Search

Quasar absorption systems encode a wealth of information about the abundances, ionization structure, and physical conditions in intergalactic and circumgalactic media. Simple (often single-phase) photoionization models are frequently used to decode such data. Using five discrete absorbers from the COS Absorption Survey of Baryon Harbors (CASBaH) that exhibit a wide range of detected ions (e.g. Mg  ii , S  ii – S  vi , O  ii – O  vi , Ne  viii), we show several examples where single-phase ionization models cannot reproduce the full set of measured column densities. To explore models that can self-consistently explain the measurements and kinematic alignment of disparate ions, we develop a Bayesian multiphase ionization modelling framework that characterizes discrete phases by their unique physical conditions and also investigates variations in the shape of the UV flux field, metallicity, and relative abundances. Our models require at least two (but favour three) distinct ionization phases ranging from T ≈ 104 K photoionized gas to warm-hot phases at T ≲ 105.8 K. For some ions, an apparently single absorption 'component' includes contributions from more than one phase, and up to 30 per cent of the H  i is not from the lowest ionization phase. If we assume that all of the phases are photoionized, we cannot find solutions in thermal pressure equilibrium. By introducing hotter, collisionally ionized phases, however, we can achieve balanced pressures. The best models indicate moderate metallicities, often with subsolar N/ α , and, in two cases, ionizing flux fields that are softer and brighter than the fiducial Haardt & Madau UV background model. [ABSTRACT FROM AUTHOR]

Davies et al. established that for L * galaxies the fraction of baryons in the circumgalactic medium (CGM) is inversely correlated with the mass of their central supermassive black holes (BHs) in the EAGLE hydrodynamic simulation. The interpretation is that, over time, a more massive BH has provided more energy to transport baryons beyond the virial radius, which additionally reduces gas accretion and star formation. We continue this research by focusing on the relationship between the (1) BH masses (M BH), (2) physical and observational properties of the CGM, and (3) galaxy colours for Milky Way-mass systems. The ratio of the cumulative BH feedback energy over the gaseous halo binding energy is a strong predictor of the CGM gas content, with BHs injecting significantly higher than the binding energy resulting in gas-poor haloes. Observable tracers of the CGM, including |$\rm {C\, \small{IV}}$|⁠ , |$\rm {O\, \small{VI}}$|⁠ , and |${\rm {H\, \small{I}}}$| absorption line measurements, are found to be effective tracers of the total z ∼ 0 CGM halo mass. We use high-cadence simulation outputs to demonstrate that BH feedback pushes baryons beyond the virial radius within 100 Myr time-scales, but that CGM metal tracers take longer (0.5–2.5 Gyr) to respond. Secular evolution of galaxies results in blue, star-forming or red, passive populations depending on the cumulative feedback from BHs. The reddest quartile of galaxies with M * = 1010.2−10.7 M⊙ (median u − r  = 2.28) has a CGM mass that is 2.5 times lower than the bluest quartile (u − r  = 1.59). We propose observing strategies to indirectly ascertain f CGM via metal lines around galaxies with measured M BH. We predict statistically detectable declines in |$\rm {C\, \small{IV}}$| and |$\rm {O\, \small{VI}}$| covering fractions with increasing M BH for central galaxies with M * = 1010.2−10.7M⊙. [ABSTRACT FROM AUTHOR]

We report a tentative detection of the circumgalactic medium (CGM) of Wolf–Lundmark–Melotte (WLM), an isolated, low-mass (log M */M⊙ ≈ 7.6), dwarf irregular galaxy in the Local Group (LG). We analyse an HST /COS archival spectrum of a quasar sightline (PHL2525), which is 45 kpc (0.5 virial radius) from WLM and close to the Magellanic Stream (MS). Along this sightline, two ion absorbers are detected in Si  ii , Si  iii , Si  iv , C  ii , and C  iv at velocities of ∼−220 km s−1 (Component v-220) and ∼−150 km s−1 (Component v-150). To identify their origins, we study the position–velocity alignment of the components with WLM and the nearby MS. Near the magellanic longitude of PHL2525, the MS-related neutral and ionized gas moves at ≲−190 km s−1, suggesting an MS origin for Component v-220, but not for Component v-150. Because PHL2525 passes near WLM and Component v-150 is close to WLM's systemic velocity (∼−132 km s−1), it is likely that Component v-150 arises from the galaxy's CGM. This results in a total Si mass in WLM's CGM of |$M_{\rm Si}^{\rm CGM}\sim (0.2-1.0)\times 10^5~\mathrm{M}_\odot$| using assumption from other COS dwarf studies. Comparing |$M_{\rm Si}^{\rm CGM}$| to the total Si mass synthesized in WLM over its lifetime (∼1.3 × 105 M⊙), we find ∼3 per cent is locked in stars, ∼6 per cent in the ISM, ∼15–77 per cent in the CGM, and the rest (∼14–76 per cent) is likely lost beyond the virial radius. Our finding resonates with other COS dwarf galaxy studies and theoretical predictions that low-mass galaxies can easily lose metals into their CGM due to stellar feedback and shallow gravitational potential. [ABSTRACT FROM AUTHOR]

We explore the circumgalactic metal content traced by commonly observed low ion absorbers, including |${\rm{C\,{\small II}}}$|⁠, |${\rm{Si\,{\small II}}}$|⁠, |${\rm{Si\,{\small III}}}$|⁠, |$\rm{Si\,{\small IV}}$|⁠, and |${\rm{Mg\,{\small II}}}$|⁠. We use a set of cosmological hydrodynamical zoom simulations run with the EAGLE model and including a non-equilibrium ionization and cooling module that follows 136 ions. The simulations of |$z$| ≈ 0.2 L * (M 200 = 1011.7 − 1012.3M⊙) haloes hosting star-forming galaxies and group-sized (M 200 = 1012.7 − 1013.3M⊙) haloes hosting mainly passive galaxies reproduce key trends observed by the COS–Halos survey – low ion column densities show 1) little dependence on galaxy-specific star formation rate, 2) a patchy covering fraction indicative of 104 K clumps with a small volume filling factor, and 3) a declining covering fraction as impact parameter increases from 20−160kpc. Simulated |${\rm{Si\,{\small II}}}$|⁠, |${\rm{Si\,{\small III}}}$|⁠, |$\rm{Si\,{\small IV}}$|⁠, |${\rm{C\,{\small II}}}$|⁠, and |${\rm{C\,{\small III}}}$| column densities show good (mostly within 0.3 dex) agreement with observations, while |${\rm{Mg\,{\small II}}}$| is under-predicted. Low ions trace a significant metal reservoir, ≈108M⊙, residing primarily at 10−100kpc from star-forming and passive central galaxies. These clumps preferentially flow inwards and most will accrete onto the central galaxy within the next several Gyr, while a small fraction are entrained in strong outflows. A multiphase structure describes the inner CGM (<0.5 R 200) with low-ion metal clumps surrounded by a hot, ambient medium. The dense, cool phase is separate from the |$\rm{O\,{\small VI}}$| observed by COS–Halos, which arises from the outer CGM (>0.5 R 200) tracing virial temperature gas around L * galaxies. Our simulations support previous ionization models indicating that cloud covering factors decline while densities and pressures show little decline with increasing impact parameter (typically <0.3 dex from 40 to 160 kpc). We find the cool clumps have lower pressures than the ambient medium they are embedded in, and discuss that numerical effects within the hydrodynamic solver likely play a role. [ABSTRACT FROM AUTHOR]

Cosmological simulations predict that a significant fraction of the low-z baryon budget resides in large-scale filaments in the form of a diffuse plasma at temperatures T ∼ 105 − 107 K. However, direct observation of this so-called warm-hot intergalactic medium (WHIM) has been elusive. In the Λcold dark matter paradigm, galaxy clusters correspond to the nodes of the cosmic web at the intersection of several large-scale filamentary threads. In previous work, we used HST/COS data to conduct the first survey of broad H i  Lyα absorbers (BLAs) potentially produced by WHIM in inter-cluster filaments. We targeted a single QSO, namely Q1410, whose sightline intersects seven independent inter-cluster axes at impact parameters <3 Mpc (comoving), and found a tentative excess of a factor of ∼4 with respect to the field. Here, we further investigate the origin of these BLAs by performing a blind galaxy survey within the Q1410 field using VLT/MUSE. We identified 77 sources and obtained the redshifts for 52 of them. Out of the total sample of seven BLAs in inter-cluster axes, we found three without any galaxy counterpart to stringent luminosity limits (∼4 × 108 L⊙ ∼0.01 L*), providing further evidence that these BLAs may represent genuine WHIM detections. We combined this sample with other suitable BLAs from the literature and inferred the corresponding baryon mean density for these filaments in the range $\Omega ^{\rm fil}_{\rm bar}= 0.02-0.04$. Our rough estimates are consistent with the predictions from numerical simulations but still subject to large systematic uncertainties, mostly from the adopted geometry, ionization corrections, and density profile. [ABSTRACT FROM AUTHOR]

We analyse the low-redshift (z≈0.2) circumgalactic medium (CGM) by comparing absorption-line data from the COS-Halos survey to absorption around a matched galaxy sample from two cosmological hydrodynamic simulations. The models include different prescriptions for galactic outflows, namely hybrid energy/momentum driven wind (ezw), and constant winds (cw). We compare equivalent widths, covering factors, ion ratios, and kinematics. Both wind models show generally ≲1σ agreement with these observations for H I and certain low-ionization metal lines, but poorer agreement with higher ionization metal lines including Si III and O VI that are well observed by COS-Halos. This suggests that both models predict too much cool, metal-enriched gas and not enough hot gas, and / or that metals are not sufficiently mixed. This may reflect our model assumption of ejecting outflows as cool and unmixing gas. Our ezw simulation includes a heuristic prescription to quench massive galaxies by superheating interstellar medium gas. This produces low-ionization absorption broadly consistent with observations, but substantial O VI absorption inconsistent with data, suggesting that gas around quenched galaxies in the real Universe does not cool. At impact parameters of ≲50 kpc, recycling winds dominate the absorption of low ions and even H I, while O VI generally arises from metals ejected ≳1 Gyr ago. The similarity between the wind models is surprising, since they differ substantially in the amount and phase distribution of halo gas.We show that this similarity owes mainly to our comparison at fixed stellar (not halo) mass, suggesting that CGM properties are more closely tied to galaxy's stellar (versus halo) mass. [ABSTRACT FROM AUTHOR]

Modern analyses of structure formation predict a universe tangled in a 'cosmic web' of dark matter and diffuse baryons. These theories further predict that at low z, a significant fraction of the baryons will be shock-heated to T ~ 105-105 K yielding a warm-hot intergalactic medium (WHIM), but whose actual existence has eluded a firm observational confirmation.We present a novel experiment to detect theWHIM, by targeting the putative filaments connecting galaxy clusters. We use HST/COS to observe a remarkable quasi-stellar object (QSO) sightline that passes within Δd = 3 Mpc from the seven intercluster axes connecting seven independent cluster pairs at redshifts 0.1 ≤ z ≤ 0.5.We find tentative excesses of total HI, narrow HI (NLA; Doppler parameters b < 50 km s-1), broad HI (BLA; b ≥ 50 km s-1) and OVI absorption lines within rest-frame velocities of Δv Δ 1000 km s-1 from the cluster-pairs redshifts, corresponding to ~2, ~1.7, ~6 and ~4 times their field expectations, respectively. Although the excess of OVI likely comes from gas close to individual galaxies, we conclude that most of the excesses of NLAs and BLAs are truly intergalactic. We find the covering fractions, fc, of BLAs close to cluster pairs are ~4-7 times higher than the random expectation (at the ~2σ c.l.), whereas the fc of NLAs and OVI are not significantly enhanced. We argue that a larger relative excess of BLAs compared to those of NLAs close to cluster pairs may be a signature of the WHIM in intercluster filaments. By extending this analysis to tens of sightlines, our experiment offers a promising route to detect the WHIM. [ABSTRACT FROM AUTHOR]

We present UV and optical observations from the Cosmic Origins Spectrograph on the Hubble Space Telescope and Keck of a z = 0.27395 Lyman limit system (LLS) seen in absorption against the QSO PG1630+377. We detect H I absorption with log N(H I) = 17.06 ± 0.05 as well as Mg II, C III, Si III, and O VI in this system. The column densities are readily explained if this is a multi-phase system, with the intermediate and low ions arising in a very low metallicity ([Mg/H] = -1.71 ± 0.06) photoionized gas. We identify via Keck spectroscopy and Large Binocular Telescope imaging a 0.3 L* star-forming galaxy projected 37 kpc from the QSO at nearly identical redshift (z = 0.27406 and Δv = -26 km s-1) with near solar metallicity ([O/H] = -0.20 ± 0.15). The presence of very low metallicity gas in the proximity of a near-solar metallicity, sub-L* galaxy strongly suggests that the LLS probes gas infalling onto the galaxy. A search of the literature reveals that such low-metallicity LLSs are not uncommon. We found that 50% (4/8) of the well-studied z ≲ 1 LLSs have metallicities similar to the present system and show sub-L* galaxies with ρ < 100 kpc in those fields where redshifts have been surveyed. We argue that the properties of these primitive LLSs and their host galaxies are consistent with those of cold mode accretion streams seen in galaxy simulations. [ABSTRACT FROM AUTHOR]