Search

Dark matter is usually considered responsible for the flat rotation curves observed at the outer radii of spiral galaxies. Since the existence of dark matter has not been indisputably detected by experiments, magnetic field is sometimes proposed to be substitute for dark matter in explaining the shape of rotation curves of spiral galaxies. Magnetic field that interacts with cosmic rays generates pressure, which in turn accelerates gas flow/dynamics and affects the speed of gas rotation. Some other studies discuss the possibilities that the flat or even increased rotation curves at the outer radii may be attributed both to dark matter and the galactic magnetic fields. In this work, we examined 16 rotation curve data of dwarf spiral galaxies in the Spitzer Photometry and Accurate Rotation Curves (SPARC) catalog obtained by Spitzer in 3.6 μm. We find that the shape of the rotation curves of the dwarf spiral galaxies cannot be explained by the contribution of magnetic field alone, due to stability issues. However, adding some contribution of magnetic field to the dark matter component explains the rotation curves at the large radii better, compared to dark matter without magnetic field. We therefore determined the maximum contribution of the magnetic field for the 16 dwarf spiral galaxies. [ABSTRACT FROM AUTHOR]

Dwarf spheroidal galaxies are promising targets for the indirect detection of dark matter through gamma-ray emission due to their proximity, lack of astrophysical backgrounds and high dark matter density. They are often used to place restrictive bounds on the dark matter annihilation cross section. In this report, we analyze six years of Fermi-LAT gamma-ray data from Dwarf Spheroidal galaxies that are satellites of the Milky Way, and constrain the parameter space that explain the Fermi-LAT Galactic center gamma-ray emission in 4 body final states, neutralino dark matter annihilation cross-section, and decaying dark matter lifetime for several decay channels. [ABSTRACT FROM AUTHOR]

Current cosmological models and data suggest the existence of a Cold Dark Matter (DM) component, however the nature of DM particles remains unknown. A favored candidate for DM is a Weakly Interacting Massive Particle (WIMP) in the mass range from 50 GeV to greater than 10 TeV. Nearby dwarf spheroidal galaxies (dSph) are expected to contain a high density of Dark Matter with a low gamma-ray background, and are thus promising targets for the detection of secondary gamma rays at very high energies (VHE, E > 0.1 TeV) through the annihilation of WIMPS into SM particles. Presented here are recent VERITAS observations of dSph, including a deep exposure on Segue 1. Limits are derived for various annihilating and decaying dark matter particle models. [ABSTRACT FROM AUTHOR]

Chemical abundances of very metal-poor stars record the nucleosynthesis products in the early Galaxy. Previous observational studies have revealed that the r-process is the dominant source of neutron-capture elements in extremely metal-poor stars, but the total amount of neutron-capture elements relative to lighter elements (e.g. Fe) shows large star-to-star scatter. These features are key to understanding the astrophysical sites of the r-process as well as the formation of stellar systems including extremely metal-poor stars. Such abundance studies for neutron-capture elements have been extended to metal-poor stars in globular clusters and dwarf galaxies around the Milky Way, in addition to field halo stars. We investigated the abundance ratios of neutron-capture elements for an extremely metal-poor star in the Sextans dwarf galaxy for which a significant excess of Ba was detected by a previous study, and found that the origin of heavy elements in this object is the sprocess. Although the sample size is still small, no r-process enhanced star has been found in dwarf galaxies at such low metallicity, in contrast to the Galactic halo. From ourmeasurements of neutroncapture elements in three very metal-poor globular clusters, we confirmed star-to-star scatter of Eu abundance ratios in M15, but found that these clusters are almost homogeneously enriched in lighter neutron-capture elements. [ABSTRACT FROM AUTHOR]

Here we present three adaptive mesh refinement radiation hydrodynamics simulations that illustrate the impact of momentum transfer from ionising radiation to the absorbing gas on star formation in high-redshift dwarf galaxies. Momentum transfer is calculated by solving the radiative transfer equation with a ray tracing algorithm that is adaptive in spatial and angular coordinates. We find that momentum input partially affects star formation by increasing the turbulent support to a three-dimensional rms velocity equal to the circular velocity of early haloes. Compared to a calculation that neglects radiation pressure, the star formation rate is decreased by a factor of five to 1.8 × 10-2 M⊙ yr-1 in a dwarf galaxy with a dark matter and stellar mass of 2.0 × 108 M⊙ and 4.5 × 105 M⊙, respectively, when radiation pressure is included. Its mean metallicity of 10-2.1 Z⊙ is consistent with the observed dwarf galaxy luminosity-metallicity relation. In addition to photo-heating in H II regions, radiation pressure further drives dense gas from star forming regions, so supernovae feedback occurs in a warmer and more diffuse medium, launching metal-rich outflows. [ABSTRACT FROM AUTHOR]

We investigate how radiative feedback from the first stars affects the assembly of the first dwarf galaxies. We perform cosmological zoomed smoothed particle hydrodynamics simulations of a galaxy assembling inside a halo reaching a virial mass ∼ 109 M⊙ at z = 10. The simulations follow the non-equilibrium chemistry and cooling of primordial gas and the subsequent conversion of the cool dense gas into massive metal-free stars. To quantify the radiative feedback, we compare a simulation in which stars emit both molecular hydrogen dissociating and hydrogen ionizing radiation with a simulation in which stars do not emit radiation but remain dark. Photodissociation and photoionization exert a strong negative feedback on the assembly of the galaxy inside the minihalo progenitor, impeding gas condensation and suppressing star formation. The radiative feedback on the gas implies a suppression of the central dark matter densities in the minihalo by factors of up to a few, which is a significant deviation from the singular isothermal density profile characterizing the dark matter distribution in the absence of radiative feedback. The properties of the galaxy become insensitive to the inclusion of radiation once the minihalo turns into an atomic cooler. The formation of a rotationally supported extended disk inside the atomically cooling galaxy therefore is a robust outcome of our simulations. Our simulations make predictions for observations with the upcoming James Webb Space Telescope. [ABSTRACT FROM AUTHOR]

Observations seem to support that dwarf Spheroidal (dSph) galaxies have a cored halo. Ursa Minor (UMi) is one of the most dark-matter dominated dSph in the Local Group and the long survival time of its cold stellar clump can be explainedwith a large enough core in its density profile. Bosonic dark matter predicts cored halos in a natural way. The size of the core depends on the mass m of the dark matter boson and on the strength of the boson self-interaction λ . In this work we constrain m and m4/λ such as the core in bosonic halos be compatible with the survival time of UMi's clump. [ABSTRACT FROM AUTHOR]

Under the assumption that dark matter is made of new particles, annihilations of those are required to reproduce the correct dark matter abundance in the Universe. This process can occur in dense regions of our Galaxy such as the Galactic center, dwarf galaxies and other types of subhaloes. High-energy gamma-rays are expected to be produced in dark matter particle collisions and could be detected by ground-based Cherenkov telescopes such as HESS, MAGIC and VERITAS. The main experimental challenges to get constraints on particle dark matter models are reviewed, making explicit the pros and cons that are inherent to this technique, together with the current results from running observatories. Main results concerning dark matter searches towards selected targets with Cherenkov telescopes are presented. Eventually, a focus is made on a new way to perform a search for Galactic sub-halos with such telescopes, based on wide-field surveys, as well as future prospects. [ABSTRACT FROM AUTHOR]

We observed the dwarf spheroidal (dSph) galaxy Leo I for 80,000 seconds with XMM-Newton and detected 105 non Galactic X-ray sources (XRS) in its field. We examined deep optical images and catalogs, and discuss the possibility that some of the sources are Low Mass X-Ray Binaries (LMXB) intrinsic to Leo I. We also studied an archival XMM-Newton observation of the Fornax dwarf spheroidal galaxy, and compare it with Leo I. 104 non Galactic XRS were detected in the Fornax field; one of them coincides with a blue object at the outskirt of the Fornax 4 globular cluster. In both observations about a third of the XRS are identified with extragalactic objects observed in optical and radio. We discuss the similarities between the two XRS groups and the differences, which may be due to the a small number of intrinsic LMXB in the two galaxies. Fornax is at half the distance of Leo I, and has an almost 4 times larger luminous mass, but is also much more sparse on the sky. Leo I has had at least one more recent episode of star formation (Held et al. 2000), and Fornax has exceptionally retained its globular clusters (Greco et al. 2009, and references therein). Although further observations are needed to study and examine our LMXB candidates, these objects are very interesting because they represented primordial LMXB, not formed by tidal encounters unlike a large fraction of known LMXB. [ABSTRACT FROM AUTHOR]

The chemical evolution of the Galaxy and the early Universe is a key topic in modern astrophysics. Since the most metal-poor Galactic stars are the local equivalent of the high-redshift Universe, they can be employed to reconstruct the onset of the chemical and dynamical formation processes of the Galaxy, the origin and evolution of the elements, and associated nucleosynthesis processes. They also provide constraints on the nature of the first stars and SNe, the initial mass function, and early star formation processes. I will discuss how the two most iron-poor stars, metal-poor stars with r-process abundance signatures, and the newly discovered extremely metal-poor stars in small dwarf galaxies are prime candidates to study the elusive first stars and their nucleosynthetic yields. [ABSTRACT FROM AUTHOR]

Extremely metal-poor stars are believed to provide a unique opportunity to investigate the detailed nucleosynthesis by first generations of stars as well as the early stage of the Milky Way formation. Such objects have been found by recent surveys of dwarf galaxies around the Milky Way. Chemical abundance studies for stars in dwarf galaxies revealed the diversity of the abundance ratios (e.g. α/Fe ratios) and their metallicity dependence, suggesting that these dwarf galaxies are not simply remnants of the building blocks of the Milky Way halo, but have original enrichment histories. More recent studies for fainter galaxies (so-called ultra-faint dwarf galaxies) suggest a higher fraction of extremely metal-poor stars in these galaxies than in classical dwarf galaxies, and the similarity of metallicity distribution to the field stars. However, chemical abundance ratios show large scatter, suggesting inhomogeneous chemical evolution. Comparisons of chemical abundance ratios between dwarf galaxies and the Milky Way halo are discussed. [ABSTRACT FROM AUTHOR]

Dwarf spheroidal (dSph) galaxies are the smallest galactic systems known. Also, under standard gravity, they are the most dark matter dominated systems known in astrophysics. The local sample of dSph satellites of the Milky Way comprise some two dozen systems, ranging from a few million, down to just a few thousand solar masses in stellar content, for the recently discovered ones. High quality observations of their stellar kinematics now allow detailed inferences of their structural and dynamical properties. We perform a modeling of the gravitational equilibrium configurations for a sample of he best studied dSphs under MOND, and find that a satisfactory description is obtained, without the need to invoke the presence of dark matter or any tidal effects, for a particular interpolation function, valid at this regime. We find also that the now well established scalings of the structural properties of these systems are naturally accounted for, while they require fine tuning under the dark matter hypothesis. In particular we find a previously unrecognized scaling between the inferred mass to light ratio of a particular dSph, with the age of most recent star formation episode for the system. Again, a trait which emerges naturally under any modified gravity scheme, and which appears as an unexplained coincidence under the dark matter hypothesis. [ABSTRACT FROM AUTHOR]

I used high resolution N-body/SPH simulations to model the new proper motion of the Large Magellanic Cloud within the Milky Way halo and investigate the effects of gravitational and hydrodynamical forces on the formation of the Magellanic Stream. [ABSTRACT FROM AUTHOR]

We use N-body simulations to study the tidal evolution of globular clusters (GCs) in dwarf spheroidal (dSph) galaxies. Our models adopt a cosmologically motivated scenario in which the dSph is approximated by a static NFW halo with a triaxial shape. For a large set of orbits and projection angles we examine the spatial and velocity distribution of stellar debris deposited during the complete disruption of stellar clusters. Our simulations show that such debris appears as shells, isolated clumps and elongated over-densities at low surface brightness (≥26 mag/arcsec2), reminiscent of substructure observed in several MW dSphs. Such features arise from the triaxiality of the galaxy potential and do not dissolve in time. Stellar over-densities reported in several MW dSphs may thus be the telltale evidence of dark matter haloes being triaxial in shape. We explore a number of kinematical signatures that would help to validate (or falsify) this scenario. The mean angular momentum of the cluster debris associated with box and resonant orbits, which are absent in spherical potentials, is null. As a result, we show that the line-of-sight velocity distribution may exhibit a characteristic “double-peak” depending on the oriention of the viewing angle with respect to the progenitor’s orbital plane. Kinematic surveys of dSphs may help to detect and identify substructures associated with the disruption of stellar clusters, as well as to address the shape of the dark matter haloes in which dSphs are embedded. [ABSTRACT FROM AUTHOR]

Using a high resolution N-body simulation of a two-component dwarf galaxy orbiting in the potential of the Milky Way, we study two effects that lead to significant biases in mass estimates of dwarf spheroidal galaxies. Both are due to the strong tidal interaction of initially disky dwarfs with their host. The tidal stripping of dwarf stars leads to the formation of strong tidal tails that are typically aligned with the line of sight of an observer positioned close to the host center. The stars from the tails contaminate the kinematic samples leading to a velocity dispersion profile increasing with the projected radius and resulting in an overestimate of mass. The tidal stirring of the dwarf leads to the morphological transformation of the initial stellar disk into a bar and then a spheroid. The distribution of stars in the dwarf remains non-spherical for a long time leading to an overestimate of its mass if it is observed along the major axis and an underestimate if it seen in the perpendicular direction. [ABSTRACT FROM AUTHOR]

We use Monte-Carlo Markov chain techniques to constrain acceptable parameter regions for the Munich L-Galaxies semi-analytic galaxy formation model. Feedback from active galactic nuclei (AGN) is required to limit star-formation in the most massive galaxies. However, we show that the introduction of tidal stripping of dwarf galaxies as they fall into and merge with their host systems can lead to a reduction in the required degree of AGN feedback. In addition, the new model correctly reproduces both the metallicity of large galaxies and the fraction of intracluster light. [ABSTRACT FROM AUTHOR]

Pulsation powered by deuterium burning in brown dwarfs and very low mass stars has been put forth (Palla & Baraffe 2005) as a novel probe of the interiors of these objects in the 1–15 Myr age range. Previous observations have hinted at variability on the expected timescales of a few hours, suggesting but not confirming that the phenomenon is at work in young brown dwarfs. We have recently carried out a dedicated campaign to search for this putative class of pulsators among known low-mass members of five young star clusters. Our survey achieves sensitivity to periodic oscillations with photometric amplitudes down to several millimagnitudes. We present the census of variability over timescales ranging from minutes to days and discuss the current prospects for pulsation as a tool in the study of young, objects near the substellar boundary. As a byproduct, this work provides new insights into the distribution of stellar rotation periods at young ages via the detection of variability due to cool surface spots. [ABSTRACT FROM AUTHOR]

The annihilations of WIMPs produce high-energy gamma-rays in the final state which could be detected by Imaging Atmospheric Cherenkov Telescopes such as the H.E.S.S. array. Plausible targets are the Galactic Center, the center of galaxy clusters, dwarf spheroidal galaxies or dark matter substructures in galactic haloes. We present here the H.E.S.S. observations on the nearby dwarf galaxies Sagittarius and Canis Major. Constraints on the velocity-weighted annihilation cross section of dark matter particles are derived in the framework of Supersymmetric and Kaluza-Klein models. An experimental flux sensitivity map to dark matter annihilation in a wide field of view is presented from the H.E.S.S. Galactic plane survey. A search for dark matter mini-spikes around intermediate mass black holes is presented as well as constraints on the particle physics parameters in various scenarios. [ABSTRACT FROM AUTHOR]

At present, dwarf spheroidal galaxies satellites of the Milky Way may represent the best astrophysical objects for dark matter (DM) searches with gamma-ray telescopes. They present the highest mass-to-light ratios known in the Universe. Furthermore, many of them are near enough from the Earth to be able to yield high predicted DM annihilation fluxes that might be observed by current gamma-ray instruments like MAGIC. The picture has become even better with the recent discovery of new dwarfs. These new objects are expected to yield even higher DM annihilation fluxes, since most of them are nearer than the previously known dwarfs and are even more DM dominated systems. Here a tentative list of the best candidates is given. The observational results obtained with MAGIC from the Draco dwarf as well as the observation of other dwarfs carried out by other Cherenkov telescopes are presented as well. Finally, we discuss the detection prospects of such kind of objects in the context of DM searches. [ABSTRACT FROM AUTHOR]

We present results of our search for X-ray line emission associated with the radiative decay of the sterile neutrino, a well-motivated dark matter candidate, in Suzaku Observatory spectra of the Ursa Minor dwarf spheroidal galaxy. These data represent the first deep observation of one of these extreme mass-to-light systems and the first dedicated dark matter search using an X-ray telescope. No such emission line is positively detected; and, we derive new upper limits that match or approach the best previous results over the entire 1–20 keV mass range from a single Suzaku observation. These are used to place constraints on the existence of sterile neutrinos with given combinations of mass and active-sterile neutrino oscillation mixing angle in the general case, and in the case where they are assumed to constitute all of the dark matter. The allowed range implies that sterile neutrinos remain a viable candidate to make up some—or all—of the dark matter and also explain pulsar kicks and various other astrophysical phenomena. [ABSTRACT FROM AUTHOR]

We present recent results from our survey of diffuse O VI-emitting gas in the interstellar medium with the Far Ultraviolet Spectroscopic Explorer (FUSE). These observations reveal the large-scale structure of the O VI-bearing gas in the quadrant of the sky centered on the Magellanic Clouds. The most prominent feature is a layer of low-velocity O VI emission extending more than 70° from the Galactic plane. At low latitudes (|b|<30°), the emission comes from narrow, high-density interfaces in the local ISM. At high latitudes, the emission is from extended, low-density regions in the Galactic halo. [ABSTRACT FROM AUTHOR]

We present O VI λ 1032 emission line measurements along several sight lines towards the Large Magellanic Cloud (LMC) and the Magellanic Interface region, obtained with the Far Ultraviolet Spectroscopic Explorer. The LMC targets include supernova remnants, superbubbles and diffuse interstellar regions. The observed O VI surface brightnesses form a near-continuous distribution spanning two-and-a-half orders of magnitude, with the diffuse gas at the faint end, and the strongly interacting supernova remnants at the bright end. The O VI emission detected from the Magellanic Interface region is evidence that the H I bearing gas is interacting with its surroundings. This, in turn, suggests that hot gas is present in the vicinity of the Magellanic Bridge, about 40 kpc away from the Galaxy. [ABSTRACT FROM AUTHOR]

The FUSE data for 287 OB stars in the Magellanic Clouds are an unprecedented resource to investigate the behavior of the stellar winds as a function of spectral type, luminosity class, and metallicity. The smaller extinction and interstellar absorption relative to Galactic counterparts render the MC sample more powerful, especially below 1100 Å. IUE provided a comparable Galactic sample at longer UV wavelengths, but HST has yet to do the same in the MCs. An initial FUSE atlas of 47 objects published in 2002 charted several basic trends, including the luminosity (or wind-density) sensitivity of the S IV, P V, and C III profiles, analogous to the previously unique Si IV behavior at longer UV wavelengths; this effect is due to the relatively low ionization of these species except for P V, for which the much lower abundance prevents saturation at lower luminosities. Now we are augmenting our sample by about 150 more objects, to provide better coverage of the parameters and redundancy at some spectral types, as well as to investigate categories of peculiar objects. Some outstanding early results are increased representation of the earliest spectral types (O2–O4), and the extension of the CNO dichotomy between O2 and ON2 spectra to the superionized O VI profiles. This work and subsequent quantitative modeling will improve our understanding of stellar winds, massive stellar evolution, and the integrated spectra of distant starbursts. [ABSTRACT FROM AUTHOR]

Over the past years observations of young and populous star clusters have shown that the stellar IMF appears to be an invariant featureless Salpeter power-law with an exponent α = 2.35 for stars more massive than a few M⊙. A consensus has also emerged that most, if not all, stars form in stellar groups and star clusters, and that the mass function of young star clusters in the solar-neighbourhood and in interacting galaxies can be described, over the mass range of a few 10M⊙ to 107M⊙, as a power-law with an exponent β≈2. Under the assumption of a relation between the most-massive star in a cluster and the cluster mass these two results imply that integrated galactic-field IMFs for early-type stars cannot be a Salpeter power-law, but that they must have a steeper exponent [1]. Furthermore, the empirical correlation between galactic star-formation rate (SFR) and the mass of the most massive cluster that forms in a galaxy [2] implies that dwarf galaxies that have low average star-formation rates will have substantially different integrated IMFs than massive galaxies such as our Milky Way [3]. This has important consequences for the distribution of stellar remnants and for the chemo-dynamical and photometric evolution of galaxies. These results suggest that dwarf galaxies ought to show large variations in their integrated field-star IMFs and consequently in their chemical enrichment, while massive galaxies tend to have the same integrated field IMFs with a corresponding smaller inter-galactic scatter in chemical abundances. [ABSTRACT FROM AUTHOR]

The ACS Survey of Galactic Globular Clusters is a Hubble Space Telescope (HST) Treasury program designed to provide a new large, deep and homogeneous photometric database. Based on observations from this program, we have measured precise relative ages for a sample of 64 Galactic globular clusters by comparing the relative position of the clusters’ main sequence turn offs, using main-sequence fitting to cross-compare clusters within the sample. This method provides relative ages to a formal precision of 2–7%. We demonstrate that the calculated relative ages are independent of the choice of theoretical model. We find that the Galactic globular cluster sample can be divided into two groups—a population of old clusters with an intrinsic age dispersion of ∼3% and no age-metallicity relation, and a group of younger clusters with an age-metallicity relation similar to that of the globular clusters associated with the Sagittarius dwarf galaxy. These results are consistent with the Milky Way halo having formed in two phases. The first phase would be compatible with a rapid (<0.8 Gyr) assembling process of the halo, in which the clusters in the old group were formed. The second phase lasted much longer in time and resulted in a group of globular clusters with a clear age-metallicity relation. It is very tempting to argue that the origin of this second group of clusters is related to the accretion of Milky Way satellite galaxies, but the origin of the age-metallicity relation remains unclear. [ABSTRACT FROM AUTHOR]

We derive ages, metallicities and [α/Fe] ratios for dwarf early-type galaxies (dE/dS0) in the center and a region believed to be infalling into the Coma cluster. We find that a set of metallic indices cannot be described by linear relations with velocity dispersion (σ), while another group of indices exhibits linear index-σ relations. dE/dS0 galaxies in the core of the cluster are on average younger, and have lower metallicities and lower α-ratios (solar to sub-solar) than the more luminous elliptical galaxies. On the other hand, dEs/dS0s in the infall region of Coma form a less homogeneous population. We find an unusually high fraction of dE/dS0s with high [α/Fe] ratios, suggesting short time scales for the star formation histories for these galaxies. Furthermore, the dE/dS0s with high α-ratios have a range of ages and metallicities implying multiple formation scenarios where some galaxies have experienced their short star formation bursts at more recent epochs. [ABSTRACT FROM AUTHOR]

During the 2007/2008 season, VERITAS was used for observations at E>200 GeV of several extragalactic non-blazar objects such as galaxy clusters, starburst and interacting galaxies, dwarf galaxies, and nearby galaxies. In these proceedings, we present preliminary results from our observations of dwarf galaxies and M87. Results from observation of other non-blazar sources are presented in separate papers in the proceedings. [ABSTRACT FROM AUTHOR]

WIMP pair annihilations produce high energy gamma-rays in the final state, which can be detected by Imaging Atmospheric Cherenkov Telescopes such as the H.E.S.S. array. We focus in this contribution on searches towards dwarf galaxies and mini-spikes around intermediate mass black holes (IMBHs) in the Galactic halo. H.E.S.S. observations towards the nearby dwarf galaxies Sagittarius and Canis Major are presented. Using realistic modellings for the dark matter (DM) density profiles, constraints on the velocity-weighted annihilation cross section σv of DM particles are derived in the framework of Supersymmetric and Kaluza-Klein models. A search for DM mini-spikes around IMBHs is described as well as constraints on the particle physics parameters. [ABSTRACT FROM AUTHOR]

The last years saw numerous detections of substructure in the Galactic halo, largely due to wide area surveys like the Sloan Digital Sky Survey (SDSS) or the Two Micron All Sky Survey (2MASS). The analysis of SDSS data revealed the ‘Field of Streams’ which encompasses, among other features, the broad and extended Sagittarius tidal stream as well as the Monoceros and Orphan stellar streams. In this contribution we present first results from our analysis of Galactic tidal streams based on SDSS Data Release 6 (DR6) stellar photometry. We use the F star method to derive stellar density maps for various distance intervals. These maps reveal a heliocentric distance to the Sagittarius stream near the Galactic anticenter of ∼13 kpc which is lower than previously claimed. The value also is smaller than predicted by current state-of the art N-body simulations of the disruption of the Sagittarius dwarf galaxy in the Galactic halo. Our maps furthermore yield a distance between 13 kpc and 16 kpc to the nearest part of the Virgo overdensity. Rather surprisingly, we found evidence for a discontinuity in the Orphan stream. The ‘disconnected’ region extends over a projected length of ∼6 degrees and appears at distances exceeding ∼25 kpc. If confirmed, this feature would pose strong constraints on theoretical models trying to explain the origin of the Orphan stream. [ABSTRACT FROM AUTHOR]

Large scale surveys are providing vast amounts of data that can help us understand and study tidal debris more easily and accurately. A maximum likelihood method for determining the spatial properties of this tidal debris and the stellar Galactic spheroid has been developed to take advantage of these huge datasets. We present the results of studying the Sagittarius dwarf tidal stream in two SDSS stripes taken in the southern Galactic Cap using this method. This study was done using stars with the colors of blue F turnoff stars in SDSS. We detected Sagittarius debris at the positions (l,b,R) = (163.311°,-48.400°,30.23 kpc) and (l,b,R) = (34.775°,-72.342°,26.08 kpc). These debris pieces were found to have a FWHM of 6.53±0.54 kpc and 5.71±0.26 kpc and also to contain ≈9,500 and ≈16,700 F turnoff stars, respectively. The debris pieces were also found to have (spatial) directions of (X,Ŷ,Z) = (0.758,0.254,-0.600) and (X,Ŷ,Z) = (0.982,0.945,0.167), respectively. Using the results of the algorithm, we have also probabilistically separated the tidal debris from the stellar spheroid and present those results as well. [ABSTRACT FROM AUTHOR]

In this short article we show how the kernel-based density estimation method are usually applied to search for clusters/dwarf galaxies in large surveys. These method proved to be very effective in last few years. Some simple modifications of these methods are discussed, which allow to perform deeper searches for the substructure in the surveys. Also I show how the generalized Hough transform can be applied to the large stellar catalogs to find stellar streams in the MW halo. The Hough transform of the SDSS data allows to detect all know large stellar streams in SDSS. [ABSTRACT FROM AUTHOR]

The dynamics on scales from dwarf galaxies to the entire universe is dominated by gravitating mass and energy that can not be accounted for by ordinary matter. We review the astrophysical and cosmological evidence for dark matter and dark energy. While there is no obvious interpretation for dark energy, dark matter presumably consists of some new form of weakly interacting elementary particles. We discuss certain candidate particles and search strategies. [ABSTRACT FROM AUTHOR]

Since ultraluminous X-ray sources are typically found in late-type systems, particularly in interacting or starburst galaxies, they are thought to be associated with young stellar populations. We find a significant population of ULXs in our sample of 94 nearby elliptical and lenticular galaxies, using the superior resolution of the Chandra X-ray observatory, far exceeding the number expected from background AGN. We show that though some of these ULXs are associated with young stellar populations in the cores of early-type galaxies, not all of the host galaxies show evidence of recent or ongoing star formation. We suggest that these ULXs come from a variety of sources, some being bright LMXBs, while some of them correspond to nuclei of recently accreted dwarf galaxies. [ABSTRACT FROM AUTHOR]

Halo clouds have been found about the three largest galaxies of the Local Group and in the halos of nearby spirals. This suggests they are a relatively generic feature of the galaxy evolution process and a source of fuel for galaxy disks. Here we discuss two major sources of star formation fuel for the Galaxy, satellite material and clouds condensing from the hot halo medium. Evidence for both methods of fueling are presented and their contribution quantified. [ABSTRACT FROM AUTHOR]

The HI content of the newly discovered satellites of the Milky Way has not been previously studied. We use HIPASS (HI Parkes All Sky Survey), LAB (Leiden/Argentine/Bonn), and GALFA (Galactic Arecibo L-band Feed Array) survey data to explore HI in the environment of the newly discovered dwarf galaxies and add this information to what was previously known about HI in the dwarf spheroidal and lower mass dwarf irregular galaxies of the Local Group. All of the new satellites discovered in the Sloan Digital Sky Survey data have limits on their HI masses which range from <13 M⊙ to 3×104 M⊙ except for Leo T, which has and HI mass of approximately 105 M⊙. We find that galaxies within 300 kpc of the Milky Way or Andromeda are all undetected in HI to low limits, or have ambiguous detections, while those further than 300 kpc are predominantly detected with masses >105 M⊙. The most favored explanation for the lack of HI in dwarf galaxies at small galactocentric distances is ram pressure stripping of the gas in the dwarf galaxy by the larger galaxy's hot halo gas. Finally, we discuss the discovery in the GALFA data of discrete HI clouds with characteristics similar to known dwarf galaxies. [ABSTRACT FROM AUTHOR]

Substantial star formation often occurs in the tidal debris of interacting galaxy pairs and groups. However, the packaging of the star formation is varied. Sometimes stars form in clusters reminiscent of the predecessors of globular clusters, and other times they form primarily in a tidal clump that is the size and mass of a dwarf galaxy. I report on a few recent projects that I have done with collaborators. We have been trying to identify the factors that determine whether star clusters or tidal dwarfs form in the debris of interacting pair. We have also undertaken a multiwavelength program to study Hickson compact groups. We suggest that Hickson compact groups can be on one of two evolutionary sequences depending on whether gas is converted within the galaxies or outside of the galaxies. [ABSTRACT FROM AUTHOR]

The Arecibo Legacy Fast ALFA (ALFALFA) survey is a program aimed at obtaining a census of HI-bearing objects over a cosmologically significant volume of the local universe. When complete in ∼3-4 years, it will cover 7000 square degrees of high latitude sky using the 305 m telescope and the seven-beam Arecibo L-band feed array (ALFA). As of May 1, 2008, almost 60% of the required observations are complete and a catalog exists in preliminary form for 25% of the final sky area. ALFALFA is detecting about twice as many HI sources as predicted based on previously published HI mass functions and should deliver a final catalog of >25000 extragalactic HI sources. ALFALFA will detect hundreds of galaxies with HI masses less than 107.5 M⊙ and similarly large numbers greater than 1010.3 M⊙. Its centroiding accuracy allows for the immediate identification of highly probably optical counterparts to each HI detection. Fewer than 3% of all extragalactic HI sources, and <1% of ones with MHI>109.5 M⊙ cannot be identified with a stellar counterpart. The hundreds of HI sources with observed line widths of 20-30 km s-1 includes a population of optically faint dwarf galaxies. The objects with highest HI masses exhibit a range of morphologies, optical colors and surface brightnesses, but all appear to be massive disk systems. The latter represent the population likely to dominate future studies of HI at high redshift. [ABSTRACT FROM AUTHOR]

The redshifted 21-cm line of distant neutral H atoms provides a probe of the cosmic “dark ages” and the epoch of reionization (“EOR”) which ended them, within the first billion years of cosmic time. The radio continuum produced by this redshifted line can be seen in absorption or emission against the cosmic microwave background (“CMB”) at meterwaves, yielding information about the thermal and ionization history of the universe and the primordial density perturbation spectrum that led to galaxy and large-scale structure formation. Observing this 21-cm background is a great challenge, as it is necessary to detect a diffuse signal at a brightness temperature that differs from that of the CMB at millikelvin levels and distinguish this from foreground continuum sources. A new generation of low-frequency radio arrays is currently under development to search for this background. Accurate theoretical predictions of the spectrum and anisotropy of this background, necessary to guide and interpret future observations, are also quite challenging. Toward this end, it is necessary to model the inhomogeneous reionization of the intergalactic medium and determine the spin temperature of the 21-cm transition and its variations in time and space as it decouples from the temperature of the CMB. In my talk, I summarized some of the theoretical progress in this area. Here, I will focus on just a few of the predictions for the 21-cm background from the EOR, based on our newest, large-scale simulations of patchy reionization. These simulations are the first with enough N-body particles (from 5 to 29 billion) and radiative transfer rays to resolve the formation of and trace the ionizing radiation from each of the millions of dwarf galaxies believed responsible for reionization, down to 108 M⊙, in a cubic volume large enough (90 and 163 comoving Mpc on a side) to make meaningful statistical predictions of the fluctuating 21-cm background. [ABSTRACT FROM AUTHOR]

There have been many observational programs exploring the connection between the HI distribution and the presence of star formation in galaxies. Since the molecular phase of the ISM is most closely associated with star formation, the results of these investigations depend greatly on the type of galaxy being observed. For low mass, low metallicity galaxies, where the molecular gas in only found associated with the densest concentrations of HI, the correlation between HI maxima and star formation is very good. For high mass, high metallicity galaxies, star formation correlates strongly with densest concentrations of molecular gas, and the majority of the HI gas can be unrelated to star formation. Understanding the HI/star formation connection requires observations of the quality being produced by the THINGS program. The THINGS sample is especially promising due to the abundance and quality of the ancillary observational databases. As a forward look, I pose the question: How many galaxies do we need to observe with this quality of data in order to answer our fundamental questions concerning the star formation process? An important part of the answer is that we need to carefully study those galaxies and regions of galaxies which do not show current star formation in order to understand the absence of star formation as well as the presence of star formation. I describe a method of recreating star formation histories of galaxies over the last ∼500 Myr which will help us to understand how the gas has come to its present state. [ABSTRACT FROM AUTHOR]

Using Chandra, XMM-Newton and optical data (published photometric catalogs and our 2dF spectroscopic survey) we study the X-ray binary (XRB) populations of the Small Magellanic Cloud (SMC). We find that the number of Be/X-ray binaries (Be-XRBs) peaks at the age of maximum Be-star formation ∼30–70 Myr ago. We also examine the “overabundance” of Be-XRBs in the SMC fields covered by Chandra, in comparison with the Milky Way. Even after taking into account the difference in the formation rate of OB stars, we estimate that Be-XRBs are ∼2 times more common in the SMC when compared to the Milky Way and for luminosities down to ∼1034 erg s-1. We explain this large number of SMC Be-XRBs as the result of both its low metallicity environment (∼1/5 solar) and its high star-formation (SF) rate at the age of maximum formation of Be stars. [ABSTRACT FROM AUTHOR]

Radiation from the first stars expel most of the gas from the shallow DM potential wells in which they formed. We use cosmological radiation hydrodynamical AMR simulations that include self-consistent Population III star formation and feedback. Radiative transport is accurately modelled with adaptive ray tracing. We include supernova (SN) explosions and follow the metal enrichment of the intergalactic medium and surrounding star forming halos. We focus on the formation of several dwarf galaxies and their progenitors. Baryon fractions in Tvir = 104 K halos decrease to 10% with stellar feedback and 5% with SN explosions. We find that gaseous spin parameters increase up to a factor of 5 when SN feedback is included. [ABSTRACT FROM AUTHOR]

The article offers information on where metal poor stars exist and how to interpret the metallicity of a star. Galactic halo serves as a great place to look for metal poor stars because it is where they are found historically. Searching for the first stars in the Galactic halo and dwarf galaxies require them to form over several epochs. The intrinsic metallicities of stars are based on their carbon, nitrogen, and oxygen abudances.

A sample consisting of 1040 stars, mainly dwarfs of late spectral types, from the solar neighbourhood is studied. Among various spectroscopic data available for all of them we use the line-of-sight velocity and metallicity. A clear correlation between these two quantities is found. Based on it we establish the fractions of galactic subsystems in the sample. It is found: 93% for the thin disc, 6% for the thick disc and 1% for the halo. The halo stars are in the present sample the most easy to be separated, they are the metal-poorest in the sample. For the thin disc the velocity ellipsoid is determined. [ABSTRACT FROM AUTHOR]

The supernova SN 2006aj associated with GRB 060218 is the second-closest GRB-SN observed to date (z=0.033) and is the clearest example of a SN associated with a Swift GRB with the earliest optical spectroscopy. Its optical data showed that this is the fastest evolving and among the least luminous GRB-SNe (70% as luminous as SN1998bw). However, its expansion velocity and a comparison with other stripped-envelope SNe suggest that SN2006aj is an intermediate object between Type Ic GRB-SNe and those not accompained by a GRB. High-resolution optical spectroscopy together with SDSS pre-burst observations revealed that the host galaxy of SN2006aj is a low-luminosity, metal-poor star-forming dwarf galaxy. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

A new revision of the gamma flux that we expect to detect in Imaging Atmospheric Cherenkov Telescopes (IACTs) from SUSY dark matter annihilation in the Draco dSph is presented using the dark matter density profiles compatible with the latest observations. This revision takes also into account the important effect of the Point Spread Function (PSF) of the Cherenkov telescope. We show that this effect is crucial in the way we will observe and interpret a possible signal profile in the telescope. Given these new considerations, some light can be shed on the recent signal excess reported by the CACTUS experiment. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We study a dark matter halo model from two points of view: the “stellar polytrope” (SP) model coming from Tsallis’ thermodynamics, and the one coming from the Navarro-Frenk-White (NFW) paradigm. We make an appropriate comparison between both halo models and analyzing the relations between the global physical parameters of observed galactic disks, coming from a sample of actual galaxies, with the ones of the unobserved dark matter halos, we conclude that the SP model is favored over the NFW model in such a comparison. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We have detected several new HII regions in two Local Group galaxies (IC10 and Sex B). The hotstars method, was used to define the HII regions. We also present preliminary results about some physical parameters of these galaxies. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Classical models of galactic evolution predict a smooth rise in heavy-element abundance (metallicity) with time. We test this prediction with a new, large and unbiased sample of long-lived stars in the solar neighbourhood and find that several of the key tests fail to support the classical predictions. In agreement with earlier studies, our observed metallicity distribution function is deficient in low-mass metal-poor stars from the generation that produced the heavy elements seen in the Sun and younger stars. In contrast to some earlier studies, we find no clear rise in overall metallicity with time in the Solar neighbourhood; we also find that the galactic disk has experienced kinematic heating throughout its life, and identify groups of stars that may be traces of dwarf galaxies that have merged with the Milky Way. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Dwarf galaxies are important to study because they potentially provide a direct connection to the earliest epochs of galaxy formation. They also allow us to study the star formation process at relatively low abundances. Our understanding of the origins and evolution of dwarf galaxies has been changing very rapidly. New observations are giving better insight into the relationship between the two main families of dwarf galaxies, the dwarf ellipticals and the dwarf irregulars. Here I review a broad range of topics related to the formation and evolution of dwarf galaxies. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present the latest results from our project to search for new planetary nebulae in nearby galaxies using Sloan Digital Sky Survey (SDSS) imaging data. Our method is based on photometric criteria and can be applied to galaxies where PNe appear as point sources. We applied these criteria to the whole area of M31 as scanned by SDSS, detecting 130 new PN candidates and 30 known PNe. All selected PNe candidates are located in the outer regions of M31. For 85 candidates follow-up spectroscopy was obtained with the 2.2m telescope at Calar Alto Observatory. The observations show that our method has a detection efficiency of about 82%. We discuss the 2D velocity field of the outer part of M31 based on our observed PN data. The PNe suggest an exponential disk scale length of 13 kpc along the minor axis. We discovered two PNe along the line of sight to Andromeda NE, a very low surface brightness giant stellar structure in the outer halo of M31. These two PNe are located at projected distances of ∼48 kpc and ∼41 kpc from the center of M31 and are the most distant PNe in M31 found up to now. Our data support the idea that Andromeda NE is located at the distance of M31. No PNe associated with other M31 satellites observed by the SDSS were found. Applying our method to other SDSS regions we checked data for the Local Group galaxies Sextans, Draco, Leo I, Pegasus, Sextans B and Leo A and recovered a known PN in Leo A. We re-measured its O/H and for the first time determined abundances of N/H, S/H, He/H as well as the electron number density Ne. We argue that the PN progenitor was formed ≈1.5 Gyr ago during the strongest episode of star formation in Leo A. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]