dynamical length is a more faithful tracer of the secular evolution and influence of the bar. We demonstrate the success of the metric for recovering the maximal bar-parenting orbit in a range of simulations, and to show its promise we perform its measurement on a real galaxy. We also study the difference between traditionally used ellipse fit approaches to determine bar length and the dynamical length proposed here in a wide range of bar-forming N-body simulations of a stellar disc and dark matter halo. We find that ellipse fitting may severely overestimate measurements of the bar length by a factor of 1.5-2.5 relative to the extent of the orbits that are trapped and actually comprise the bar. This bias leads to overestimates of both bar mass and the ratio of corotation radius to bar length, i.e. the bar speed, affecting inferences about the evolution of bars in the real universe., Submitted to MNRAS, comments welcome. arXiv admin note: text overlap with arXiv:1903.08203

The bimodal absorption system imaging campaign (BASIC) aims to characterize the galaxy environments of a sample of 36 HI-selected partial Lyman limit systems (pLLSs) and Lyman limit systems (LLSs) in 23 QSO fields at $z \lesssim 1$. These pLLSs/LLSs provide a unique sample of absorbers with unbiased and well-constrained metallicities, allowing us to explore the origins of metal-rich and low-metallicity circumgalactic medium (CGM) at $z 9.0$, whereas low-metallicity pLLSs/LLSs are found in more diverse locations, with one population arising in the CGM of galaxies and another more broadly distributed in overdense regions of the universe. Using absorbers not associated with galaxies, we estimate the unweighted geometric mean metallicity of the intergalactic medium to be [X/H] $\lesssim -2.1$ at $z, Key figures: 13 and 18. Modified abstract. Accepted for publication in ApJ. Updated after referee comments, expanded subsection 4.4

In the local Universe, OH megamasers (OHMs) are detected almost exclusively in infrared-luminous galaxies, with a prevalence that increases with IR luminosity, suggesting that they trace gas-rich galaxy mergers. Given the proximity of the rest frequencies of OH and the hyperfine transition of neutral atomic hydrogen (HI), radio surveys to probe the cosmic evolution of HI in galaxies also offer exciting prospects for exploiting OHMs to probe the cosmic history of gas-rich mergers. Using observations for the Looking At the Distant Universe with the MeerKAT Array (LADUMA) deep HI survey, we report the first untargeted detection of an OHM at $z > 0.5$, LADUMA J033046.20$-$275518.1 (nicknamed "Nkalakatha"). The host system, WISEA J033046.26$-$275518.3, is an infrared-luminous radio galaxy whose optical redshift $z \approx 0.52$ confirms the MeerKAT emission line detection as OH at a redshift $z_{\rm OH} = 0.5225 \pm 0.0001$ rather than HI at lower redshift. The detected spectral line has 18.4$σ$ peak significance, a width of $459 \pm 59\,{\rm km\,s^{-1}}$, and an integrated luminosity of $(6.31 \pm 0.18\,{\rm [statistical]}\,\pm 0.31\,{\rm [systematic]}) \times 10^3\,L_\odot$, placing it among the most luminous OHMs known. The galaxy's far-infrared luminosity $L_{\rm FIR} = (1.576 \pm 0.013) \times 10^{12}\,L_\odot$ marks it as an ultra-luminous infrared galaxy; its ratio of OH and infrared luminosities is similar to those for lower-redshift OHMs. A comparison between optical and OH redshifts offers a slight indication of an OH outflow. This detection represents the first step towards a systematic exploitation of OHMs as a tracer of galaxy growth at high redshifts., 10 pages, 4 figures. Accepted to ApJ Letters

We study the mechanisms and evolutionary phases of bar formation in n-body simulations of a stellar disc and dark matter halo system using harmonic basis function expansion analysis to characterize the dynamical mechanisms in bar evolution. We correlate orbit families with phases of bar evolution by using empirical orthogonal functions that act as a spatial filter and form the gravitational potential basis. In both models we find evidence for three phases in evolution with unique harmonic signatures. We recover known analytic results, such as bar slowdown owing to angular momentum transfer. We also find new dynamical mechanisms for bar evolution: a steady-state equilibrium configuration and harmonic interaction resulting in harmonic mode locking, both of which may be observable. Additionally, we find that ellipse fitting may severely overestimate measurements of bar length by a factor of two relative to the measurements based on orbits that comprise the true backbone supporting the bar feature. The bias will lead to overestimates of both bar mass and bar pattern speed, affecting inferences about the evolution of bars in the real universe, such as the fraction of bars with fast pattern speeds. We propose a direct observational technique to compute the radial extent of trapped orbits and determine a dynamical length for the bar., Submitted to MNRAS, comments welcome

We explore the application of Bayesian image analysis to infer the properties of an SDSS early-type galaxy sample including AGN. We use GALPHAT (Yoon et al. 2010) with a Bayes-factor model comparison to photometrically infer an AGN population and verify this using spectroscopic signatures. Our combined posterior sample for the SDSS sample reveals distinct low and high concentration modes after the point-source flux is modeled. This suggests that ETG parameters are intrinsically bimodal. The bimodal signature was weak when analyzed by GALFIT (Peng et al. 2002, 2010). This led us to create several ensembles of synthetic images to investigate the bias of inferred structural parameters and compare with GALFIT. GALPHAT inferences are less biased, especially for high-concentration profiles: GALPHAT S��rsic index $n$, $r_{e}$ and MAG deviate from the true values by $6\%$, $7.6\%$ and $-0.03 \,\mathrm{mag}$, respectively, while GALFIT deviates by $15\%$, $22\%$ and $-0.09$\, mag, respectively. In addition, we explore the reliability for the photometric detection of AGN using Bayes factors. For our SDSS sample with $r_{e}\ge 7.92\,$arcsec, we correctly identify central point sources with $\mathrm{Mag_{PS}}-\mathrm{Mag_{Sersic}}\le 5$ for $n\le6$ and $\mathrm{Mag_{PS}}-\mathrm{Mag_{Sersic}}\le 3$ for $n>6$. The magnitude range increases and classification error decreases with increasing resolution, suggesting that this approach will excel for upcoming high-resolution surveys. Future work will extend this to models that test hypotheses of galaxy evolution through the cosmic time.

We examine subhaloes and galaxies residing in a simulated LCDM galaxy cluster ($M^{\rm crit}_{200}=1.1\times10^{15}M_\odot/h$) produced by hydrodynamical codes ranging from classic Smooth Particle Hydrodynamics (SPH), newer SPH codes, adaptive and moving mesh codes. These codes use subgrid models to capture galaxy formation physics. We compare how well these codes reproduce the same subhaloes/galaxies in gravity only, non-radiative hydrodynamics and full feedback physics runs by looking at the overall subhalo/galaxy distribution and on an individual objects basis. We find the subhalo population is reproduced to within $\lesssim10\%$ for both dark matter only and non-radiative runs, with individual objects showing code-to-code scatter of $\lesssim0.1$ dex, although the gas in non-radiative simulations shows significant scatter. Including feedback physics significantly increases the diversity. Subhalo mass and $V_{max}$ distributions vary by $\approx20\%$. The galaxy populations also show striking code-to-code variations. Although the Tully-Fisher relation is similar in almost all codes, the number of galaxies with $10^{9}M_\odot/h\lesssim M_*\lesssim 10^{12}M_\odot/h$ can differ by a factor of 4. Individual galaxies show code-to-code scatter of $\sim0.5$ dex in stellar mass. Moreover, strong systematic differences exist, with some codes producing galaxies $70\%$ smaller than others. The diversity partially arises from the inclusion/absence of AGN feedback. Our results combined with our companion papers demonstrate that subgrid physics is not just subject to fine-tuning, but the complexity of building galaxies in all environments remains a challenge. We argue even basic galaxy properties, such as the stellar mass to halo mass, should be treated with errors bars of $\sim0.2-0.4$ dex., 17 pages (+4 page appendix), 16 figures, 2 tables; accepted for publication in MNRAS

We investigate the time evolution of non-parametric morphological quantities and their relationship to major mergers between $4\geq z \geq 2$ in high-resolution cosmological zoom simulations of disk galaxies that implement kinetic wind feedback, $H_2$-based star formation, and minimal ISM pressurisation. We show that the resulting galaxies broadly match basic observed physical properties of $z\sim 2$ objects. We measure the galaxies' concentrations ($C$), asymmetries ($A$), and $Gini$ ($G$) and $M_{20}$ coefficients, and correlate these with major merger events identified from the mass growth history. We find that high values of asymmetry provide the best indicator for identifying major mergers of $>1:4$ mass ratio within our sample, with $Gini$-$M_{20}\,$ merger classification only as effective for face-on systems and much less effective for edge-on or randomly-oriented galaxies. The canonical asymmetry cut of $A\geq0.35$, however, is only able to correctly identify major mergers $\sim 10\%$ of the time, while a higher cut of $A\geq 0.8$ more efficiently picks out mergers at this epoch. We further examine the temporal correlation between morphological statistics and mergers, and show that for randomly-oriented galaxies, half the galaxies with $A\geq0.8$ undergo a merger within $\pm0.2\,{\rm Gyr}$, whereas $Gini$-$M_{20}\,$ identification only identifies about a third correctly. The fraction improves further using $A\geq 1.5$, but about the half the mergers are missed by this stringent cut., 19 pages, 19 figures, submitted

We examine the star formation histories (SFHs) of galaxies in smoothed particle hydrodynamics (SPH) simulations, compare them to parametric models that are commonly used in fitting observed galaxy spectral energy distributions, and examine the efficacy of these parametric models as practical tools for recovering the physical parameters of galaxies. The commonly used tau-model, with SFR ~ exp(-t/tau), provides a poor match to the SFH of our SPH galaxies, with a mismatch between early and late star formation that leads to systematic errors in predicting colours and stellar mass-to-light ratios. A one-parameter lin-exp model, with SFR ~ t*exp(-t/tau), is much more successful on average, but it fails to match the late-time behavior of the bluest, most actively star-forming galaxies and the passive, "red and dead" galaxies. We introduce a 4-parameter model, which transitions from lin-exp to a linear ramp after a transition time, which describes our simulated galaxies very well. We test the ability of these parametrised models to recover (at z=0, 0.5, and 1) the stellar mass-to-light ratios, specific star formation rates, and stellar population ages from the galaxy colours, computed from the full SPH star formation histories using the FSPS code of Conroy et al. (2009). Fits with tau-models systematically overestimate M/L by ~ 0.2 dex, overestimate population ages by ~ 1-2 Gyr, and underestimate sSFR by ~ 0.05 dex. Fits with lin-exp are less biased on average, but the 4-parameter model yields the best results for the full range of galaxies. Marginalizing over the free parameters of the 4-parameter model leads to slightly larger statistical errors than 1-parameter fits but essentially removes all systematic biases, so this is our recommended procedure for fitting real galaxies., Comment: 28 pages, 18 figures

Recent theoretical work has implicated fast bar formation modes and subsequent evolution as the creation mechanism for the observed peanut-shaped bulges in some edge-on disk galaxies. We demonstrate an N-body simulation of a disk undergoing a contrasting slow mode of bar growth, unsubjected to a buckling instability, which nonetheless grows the 4:1 orbit family responsible for a peanut-shaped bulge. We also present a simulation with fast mode bar growth, which exhibits thickening similar to other work. A novel orbit classification method that finds dynamically distinct families is presented, allowing for a detailed analysis of angular momentum transfer channels within the disk., 4 pages, 3 figures. To appear in the proceedings of the conference "The Structure and Dynamics of Disk Galaxies." M.S. Seigar and P. Treuthardt, eds

Weinberg (2012) described a constructive algorithm for computing the marginal likelihood, Z, from a Markov chain simulation of the posterior distribution. Its key point is: the choice of an integration subdomain that eliminates subvolumes with poor sampling owing to low tail-values of posterior probability. Conversely, this same idea may be used to choose the subdomain that optimizes the accuracy of Z. Here, we explore using the simulated distribution to define a small region of high posterior probability, followed by a numerical integration of the sample in the selected region using the volume tessellation algorithm described in Weinberg (2012). Even more promising is the resampling of this small region followed by a naive Monte Carlo integration. The new enhanced algorithm is computationally trivial and leads to a dramatic improvement in accuracy. For example, this application of the new algorithm to a four-component mixture with random locations in 16 dimensions yields accurate evaluation of Z with 5% errors. This enables Bayes-factor model selection for real-world problems that have been infeasible with previous methods., 14 pages, 3 figures, submitted to Bayesian Analysis

We investigate the metallicity evolution and content of the intergalactic medium (IGM) and galactic halo gas from z=2->0 using 110-million particle cosmological hydrodynamic simulations. We focus on the detectability and physical properties of UV resonance metal-line absorbers observable with Hubble's Cosmic Origins Spectrograph (COS). We confirm that galactic superwind outflows are required to enrich the IGM to observed levels down to z=0 using three wind prescriptions contrasted to a no-wind simulation. Our favoured momentum-conserved wind prescription deposits metals closer to galaxies owing to its moderate energy input, while the more energetic constant wind model enriches the warm-hot IGM 6.4x more. Despite these significant differences, all wind models produce metal-line statistics within a factor of two of existing observations. This is because OVI, CIV, SiIV, and NeVIII absorbers primarily arise from T, Accepted to MNRAS, 34 pages, 21 figures, 3 tables

We conduct Bayesian model inferences from the observed K-band luminosity function of galaxies in the local Universe, using the semi-analytic model (SAM) of galaxy formation introduced in Lu et al (2011). The prior distributions for the 14 free parameters include a large range of possible models. We find that some of the free parameters, e.g. the characteristic scales for quenching star formation in both high-mass and low-mass halos, are already tightly constrained by the single data set. The posterior distribution includes the model parameters adopted in other SAMs. By marginalising over the posterior distribution, we make predictions that include the full inferential uncertainties for the colour-magnitude relation, the Tully-Fisher relation, the conditional stellar mass function of galaxies in halos of different masses, the HI mass function, the redshift evolution of the stellar mass function of galaxies, and the global star formation history. Using posterior predictive checking with the available observational results, we find that the model family (i) predicts a Tully-Fisher relation that is curved; (ii) significantly over predicts the satellite fraction; (iii) vastly over predicts the HI mass function; (iv) predicts high-z stellar mass functions that have too many low mass galaxies and too few high mass ones. and (v) predicts a redshift evolution of the stellar mass density and the star formation history that are in moderate disagreement. These results suggest that some important processes are still missing in the current model family and we discuss a number of possible solutions to solve the discrepancies, such as interactions between galaxies and dark matter halos, tidal stripping, the bimodal accretion of gas, preheating, and a redshift-dependent initial mass function., Comment: 45 pages, 12 figures, MNRAS in press

We introduce a new galaxy image decomposition tool, GALPHAT (GALaxy PHotometric ATtributes), to provide full posterior probability distributions and reliable confidence intervals for all model parameters. GALPHAT is designed to yield a high speed and accurate likelihood computation, using grid interpolation and Fourier rotation. We benchmark this approach using an ensemble of simulated Sersic model galaxies over a wide range of observational conditions: the signal-to-noise ratio S/N, the ratio of galaxy size to the PSF and the image size, and errors in the assumed PSF; and a range of structural parameters: the half-light radius $r_e$ and the Sersic index $n$. We characterise the strength of parameter covariance in Sersic model, which increases with S/N and $n$, and the results strongly motivate the need for the full posterior probability distribution in galaxy morphology analyses and later inferences. The test results for simulated galaxies successfully demonstrate that, with a careful choice of Markov chain Monte Carlo algorithms and fast model image generation, GALPHAT is a powerful analysis tool for reliably inferring morphological parameters from a large ensemble of galaxies over a wide range of different observational conditions. (abridged), Submitted to MNRAS. The submitted version with high resolution figures can be downloaded from http://www.astro.umass.edu/~iyoon/GALPHAT/galphat1.pdf

The intergalactic medium (IGM) is the dominant reservoir of baryons at all cosmic epochs. We investigate the evolution of the IGM from z=2-0 in 48 Mpc/h, 110-million particle cosmological hydrodynamic simulations using three prescriptions for galactic outflows. We focus on the evolution of IGM physical properties, and how such properties are traced by Ly-alpha absorption as detectable using HST/COS. Our results broadly confirm the canonical picture that most Ly-alpha absorbers arise from highly ionized gas tracing filamentary large-scale structure. Growth of structure causes gas to move from the diffuse photoionized IGM into other cosmic phases, namely stars, cold and hot gas within galaxy halos, and the unbound and shock-heated warm-hot intergalactic medium (WHIM). By today, baryons are roughly equally divided between bound phases (35%), the diffuse IGM (41%), and the WHIM (24%). Here we (re)define the WHIM as gas with overdensities lower than that in halos and temperatures >10^5 K, in order to more closely align it with "missing baryons". When we tune our photoionizing background to match the observed evolution of the Ly-alpha mean flux decrement, we obtain a line count evolution that broadly agrees with available data. We predict a column density distribution slope of -1.70 for our favored momentum-driven wind model, in agreement with recent observations, and it becomes shallower with redshift. With improved statistics, the frequency of strong lines can be a valuable diagnostic of outflows, and our favored wind model matches existing data best among our models. The relationship between column density and physical density is fairly tight from z=2-0, and evolves as rho N_HI^0.74 10^(-0.37z) for diffuse absorbers. Linewidths only loosely reflect the temperature of the absorbing gas, which will hamper attempts to quantify the WHIM using broad Ly-alpha absorbers. [Abridged], 22 pages, MNRAS, accepted

We study the behaviour of multiple radiative cooling algorithms implemented in seven Semi-Analytic Models (SAMs) of galaxy formation, including a new model we propose in this paper. We use versions of the models without feedback and apply them to dark matter haloes growing in a cosmological context, which have final masses that range from 10^{11}Msun to 10^{14}Msun. First, using simplified smoothly-growing halo models, we demonstrate that the different algorithms predict cooling rates and final cold gas masses that differ by a factor of ~5 for massive haloes (>10^{12}Msun). The algorithms are in better agreement for less massive haloes because they cool efficiently and, therefore, their cooling rates are largely limited by the halo accretion rate. However, for less massive haloes, all the SAMs predict less cooling than corresponding 1D hydrodynamic models. Second, we study the gas accretion history of the central galaxies of dark matter haloes using merger trees. The inclusion of mergers alters the cooling history of haloes by locking up gas in galaxies within small haloes at early times. For realistic halo models, the dispersion in the cold gas mass predicted by the algorithms is 0.5 dex for high mass haloes and 0.1 dex for low mass haloes, while the dispersion in the accretion rate is about two times larger. Comparing to cosmological SPH simulations, we find that most SAMs systematically under-predict the gas accretion rates for low-mass haloes but over-predict the gas accretion rates for massive haloes. Although the models all include both "rapid" and "slow" mode accretion, the transition between the two accretion modes varies between models and also differs from the simulations. Finally, we construct a new model that explicitly incorporates cold halo gas to illustrate that such a class of models can better match the results from cosmological hydrodynamic simulations., 1 figure added, MORGANA model added into the comparison, published in MNRAS

Recently Menard et al. detected a subtle but systematic change in the mean color of quasars as a function of their projected separation from foreground galaxies, extending to comoving separations of ~10Mpc/h, which they interpret as a signature of reddening by intergalactic dust. We present theoretical models of this remarkable observation, using SPH cosmological simulations of a (50Mpc/h)^3 volume. Our primary model uses a simulation with galactic winds and assumes that dust traces the intergalactic metals. The predicted galaxy-dust correlation function is similar in form to the galaxy-mass correlation function, and reproducing the MSFR data requires a dust-to-metal mass ratio of 0.24, about half the value in the Galactic ISM. Roughly half of the reddening arises in dust that is more than 100Kpc/h from the nearest massive galaxy. We also examine a simulation with no galactic winds, which predicts a much smaller fraction of intergalactic metals (3% vs. 35%) and therefore requires an unphysical dust-to-metal ratio of 2.18 to reproduce the MSFR data. In both models, the signal is dominated by sightlines with E(g-i)=0.001-0.1. The no-wind simulation can be reconciled with the data if we also allow reddening to arise in galaxies up to several x 10^10 Msun. The wind model predicts a mean visual extinction of A_V ~0.0133 mag out to z=0.5, with a sightline-to-sightline dispersion similar to the mean, which could be significant for future supernova cosmology studies. Reproducing the MSFR results in these simulations requires that a large fraction of ISM dust survive its expulsion from galaxies and its residence in the intergalactic medium. Future observational studies that provide higher precision and measure the dependence on galaxy type and environment will allow detailed tests for models of enriched galactic outflows and the survival of IG dust., Comment: Matches version accepted by MNRAS

Subhalo abundance matching (SHAM) is a technique for populating simulated dark matter distributions with galaxies, assuming a monotonic relation between a galaxy's stellar mass or luminosity and the mass of its parent dark matter halo or subhalo. We examine the accuracy of SHAM in two cosmological SPH simulations, one of which includes momentum-driven winds. The SPH simulations indeed show a nearly monotonic relation between stellar mass and halo mass provided that, for satellite galaxies, we use the mass of the subhalo at the epoch when it became a satellite. In each simulation, the median relation for central and satellite galaxies is nearly identical, though a somewhat larger fraction of satellites are outliers. SHAM-assigned masses (at z=0-2), luminosities (R-band at z=0), or star formation rates (at z=2) have a 68% scatter of 0.09-0.15 dex relative to the true simulation values. When we apply SHAM to the subhalo population of collisionless N-body simulation with the same initial conditions as the SPH runs, we find generally good agreement for the halo occupation distributions and halo radial profiles of galaxy samples defined by thresholds in stellar mass. However, because a small fraction of SPH galaxies suffer severe stellar mass loss after becoming satellites, SHAM slightly overpopulates high mass halos; this effect is more significant for the wind simulation, which produces galaxies that are less massive and more fragile. SHAM recovers the two-point correlation function of the SPH galaxies in the no-wind simulation to better than 10% at scales 0.1 < r < 10 Mpc/h. For the wind simulation, agreement is better than 15% at r > 2 Mpc/h, but overpopulation of massive halos increases the correlation function by a factor of ~2.5 on small scales., 21 pages, 11 figures, Accepted by MNRAS

The kinematics of damped Lyman alpha absorbers (DLAs) are difficult to reproduce in hierarchical galaxy formation models, particularly the preponderance of wide systems. We investigate DLA kinematics at z=3 using high-resolution cosmological hydrodynamical simulations that include a heuristic model for galactic outflows. Without outflows, our simulations fail to yield enough wide DLAs, as in previous studies. With outflows, predicted DLA kinematics are in much better agreement with observations. Comparing two outflow models, we find that a model based on momentum-driven wind scalings provides the best match to the observed DLA kinematic statistics of Prochaska & Wolfe. In this model, DLAs typically arise a few kpc away from galaxies that would be identified in emission. Narrow DLAs can arise from any halo and galaxy mass, but wide ones only arise in halos with mass >10^11 Mo, from either large central or small satellite galaxies. This implies that the success of this outflow model originates from being most efficient at pushing gas out from small satellite galaxies living in larger halos. This increases the cross-section for large halos relative to smaller ones, thereby yielding wider kinematics. Our simulations do not include radiative transfer effects or detailed metal tracking, and outflows are modeled heuristically, but they strongly suggest that galactic outflows are central to understanding DLA kinematics. An interesting consequence is that DLA kinematics may place constraints on the nature and efficiency of gas ejection from high-z galaxies., submitted to MNRAS

We analyse cosmological hydrodynamic simulations that include observationally-constrained prescriptions for galactic outflows. If these simulated winds accurately represent winds in the real Universe, then material previously ejected in winds provides the dominant source of gas infall for new star formation at redshifts z, 16 pages, 7 figures, accepted by MNRAS

We present predictions for the fluorescent Lyman-alpha emission signature arising from photoionized, optically thick structures in Smoothed Particle Hydrodynamic (SPH) cosmological simulations of a Lambda-CDM universe using a Monte Carlo Lyman-alpha radiative transfer code. We calculate the expected Lyman-alpha image and 2-dimensional spectra for gas exposed to a uniform ultraviolet ionizing background as well as gas exposed additionally to the photoionizing radiation from a local quasar, after correcting for the self-shielding of hydrogen. As a test of our numerical methods and for application to current observations, we examine simplified analytic structures that are uniformly or anisotropically illuminated. We compare these results with recent observations. We discuss future observing campaigns on large telescopes and realistic strategies for detecting fluorescence owing to the ambient metagalactic ionization and in regions close to bright quasars. While it will take hundreds of hours on the current generation of telescopes to detect fluorescence caused by the ultraviolet background (UVB) alone, our calculations suggest that of order ten sources of quasar-induced fluorescent Lyman-alpha emission should be detectable after a 10 hour exposure in a 10 arcmin^2 field around a bright quasar. These observations will help probe the physical conditions in the densest regions of the intergalactic medium as well as the temporal light curves and isotropy of quasar radiation., Submitted to ApJ. 64 pages including 25 figures. High resolution version available at http://www.ociw.edu/~jak/lya_volume1.pdf

We introduce a novel image decomposition package, GALPHAT, that provides robust estimates of galaxy surface brightness profiles using Bayesian Markov Chain Monte Carlo. The GALPHAT-determined posterior distribution of parameters enables us to assign rigorous statistical confidence intervals to maximum a posteriori estimates and to test complex galaxy formation and evolution hypotheses. We describe the GALPHAT algorithm, assess its performance using test image data, and demonstrate that it has sufficient speed for production analysis of a large galaxy sample. Finally we briefly introduce our ongoing science program to study the distribution of galaxy structural properties in the local universe using GALPHAT., To appear in the proceedings of "Galaxy Evolution: Emerging Insights and Future Challenges", Shardha Jogee, Lei Hao, Guillermo Blanc, Irina Marinova, eds., ASP Conference Series

We compare the properties of galaxies that form in a cosmological simulation without strong feedback to observations at z=0. We confirm previous findings that models without strong feedback overproduce the observed galaxy baryonic mass function, especially at the low and high mass extremes. Through post-processing we investigate what kinds of feedback would be required to reproduce observed galaxy masses and star formation rates. To mimic an extreme form of "preventive" feedback (e.g., AGN radio mode) we remove all baryonic mass that was originally accreted via "hot mode" from shock-heated gas. This does not bring the high mass end of the galaxy mass function into agreement with observations because much of the stellar mass in these systems formed at high redshift from baryons that originally accreted via "cold mode" onto lower mass progenitors. An efficient "ejective" feedback mechanism, such as supernova driven winds, must reduce the masses of these progenitors. Feedback must also reduce the masses of lower mass z=0 galaxies, which assemble at lower redshifts and have much lower star formation rates. If we monotonically re-map galaxy masses to reproduce the observed mass function, but retain the simulation's predicted star formation rates, we obtain fairly good agreement with the observed sequence of star-forming galaxies but fail to recover the observed population of passive, low star formation rate galaxies. Suppressing all hot mode accretion improves agreement for high mass galaxies but worsens the agreement at intermediate masses. Reproducing these z=0 observations requires a feedback mechanism that dramatically suppresses star formation in a fraction of galaxies, increasing with mass, while leaving star formation rates of other galaxies essentially unchanged., MNRAS in press. 15 pages, 5 figures, minimal changes from the first version

We study the formation of galaxies in a (50 Mpc/h)^3 cosmological simulation (2x288^3 particles), evolved using the entropy conserving SPH code Gadget-2. Most of the baryonic mass in galaxies of all masses is originally acquired through filamentary "cold mode" accretion of gas that was never shock heated to its halo virial temperature, confirming the key feature of our earlier results obtained with a different SPH code (Keres et al. 2005). Atmospheres of hot, virialized gas develop in halos above ~2.5e11 Msun, a transition mass that is nearly constant from z=3 to z=0. Cold accretion persists in halos above the transition mass, especially at z>=2. It dominates the growth of galaxies in low mass halos at all times, and it is the main driver of the cosmic star formation history. Satellite galaxies have accretion rates similar to central galaxies of the same baryonic mass at high redshifts, but they have less accretion than comparable central galaxies at low redshift. Relative to our earlier results, the Gadget-2 simulations predict much lower rates of "hot mode" accretion from the virialized gas component of massive halos. At z, MNRAS in press, 24 pages, 11 figures. Minor changes and updated references. A copy with high resolution figures (Highly Recommended) can be found at http://www.cfa.harvard.edu/~dkeres/coldmode/ms_keres_rv1.ps.gz

(Abbreviated) We investigate the dynamical mechanisms responsible for producing tidal tails from dwarf satellites using N-body simulations. We identify two important dynamical co-conspirators: 1) the points where the attractive force of the host halo and satellite are balanced do not occur at equal distances from the satellite centre or at the same equipotential value for massive satellites, breaking the morphological symmetry of the leading and trailing tails; and 2) the escaped ejecta in the leading (trailing) tail continues to be decelerated (accelerated) by the satellite's gravity leading to large offsets of the ejecta orbits from the satellite orbit. The effect of the satellite's self gravity decreases only weakly with a decreasing ratio of satellite mass to host halo mass, demonstrating the importance of these effects over a wide range of subhalo masses. Not only will the morphology of the leading and trailing tails for massive satellites be different, but the observed radial velocities of the tails will be displaced from that of the satellite orbit; both the displacement and the peak radial velocity is proportional to satellite mass. If the tails are assumed to follow the progenitor satellite orbits, the tails from satellites with masses greater than 0.0001 of the host halo virial mass in a spherical halo will appear to indicate a flattened halo. Therefore, a constraint on the Milky Way halo shape using tidal streams requires mass-dependent modelling. Similarly, we compute the the distribution of tail orbits both in E_{r}-r^{-2} space and in E-L_{z} space, advocated for identifying satellite stream relics. The acceleration of ejecta by a massive satellite during escape spreads the velocity distribution and obscures the signature of a well-defined ``moving group'' in phase space., 16 pages, 25 figures, accepted for publication in MNRAS. A full resolution version is available at http://www.astro.umass.edu/~jhchoi/Tail/Tail_HR.ps

We present an overview of some of the issues surrounding current models of galaxy formation, highlighting recent insights obtained from cosmological hydrodynamic simulations. Detailed examination of gas accretion processes show a hot mode of gas cooling from near the halo's virial temperature, and a previously underappreciated cold mode where gas flows in along filaments on dynamical timescales, emitting its energy in line radiation. Cold mode dominates in systems with halo masses slightly smaller than the Milky Way and below, and hence dominates the global accretion during the heydey of galaxy formation. This rapid accretion path enables prompt assembly of massive galaxies in the early universe, and results in $z\sim 4$ galaxy properties in broad agreement with observations, with the most massive galaxies being the most rapid star formers. Massive galaxies today are forming stars at a much reduced rate, a trend called downsizing. The trend of downsizing is naturally reproduced in simulations, owing to a transition from cold mode accretion in the early growth phase to slower hot mode accretion once their halos grow large. However, massive galaxies at the present epoch are still observed to have considerably redder colors than simulations suggest, suggesting that star formation is not sufficiently truncated in models by the transition to hot mode, and that another process not included in current simulations is required to suppress star formation., 10 pages, to appear in proceedings of "The Fabulous Destiny of Galaxies: Bridging Past and Present", Marseille, 2005

We use recent observations of the HI-mass function to constrain galaxy formation. The data conflicts with the standard model where most of the gas in a low-mass dark matter halo is assumed to settle into a disk of cold gas that is depleted by star formation and supernova-driven outflows until the disk becomes gravitationally stable. A consistent model can be found if low-mass haloes are embedded in a preheated medium, with a specific gas entropy ~ 10Kev cm^2. Such a model simultaneously matches the faint-end slope of the galaxy luminosity function. We propose a preheating model where the medium around low-mass haloes is preheated by gravitational pancaking. Since gravitational tidal fields suppress the formation of low-mass haloes while promoting that of pancakes, the formation of massive pancakes precedes that of the low-mass haloes within them. We demonstrate that the progenitors of present-day dark matter haloes with M, 13 pages, 3 figures. Accepted for publication in MNRAS

We analyze the flux power spectrum and its covariance using simulated Lyman alpha forests. We find that pseudo-hydro techniques are good approximations of hydrodynamical simulations at high redshift. However, the pseudo-hydro techniques fail at low redshift because they are insufficient for characterizing some components of the low-redshift intergalactic medium, notably the warm-hot intergalactic medium. Hence, to use the low-redshift Lyman alpha flux power spectrum to constrain cosmology, one would need realistic hydrodynamical simulations. By comparing one-dimensional mass statistics with flux statistics, we show that the nonlinear transform between density and flux quenches the fluctuations so that the flux power spectrum is much less sensitive to cosmological parameters than the one-dimensional mass power spectrum. The covariance of the flux power spectrum is nearly Gaussian. As such, the uncertainties of the underlying mass power spectrum could still be large, even though the flux power spectrum can be precisely determined from a small number of lines of sight., 10 pages, 15 figures, accepted for publication in MNRAS

We examine the temperature history of gas accreted by forming galaxies in SPH simulations. About half the gas shock heats to roughly the virial temperature of the galaxy potential well before cooling, condensing, and forming stars, but the other half radiates its acquired gravitational energy at much lower temperatures, typically T, 1 figure added, Appendix discussing SAMs added, some text changes. Matches the version accepted by MNRAS. 31 pages (MNRAS style), 21 figures,For high resolution version of the paper (highly recommended) follow http://www.astro.umass.edu/~keres/paper/ms2.ps.gz

We investigate galaxy clustering and galaxy-mass correlations in the LCDM cosmological model using a large volume SPH simulation. For the most part, the predicted biases between galaxies and dark matter lead to good agreement with current observations, including: (1) a nearly constant comoving correlation length from z=3 to z=0 for mass-selected galaxy samples of constant comoving space density; (2) an rms bias factor b~1 at z=0; (3) a scale-dependent bias on small scales that transforms the curved dark matter correlation function into a nearly power-law galaxy correlation function; (4) galaxy pairwise dispersion and hierarchical skewness ratio S_3 in good agreement with observed values, and lower than values for the dark matter by ~20%; (5) a ratio of galaxy-galaxy to galaxy-mass correlation functions consistent with recent measurements from the RCS survey; and (6) a mean excess mass dM(260/h kpc) approximately proportional to galaxy baryon mass M_b, in agreement with lensing estimates from the SDSS. These clustering properties vary with galaxy baryon mass and, more strongly, with stellar population age, in good qualitative agreement with the observed dependence of galaxy clustering and galaxy-mass correlations on galaxy type. The predicted ratio dM(260)/M_b is lower than the SDSS estimates for galaxies with M_b>~2e11 Msun, but numerical resolution effects account for most of this discrepancy, leaving little room for feedback or other astrophysical processes to reduce the stellar masses of luminous galaxies, at least given our adopted cosmological parameters. Our results show that the LCDM model and the galaxy formation physics incorporated in the SPH simulation give a good account of observed galaxy clustering, but anticipated improvements in clustering and weak lensing measurements will soon test this picture in much more detail., Submitted to ApJ, 49 pages, 14 figures, version with full resolution Figs 1 and 3 at http://www.astronomy.ohio-state.edu/~dhw/Preprints/gcbias.ps.gz

Not the way one might have thought. In hydrodynamic simulations of galaxy formation, some gas follows the traditionally envisioned route, shock heating to the halo virial temperature before cooling to the much lower temperature of the neutral ISM. But most gas enters galaxies without ever heating close to the virial temperature, gaining thermal energy from weak shocks and adiabatic compression, and radiating it just as quickly. This ``cold mode'' accretion is channeled along filaments, while the conventional, ``hot mode'' accretion is quasi-spherical. Cold mode accretion dominates high redshift growth by a substantial factor, while at z, 7 pages, 1 figure. To be published in the proceedings of the IGM/Galaxy Connection- The Distribution of Baryons at z=0 conference

We use hydrodynamic simulations to predict correlations between Lya forest absorption and galaxies at redshift z~3. The probability distribution function (PDF) of Lya flux decrements shifts systematically towards higher values in the vicinity of galaxies, reflecting the overdense environments in which these galaxies reside. The predicted signal remains strong in spectra smoothed over 50-200 km/s, allowing tests with moderate resolution quasar spectra. The strong bias of high redshift galaxies towards high density regions imprints a clear signature on the flux PDF, but the predictions are not sensitive to galaxy baryon mass or star formation rate, and they are similar for galaxies and for dark matter halos. The dependence of the flux PDF on galaxy proximity is sensitive to redshift determination errors, with rms errors of 150-300 km/s substantially weakening the predicted trends. On larger scales, the mean galaxy overdensity in a cube of 5 or 10 Mpc/h (comoving) is strongly correlated with the mean Lya flux decrement on a line of sight through the cube center. The slope of the correlation is ~3 times steeper for galaxies than for dark matter as a result of galaxy bias. The predicted large scale correlation is in qualitative agreement with recently reported observational results. However, observations also show a drop in absorption in the immediate vicinity of galaxies, which our models do not predict even if we allow the galaxies or AGNs within them to be ionizing sources. This decreased absorption could be a signature of galaxy feedback on the surrounding IGM, perhaps via galactic winds. Peculiar velocities often allow gas at comoving distances ~1.5 Mpc/h to produce saturated absorption at the galaxy redshift, so any feedback mechanism must suppress neutral hydrogen out to these radii to match the data. (Abridged), Comment: 54 pages, 19 figures. Accepted for publication in ApJ. Includes discussion of simple wind models

Using a hydrodynamic simulation of a LCDM universe, we investigate the "X-ray forest" absorption imprinted on the spectra of background quasars by the intervening intergalactic medium (IGM). In agreement with previous studies, we find that OVII and OVIII produce the strongest absorption features. The strong oxygen absorbers that might be detectable with Chandra or XMM-Newton arise in gas with T ~ 10^6 K and overdensities delta >~ 100 that are characteristic of galaxy groups. Future X-ray missions could detect weaker oxygen absorption produced by gas with a wider range of temperatures and the lower densities of unvirialized structures; they could also detect X-ray forest absorption by C, N, Ne, Fe, and possibly Si. If the IGM metallicity is 0.1 solar, then the predicted number of systems strong enough for a ~5\sigma detection with Chandra or XMM-Newton is extremely low, though scatter in metallicity would increase the number of strong absorbers even if the mean metallicity remained the same. Our simulation reproduces the high observed incidence of OVI absorbers (in the UV), and the most promising strategy for finding the X-ray forest is to search at the redshifts of known OVI systems, thus reducing the signal-to-noise threshold required for a significant detection. However, while many OVI absorbers have associated OVII or OVIII absorption, the OVI systems trace only the low temperature phases of the X-ray forest, and a full accounting of the strong OVII and OVIII systems will require a mission with the anticipated capabilities of Constellation-X. The large effective area of the XEUS satellite would make it an extremely powerful instrument for studying the IGM, measuring X-ray forest absorption by a variety of elements and revealing the shock-heated filaments that may be an important reservoir of cosmic baryons., Comment: 41 pages including 16 figures, replaced with version accepted by ApJ. Includes clarification of several points and brief discussion of recent observational results

We study the predicted sub-mm emission from massive galaxies in a Lambda-CDM universe, using hydrodynamic cosmological simulations. Assuming that most of the emission from newly formed stars is absorbed and reradiated in the rest-frame far-IR, we calculate the number of galaxies that would be detected in sub-mm surveys conducted with SCUBA. The predicted number counts are strongly dependent on the assumed dust temperature and emissivity law. With plausible choices for SED parameters (e.g., T=35 K, beta=1.0), the simulation predictions reproduce the observed number counts above ~ 1 mJy. The sources have a broad redshift distribution with median z ~ 2, in reasonable agreement with observational constraints. However, the predicted count distribution may be too steep at the faint end, and the fraction of low redshift objects may be larger than observed. In this physical model of the sub-mm galaxy population, the objects detected in existing surveys consist mainly of massive galaxies (several M_*) forming stars fairly steadily over timescales ~ 10^8-10^9 years, at moderate rates ~100 Msun/yr. The typical descendants of these sub-mm sources are even more massive galaxies, with old stellar populations, found primarily in dense environments. While the resolution of our simulations is not sufficient to determine galaxy morphologies, these properties support the proposed identification of sub-mm sources with massive ellipticals in the process of formation. The most robust and distinctive prediction of this model, stemming directly from the long timescale and correspondingly moderate rate of star formation, is that the far-IR SEDs of SCUBA sources have a relative high 850 micron luminosity for a given bolometric luminosity. [Abridged], Comment: Submitted to ApJ. 34 pages including 8 PS figures

We investigate the X-ray vs. optical scaling relations of poor groups to small clusters (sigma~100-700 km/s) identified in a cosmological hydrodynamic simulation of a Lambda-CDM universe, with cooling and star formation but no pre-heating. We find that the scaling relations between X-ray luminosity, X-ray temperature, and velocity dispersion show significant departures from the relations predicted by simple hydrostatic equilibrium models or simulations without cooling, having steeper L_X-sigma and L_X-T_X slopes and a "break" at \~200 km/s (~0.3 keV). These departures arise because the hot (X-ray emitting) gas fraction varies substantially with halo mass in this regime. Our predictions roughly agree with observations. Thus radiative cooling is a critical physical process in modeling galaxy groups, and may present an alternative to ad hoc models such as pre-heating or entropy floors for explaining X-ray group scaling relations., Comment: 4 pages, to be puplished in the proceedings of the ''Sesto 2001-Tracing Cosmic Evolution with Galaxy Clusters''

We analyze high-redshift structure in three hydrodynamic simulations that have identical initial conditions and cosmological parameters and differ only in the value of the baryon density parameter, Omega_b=0.02, 0.05, 0.125. Increasing Omega_b does not change the fraction of baryons in the diffuse (unshocked) phase of the intergalactic medium, but it increases cooling rates and therefore transfers some baryons from the shocked intergalactic phase to the condensed phase associated with galaxies. Predictions of Lyman-alpha forest absorption are almost unaffected by changes of Omega_b provided that the UV background intensity is adjusted so that the mean opacity of the forest matches the observed value. The required UV background intensity scales as Omega_b^1.7, and the higher photoionization rate increases the gas temperature in low density regions. Damped Lyman-alpha absorption and Lyman limit absorption both increase with increasing Omega_b, though the impact is stronger for damped absorption and is weaker at z=4 than at z=2-3. The mass of cold gas and stars in high-redshift galaxies increases faster than Omega_b but slower than Omega_b^2, and the global star formation rate scales approximately as Omega_b^1.5. In the higher Omega_b models, the fraction of baryonic material within the virial radius of dark matter halos is usually higher than the universal fraction, indicating that gas dynamics and cooling can lead to over-representation of baryons in virialized systems. On the whole, our results imply a fairly intuitive picture of the influence of Omega_b on high-redshift structure, and we provide scalings that can be used to estimate the impact of Omega_b uncertainties on the predictions of hydrodynamic simulations., Comment: 22 pages, 9 figures, submitted to ApJ, high-resolution version availible at http://lahmu.phyast.pitt.edu/~gardner/publications/omegab.ps.gz

We combine COBE-DMR measurements of cosmic microwave background anisotropy with a recent measurement of the mass power spectrum at redshift z=2.5 from Lya forest data to derive constraints on cosmological parameters and test the inflation+CDM scenario of structure formation. By treating the inflationary spectral index n as a free parameter, we can find successful fits to the COBE and Lya forest constraints in Omega_m=1 models with and without massive neutrinos and in low-Omega_m models with and without a cosmological constant. Within each class of model, the combination of COBE and the Lya forest P(k) constrains a parameter combination of the form (Omega_m h^a n^b Omega_b^c), with different indices for each case. This new constraint breaks some of the degeneracies in cosmological parameter determinations from other measurements. The Lya forest P(k) provides the first measurement of the slope of the linear mass power spectrum on ~Mpc scales, and it confirms a basic prediction of the inflationary CDM scenario: a nearly scale-invariant spectrum of primeval fluctuations (n~1) that bends towards k^{n-4} on small scales. Considering additional observational data, we find that COBE-normalized, Omega_m=1 models that match the Lya forest P(k) do not match the observed masses of rich galaxy clusters and that a low-Omega_m model with a cosmological constant provides the best overall fit, even without the direct evidence for cosmic acceleration from supernovae. Modest improvements in the Lya forest P(k) measurement could greatly restrict the allowable region of parameter space for CDM models, constrain the contribution of tensor fluctuations to CMB anisotropy, and achieve a more stringent test of the current consensus model of structure formation., 26 pages, 6 figs, ApJ, in press (10/01). New version corrects some numerical results for Lambda-CDM models, updates discussion in light of new observational developments (especially CMB). Main conclusions unchanged

We examine the cooling radiation from forming galaxies in hydrodynamic simulations of the LCDM model (cold dark matter with a cosmological constant), focusing on the Ly-alpha line luminosities of high-redshift systems. Primordial composition gas condenses within dark matter potential wells, forming objects with masses and sizes comparable to the luminous regions of observed galaxies. As expected, the energy radiated in this process is comparable to the gravitational binding energy of the baryons, and the total cooling luminosity of the galaxy population peaks at z ~= 2. However, in contrast to the classical picture of gas cooling from the \sim 10^6 K virial temperature of a typical dark matter halo, we find that most of the cooling radiation is emitted by gas with T < 20,000 K. As a consequence, roughly 50% of this cooling radiation emerges in the Ly-alpha line. While a galaxy's cooling luminosity is usually smaller than the ionizing continuum luminosity of its young stars, the two are comparable in the most massive systems, and the cooling radiation is produced at larger radii, where the Ly-alpha photons are less likely to be extinguished by dust. We suggest, in particular, that cooling radiation could explain the two large (\sim 100 kpc), luminous (L_{Ly-alpha} \sim 10^{44} erg s^{-1}) ``blobs'' of Ly-alpha emission found in Steidel et al.'s (1999) narrow band survey of a z = 3 proto-cluster. Our simulations predict objects of the observed luminosity at about the right space density, and radiative transfer effects can account for the observed sizes and line widths. We discuss observable tests of this hypothesis for the nature of the Ly-alpha blobs, and we present predictions for the contribution of cooling radiation to the Ly-alpha luminosity function of galaxies as a function of redshift., Submitted to ApJ. 28 pages including 9 PS figures. Version with color figures available at http://donald.astro.umass.edu/~fardal/papers/cooling/cooling.html

We review theoretical approaches to the study of galaxy formation, with emphasis on the role of hydrodynamic simulations in modeling the high redshift galaxy population. We present new predictions for the abundance of star-forming galaxies in the Lambda + cold dark matter model (Omega_m=0.4, Omega_L=0.6), combining results from several simulations to probe a wide range of redshift. At a threshold density of one object per arcmin^2 per unit z, these simulations predict galaxies with star formation rates of 2 msun/yr (z=10), 5 msun/yr (z=8), 20 msun/yr (z=6), 70-100 msun/yr (z=4-2), and 30 msun/yr (z=0.5). For galaxies selected at a fixed comoving space density n=0.003 h^3 Mpc^{-3], a (50 Mpc/h)^3 simulation predicts a galaxy correlation function (r/5 Mpc/h)^{-1.8} in comoving coordinates, essentially independent of redshift from z=4 to z=0.5. Different cosmological models predict global histories of star formation that reflect their overall histories of mass clustering, but robust numerical predictions of the comoving space density of star formation are difficult because the simulations miss the contribution from galaxies below their resolution limit. The LCDM model appears to predict a star formation history with roughly the shape inferred from observations, but it produces too many stars at low redshift, predicting Omega_* ~ 0.015 at z=0. We conclude with a brief discussion of this discrepancy and three others that suggest gaps in our current theory of galaxy formation: small disks, steep central halo profiles, and an excess of low mass dark halos. While these problems could fade as the simulations or observations improve, they could also guide us towards a new understanding of galactic scale star formation, the spectrum of primordial fluctuations, or the nature of dark matter., 12 pages, 3 figs. To be published in "Photometric Redshifts and High Redshift Galaxies", eds. R. Weymann, L. Storrie-Lombardi, M. Sawicki & R. Brunner, (San Francisco: ASP Conference Series)

We determine the evolution from $z=3\to 0$ of the galaxy and mass correlation functions and bias factor in a $50h^{-1}$Mpc $\Lambda$CDM hydrodynamic simulation with $10h^{-1}$kpc resolution. The mass correlation function grows with time, but the galaxy correlation function shows little evolution and is well described by a power law. At early times, galaxies are biased traces of mass, with bias being higher on smaller scales. By z=0, galaxies trace the mass, and the bias shows little scale dependence., Comment: 2 pages, contribution to the Proceedings of Rencontres Internationales de l'IGRAP, Clustering at High Redshift, Marseille 1999

What type of objects are being detected as $z\sim 3$ "Lyman break galaxies"? Are they predominantly the most massive galaxies at that epoch, or are many of them smaller galaxies undergoing a short-lived burst of merger-induced star formation? We attempt to address this question using high-resolution cosmological hydrodynamic simulations including star formation and feedback. Our $\Lambda$CDM simulation, together with Bruzual-Charlot population synthesis models, reproduces the observed number density and luminosity function of Lyman break galaxies when dust is incorporated. The inclusion of dust is crucial for this agreement. In our simulation, these galaxies are predominantly the most massive objects at this epoch, and have a significant population of older stars. Nevertheless, it is possible that our simulations lack the resolution and requisite physics to produce starbursts, despite having a physical resolution of $\la 700$ pc at z=3. Thus we cannot rule out merger-induced starburst galaxies also contributing to the observed population of high-redshift objects., Comment: 5 pages, contribution to the Proceedings of Rencontres Internationales de l'IGRAP, Clustering at High Redshift, Marseille 1999

We outline the physical picture of the high-redshift Ly-alpha forest that has emerged from cosmological simulations, describe statistical characteristics of the forest that can be used to test theories of structure formation, present a preliminary comparison between simulation results and measurements from Keck HIRES spectra, and discuss a recent determination of the slope and amplitude of the linear mass power spectrum P(k) at z=2.5 from moderate resolution spectra. The physical picture is simple if each QSO spectrum is viewed as a continuous non-linear map of the line-of-sight density field rather than a collection of discrete absorption lines. The distribution of flux decrements depends mainly on the amplitude and PDF (Gaussian vs. non-Gaussian) of the primordial density fluctuations. The threshold crossing frequency, analogous to the 3-d genus curve, responds to the shape and amplitude of P(k) and to the values of Omega and Lambda. Open and Lambda-CDM models agree well with the measured flux decrement distribution at smoothing lengths of 25 km/s and 100 km/s and with the threshold crossing frequency at 100 km/s. Discrepancy with the observed threshold crossing frequency at 25 km/s may reflect the combined effects of noise in the data and limited mass resolution of the simulations. The slope of the measured P(k) agrees with the predictions of inflation+CDM models. Combining the amplitude with COBE normalization imposes a constraint on these models of the form Omega h^x n^y Omega_b^z = constant. Assuming Gaussian primordial fluctuations and a power spectrum shape parameter Gamma~0.2, consistency of the measured P(k) with the observed cluster mass function at z=0 requires Omega=0.46^{+0.12}_{-0.10} for an open universe and Omega=0.34^{+0.13}_{-0.09} for a flat universe (1-sigma errors). (Shortened), 13 pages, 9 figs. To appear in Proceedings of the MPA/ESO Conference "Evolution of Large Scale Structure: From Recombination to Garching"

We search for OVI absorption in a Keck HIRES spectrum of the z=3.62 quasar Q1422+231. Comparison of CIV measurements to cosmological simulations shows that \lya forest absorbers with N_HI > 10^{14.5} have [C/H]~=-2.5, for the UV background spectrum of Haardt & Madau (HM). Lower column density absorption arises in lower density gas, where OVI is the most sensitive metal tracer. Since OVI lines lie at wavelengths contaminated by Lyman series absorption, we interpret our Q1422 results by comparing to artificial spectra drawn from an SPH simulation of a Lambda-dominated CDM model. A search for deep, narrow features in Q1422 yields only a few candidate OVI lines, statistically consistent with the number in artificial spectra with no metals; spectra generated with the HM background and [O/H] >= -2.5 predict too many narrow lines. However, applying the optical depth ratio technique of Songaila (1998), we DO find significant OVI associated with CIV systems; matching Q1422 requires [O/C]~=+0.5, implying [O/H]~=-2.0. Taken together these results imply that (a) the metallicity in the low density IGM is at least a factor of three below that in the overdense regions where CIV absorption is detectable, and (b) oxygen is overabundant in these regions, consistent with the enrichment pattern of old halo stars. If the UV background is heavily truncated above 4 Ry, an implausibly high oxygen overabundance ([O/C]>+2) is required by the data; thus a majority of the volume of the universe must have undergone helium reionization by z=3.(Abridged), Submitted to ApJ, 48 pp including 14 ps figures, uses aaspp4.sty

We discuss simulations of gas at high redshift. We briefly review the methods used and the results for quasar absorption lines. We present gas mass functions and galaxy correlation functions for 5 different cosmological models. Galaxies should be detectable at redshifts greater than 2 by SKAI, and measurements of the gas mass functions and galaxy correlation functions could be used to discriminate between different cosmological models., 9 pages, 4 figures in text, LaTex (paspconf.sty) To appear in the conference proceedings of Highly Redshifted Radio Lines

We estimate the value of the matter density parameter \Omega_0 by combining constraints from the galaxy cluster mass function with Croft et al.'s recent measurement of the mass power spectrum, P(k), from \lya forest data. The key assumption of the method is that cosmic structure formed by gravitational instability from Gaussian primordial fluctuations. For a specified value of \Omega_0, matching the observed cluster mass function then fixes the value of \sigma_8, the rms amplitude of mass fluctuations in 8\hmpc spheres, and it thus determines the normalization of P(k) at z=0. The value of \Omega_0 also determines the ratio of P(k) at z=0 to P(k) at z=2.5, the central redshift of the \lya forest data; the ratio is different for an open universe (\Lambda=0) or a flat universe. Because the \lya forest measurement only reaches comoving scales 2\pi/k ~ 15-20\hmpc, the derived value of \Omega_0 depends on the value of the power spectrum shape parameter \Gamma, which determines the relative contribution of larger scale modes to \sigma_8. Adopting \Gamma=0.2, a value favored by galaxy clustering data, we find \Omega_0 = 0.46^{+0.12}_{-0.10} for an open universe and \Omega_0=0.34^{+0.13}_{-0.09} for a flat universe (1\sigma errors, not including the uncertainty in cluster normalization). Cluster-normalized models with \Omega_0=1 predict too low an amplitude for P(k) at z=2.5, while models with \Omega_0=0.1 predict too high an amplitude. The more general best fit parameter combination is approximately \Omega_0 + 0.2\Lambda_0 = 0.46 + 1.3(\Gamma-0.2). Analysis of larger, existing samples of QSO spectra could greatly improve the measurement of P(k) from the \lya forest, allowing a determination of \Omega_0 by this method with a precision of ~15%, limited mainly by uncertainty in the cluster mass function., Comment: Submitted to ApJ, 6 emulateapj pages w/ 2 postscript figs

A convincing physical picture for the Lyman-alpha forest has emerged from simulations and related semi-analytic studies of structure formation models. Observations can be be used in the context of this picture to study cosmology using the structure of the forest. With the availability of well motivated predictions, not only has it become possible to test models directly, but the physical processes involved appear to be simple enough that we can attempt to reconstruct aspects of the underlying cosmology from observations. We briefly summarise the method of Croft et al (1997) (astro-ph/9708018) for recovering the primordial mass power spectrum from Lyman-alpha forest data, emphasising the physical reasons that the derived P(k) is independent of unknown "bias factors". We present an illustrative application of the method to four quasar "spectra" reconstructed from published line lists., Comment: Latex (sprocl.sty), 6 pages, 2 ps figs. To appear in "Large-Scale Structure: Tracks and Traces", proceedings of the 12th Potsdam Cosmology Workshop (held Sept 1997), eds V. Muller et al., World Scientific, 1998

The metal absorption lines found in association with \lya absorbers of moderate to high HI column density contain valuable information about the metallicity and ionization conditions within the absorbers and offer a stronger test of models of the intergalactic medium at \sim 3$ than HI absorption lines alone. We have developed a method to predict the strengths of metal absorption lines within the framework of cosmological models for the \lya forest. The method consists of evaluating a quantity, the Line Observability Index, for a database of hundreds of candidate metal lines, allowing a comprehensive identification of the lines the model predicts to be detectable associated with a \lya absorber of a given HI column density and metallicity. Applying this technique to a particular class of models at \sim 2-4$, we predict that the OVI(1032 \AA, 1038 \AA) doublet is the only practical probe of the metallicity of low column density absorbers (HI \simlt 10^{14.5} cm^{-2}$), that CIV (1548 \AA) is the strongest line with rest wavelength $\lambda_r > 1216$ \AA{} regardless of HI$, and that the strongest metal lines should be CIII(977 \AA) and SiIII(1206.5 \AA), which peak at HI \sim 10^{17} cm^{-2}$., Comment: To appear in the proceedings from the 13th IAP Workshop (1-5 July 1997) "Evolution of the Intergalactic Medium From QSO Absorption Line Systems"

Any viable cosmological model must produce enough structure at early epochs to explain the amount of gas associated with high-redshift damped Ly$\alpha$ systems. We study the evolution of damped Ly$\alpha$ systems at redshifts $z\ge 2$ in cold dark matter (CDM) and cold+hot dark matter (CDM+HDM) models using both N-body and hydrodynamic simulations. Our approach incorporates the effects of gas dynamics, and we find that all earlier estimates which assumed that all the baryons in dark matter halos would contribute to damped Ly$\alpha$ absorption have overestimated the column density distribution $f(N)$ and the fraction of neutral dense gas $\Omega_g$ in damped Ly$\alpha$ systems. The differences are driven by ionization of hydrogen in the outskirts of galactic halos and by gaseous dissipation near the halo centers, and they tend to exacerbate the problem of late galaxy formation in CDM+HDM models. We only include systems up to the highest observed column density $N\sim 10^{21.8}$ cm$^{-2}$ in the estimation of $\Omega_g$ for a fair comparison with data. If the observed $f(N)$ and $\Omega_g$ inferred from a small number of confirmed and candidate absorbers are robust, the amount of gas in damped Ly$\alpha$ systems at high redshifts in the $\Omega_\nu=0.2$ CDM+HDM model falls well below the observations., Comment: 11 pages including 2 figures. AAS LaTeX v4.0. Astrophysical Journal Letters, in press

We develop a prescription for characterizing the strengths of metal lines associated with Lyman-alpha forest absorbers (LYFAs) of a given neutral hydrogen column density N_HI and metallicity [Fe/H]. This Line Observability Index (LOX) is line-specific and translates, for weak lines, into a measure of the equivalent width. It can be evaluated quickly for thousands of transitions within the framework of a given model of the Lyman-alpha forest, providing a ranking of the lines in terms of their strengths and enabling model builders to select the lines that should be detectable in observed spectra of a given resolution and signal-to-noise ratio. We compute the LOX for a large number of elements and transitions in two cosmological models of the Lyman-alpha forest at z=3 derived from a hydrodynamic simulation of structure formation, and we discuss how the LOX depends on redshift and on model parameters such as the mean baryonic density and radiation field. We find that the OVI (1032,1038) doublet is the best probe of the metallicity in low column density LYFAs N_{HI} \approx 10^{14.5} cm^{-2}). Metallicities down to [O/H] \sim -3 ([Fe/H] \sim -3.5 with the assumed [O/Fe] ratio) yield OVI absorption features that should be detectable in current high-quality spectra, provided that the expected position of the OVI feature is not contaminated by HI absorption. The strongest transitions in lower ionisation states of oxygen are OV(630), OIV(788), and OIII(833), and are likely to be detected with next generation UV instruments. Of the lines with rest wavelengths \lambda_r > 1216, which can potentially be observed redwards of the \lya forest, the CIV(1548,1551) doublet is expected to dominate in all LYFAs, regardless of the value of N_HI., Comment: Substantially revised version: larger line database, additional cosmological model analyzed. Accepted for ApJ

Leapfrog integration has been the method of choice in N-body simulations owing to its low computational cost for a symplectic integrator with second order accuracy. We introduce a new leapfrog integrator that allows for variable timesteps for each particle in large N-body simulations. Tests with single particles in fixed potentials show that it behaves as a symplectic integrator. We then examine the results of both standard leapfrog and our temporally adaptive leapfrog on full N-body integrations of clusters and large scale structure establishing accuracy criteria for both methods. The adaptive method shows significant speed-ups over single step integrations---but the integrator no longer appears to be symplectic or, in the case of large scale structure simulations, accurate. This loss of accuracy appears to be caused by the way that the timestep is chosen, not by the integrator itself. We present a related integration technique that does retain sufficient accuracy. Although it is not symplectic, it is apparently better than previous implementations and is our current integrator of choice for large astrophysical simulations. We also note that the standard leapfrog difference equations used in cosmological N-body integrations in comoving coordinates are not symplectic. We derive an implementation of leapfrog that is in comoving canonical coordinates to correct for this deficiency., Comment: 16 pages, 12 figures, submitted to Ap. J

We describe cosmological simulation techniques and their application to studies of cosmic structure formation, with particular attention to recent hydrodynamic simulations of structure in the high redshift universe. Collisionless N-body simulations with Gaussian initial conditions produce a pattern of sheets, filaments, tunnels, and voids that resembles the observed large scale galaxy distribution. Simulations that incorporate gas dynamics and dissipation form dense clumps of cold gas with sizes and masses similar to the luminous parts of galaxies. Models based on inflation and cold dark matter predict a healthy population of high redshift galaxies, including systems with star formation rates of 20 M_{\sun}/year at z=6. At z~3, most of the baryons in these models reside in the low density intergalactic medium, which produces fluctuating Lyman-alpha absorption in the spectra of background quasars. The physical description of this ``Lyman-alpha forest'' is particularly simple if the absorption spectrum is viewed as a 1-dimensional map of a continuous medium instead of a collection of lines. The combination of superb observational data and robust numerical predictions makes the Lyman-alpha forest a promising tool for testing cosmological models., Comment: Latex w/ paspconf.sty, 25 pages, 8 ps figs. To appear in Origins, eds. J. M. Shull, C. E. Woodward, & H. Thronson (ASP Conference Series)