Galaxy Impact Parameter Research Articles

Probing the Circumgalactic Medium with Fast Radio Bursts: Insights from CAMELS

Most diffuse baryons, including the circumgalactic medium (CGM) surrounding galaxies and the intergalactic medium (IGM) in the cosmic web, remain unmeasured and unconstrained. Fast radio bursts (FRBs) offer an unparalleled method to measure the electron dispersion measures (DMs) of ionized baryons. Their distribution can resolve the missing baryon problem and constrain the history of feedback theorized to impart significant energy to the CGM and IGM. We analyze the Cosmology and Astrophysics with Machine Learning Simulations using three suites, IllustrisTNG, SIMBA, and Astrid, each varying six parameters (two cosmological and four astrophysical feedback), for a total of 183 distinct simulation models. We find significantly different predictions between the fiducial models of the suites owing to their different implementations of feedback. SIMBA exhibits the strongest feedback, leading to the smoothest distribution of baryons and reducing the sight-line-to-sight-line variance in DMs between z = 0 and 1. Astrid has the weakest feedback and the largest variance. We calculate FRB CGM measurements as a function of galaxy impact parameter, with SIMBA showing the weakest DMs due to aggressive active galactic nucleus (AGN) feedback and Astrid the strongest. Within each suite, the largest differences are due to varying AGN feedback. IllustrisTNG shows the most sensitivity to supernova feedback, but this is due to the change in the AGN feedback strengths, demonstrating that black holes, not stars, are most capable of redistributing baryons in the IGM and CGM. We compare our statistics directly to recent observations, paving the way for the use of FRBs to constrain the physics of galaxy formation and evolution.

Low-mass Group Environments Have No Substantial Impact on the Circumgalactic Medium Metallicity

Abstract We explore how environment affects the metallicity of the circumgalactic medium (CGM) using 13 low-mass galaxy groups (two to five galaxies) at identified near background quasars. Using quasar spectra from the Hubble Space Telescope/COS and from Keck/HIRES or the Very Large Telescope/UVES, we measure column densities of or determine limits on CGM absorption lines. We use a Markov Chain Monte Carlo approach with Cloudy to estimate metallicities of cool (T ∼ 104 K) CGM gas within groups and compare them to CGM metallicities of 47 isolated galaxies. Both group and isolated CGM metallicities span a wide range (−2 < [Si/H] < 0), where the mean group (−0.54 ± 0.22) and isolated (−0.77 ± 0.14) CGM metallicities are similar. Group and isolated environments have similar distributions of column densities as a function of impact parameter. However, contrary to isolated galaxies, we do not find an anticorrelation between column density and the nearest group galaxy impact parameter. We additionally divided the groups by member luminosity ratios (i.e., galaxy–galaxy and galaxy–dwarf groups). While there was no significant difference in their mean metallicities, a modest increase in sample size should allow one to statistically identify a higher CGM metallicity in galaxy–dwarf groups compared to galaxy–galaxy groups. We conclude that either environmental effects have not played an important role in the metallicity of the CGM at this stage and expect that this may only occur when galaxies are strongly interacting or merging or that some isolated galaxies have higher CGM metallicities due to past interactions. Thus, environment does not seem to be the cause of the CGM metallicity bimodality.

Probing the intra-group medium of a z = 0.28 galaxy group

We present new MUSE observations of a galaxy group probed by a background quasar. The quasar sightline passes between multiple $z=0.28$ galaxies, whilst showing at the same redshift low ionised metal line species, including Ca II, Mg I, Mg II and Fe II. Based on the galaxy redshifts measured from the MUSE data, we estimate the galaxies to be part of a small galaxy group with a halo mass of $\approx6\times10^{12}$ M$_{\odot}$. We use the MUSE data to reveal the two dimensional dynamical properties of the gas and stars in the group galaxies, and relate these to the absorber kinematics. With these data we consider a number of scenarios for the nature of the gas probed by the sightline absorbers: a co-rotating gas halo associated with a single galaxy within the group; outflowing material from a single group member powered by recent star-formation; and cool dense gas associated with an intra-group medium. We find that the dynamics, galaxy impact parameters, star-formation rates, and the absorber strength suggest the cool gas can not be clearly associated with any single galaxy within the group. Instead we find that the observations are consistent with a superposition of cool gas clouds originating with the observed galaxies as they fall into the group potential, and are now likely in the process of forming the intra-group medium.

Metal-line absorption around z ≈ 2.4 star-forming galaxies in the Keck Baryonic Structure Survey★

We study metal absorption around 854 z ≈ 2.4 star-forming galaxies taken from the Keck Baryonic Structure Survey. The galaxies examined in this work lie in the fields of 15 hyperluminous background quasi-stellar objects, with galaxy impact parameters ranging from 35 proper kpc (pkpc) to 2 proper Mpc (pMpc). Using the pixel optical depth technique, we present the first galaxy-centred 2D maps of the median absorption by O vi, N v, C iv, C iii, and Si iv, as well as updated results for H i. At small galactocentric radii we detect a strong enhancement of the absorption relative to randomly located regions that extend out to at least 180 pkpc in the transverse direction, and ±240 km s−1 along the line of sight (LOS, ∼1 pMpc in the case of pure Hubble flow) for all ions except N v. For C iv (and H i) we detect a significant enhancement of the absorption signal out to 2 pMpc in the transverse direction, corresponding to the maximum impact parameter in our sample. After normalizing the median absorption profiles to account for variations in line strengths and detection limits, in the transverse direction we find no evidence for a sharp drop-off in metals distinct from that of H i. We argue instead that non-detection of some metal-line species in the extended circumgalactic medium is consistent with differences in the detection sensitivity. Along the LOS, the normalized profiles reveal that the enhancement in the absorption is more extended for O vi, C iv, and Si iv than for H i. We also present measurements of the scatter in the pixel optical depths, covering fractions, and equivalent widths as a function of projected galaxy distance. Limiting the sample to the 340 galaxies with redshifts measured from nebular emission lines does not decrease the extent of the enhancement along the LOS compared to that in the transverse direction. This rules out redshift errors as the source of the observed redshift-space anisotropy and thus implies that we have detected the signature of gas peculiar velocities from infall, outflows, or virial motions for H i, O vi, C iv, C iii, and Si iv.

Verifying the mass–metallicity relation in damped Lyman α selected galaxies at 0.1 < z < 3.2

A scaling relation has recently been suggested to combine the galaxy mass-metallicity (MZ) relation with metallicities of damped Lyman-alpha systems (DLAs) in quasar spectra. Based on this relation the stellar masses of the absorbing galaxies can be predicted. We test this prediction by measuring the stellar masses of 12 galaxies in confirmed DLA absorber - galaxy pairs in the redshift range 0.1<z<3.2. We find an excellent agreement between the predicted and measured stellar masses over three orders of magnitude, and we determine the average offset $\langle C_{[M/H]} \rangle$ = 0.44+/-0.10 between absorption and emission metallicities. We further test if $C_{[M/H]}$ could depend on the impact parameter and find a correlation at the 5.5sigma level. The impact parameter dependence of the metallicity corresponds to an average metallicity difference of -0.022+/-0.004 dex/kpc. By including this metallicity vs. impact parameter correlation in the prescription instead of $C_{[M/H]}$, the scatter reduces to 0.39 dex in log M*. We provide a prescription how to calculate the stellar mass (M*,DLA) of the galaxy when both the DLA metallicity and DLA galaxy impact parameter is known. We demonstrate that DLA galaxies follow the MZ relation for luminosity-selected galaxies at z=0.7 and z=2.2 when we include a correction for the correlation between impact parameter and metallicity.

OBSERVATIONS OF Mg II ABSORPTION NEARz∼ 1 GALAXIES SELECTED FROM THE DEEP2 REDSHIFT SURVEY

We study the frequency of Mg II absorption in the outer haloes of galaxies at z = 0.6 - 1.4 (with median z = 0.87), using new spectra obtained of ten background quasars with galaxy impact parameters of b < 100 kpc. The quasar sightlines were selected from the SDSS DR6 QSO catalog based on proximity to galaxies in the DEEP2 redshift survey. In addition to the 10 small impact systems, we examine 40 additional galaxies at 100 < b < 500 kpc serendipitously located in the same fields. We detect Mg II absorbers with equivalent width W_r = 0.15 A - 1.0 A, though not all absorbers correlate with DEEP galaxies. We find five unique absorbers within \Delta v = 500 km/s and b < 100 kpc of a DEEP galaxy; this small sample contains both early and late type galaxies and has no obvious trends with star formation rate. No Mg II is detected more than 100 kpc from galaxies; inside this radius the covering fraction scales with impact parameter and galaxy luminosity in very similar fashion to samples studied at lower redshift. In all but one case, when Mg II is detected without a spectroscopically confirmed galaxy, there exists a plausible photometric candidate which was excluded because of slit collision or apparent magnitude. We do not detect any strong absorbers with W_r > 1.0 A, consistent with other samples of galaxy-selected Mg II systems. We speculate that Mg II systems with 0.3 < W_r < 1.0 trace old relic material from galactic outflows and/or the halo assembly process, and that in contrast, systems with large W_r are more likely to reflect the more recent star forming history of their associated galaxies.

Observations of Chemically Enriched QSO Absorbers nearz∼2.3 Galaxies: Galaxy Formation Feedback Signatures in the Intergalactic Medium

We present a comparative study of galaxies and intergalactic gas toward the z = 2.73 quasar HS 1700+6416, to explore the effects of galaxy formation feedback on the IGM. Our observations and ionization simulations indicate that the volume within 100-200 h physical kpc of high-redshift galaxies is populated by very small (ΔL 1 kpc), dense (ρ/ ~ 1000), and metal-rich (Z - Z☉) absorption-line regions. These systems often contain shock-heated gas seen in O VI and may exhibit [Si/C] abundance enhancements suggestive of preferential enrichment by Type II supernovae. We argue that the absorber geometries resemble thin sheets or bubbles and that their unusual physical properties can be explained using a simple model of radiatively efficient shocks plowing through moderately overdense intergalactic filaments. The high metallicities suggest that these shocks are being expelled from, rather than falling into, star-forming galaxies. There is a drop-off in the intergalactic gas density at galaxy impact parameters of 300 physical kpc (1 comoving Mpc) that may represent boundaries of the gas structures where galaxies reside. The heavy-element enhancement near galaxies covers smaller distances: at galactocentric radii between 100 and 200 h kpc the observed abundances blend into the general metallicity field of the IGM. Our results suggest that either supernova-driven winds or dynamical stripping of interstellar gas alters the IGM near massive galaxies, even at R 100 kpc. However, only a few percent of the total mass in the Lyα forest is encompassed by this active feedback at z ~ 2.5. The effects could be more widespread if the more numerous metal-poor C IV systems at impact parameters 200 h kpc are the tepid remnants of very powerful late-time winds. However, based on present observations it is not clear that this scenario is to be favored over one involving preenrichment by smaller galaxies at z 6.

Galaxy morphology – halo gas connections

We studied a sample of 38 intermediate redshift MgII absorption-selected galaxies using (1) Keck/HIRES and VLT/UVES quasar spectra to measure the halo gas kinematics from MgII absorption profiles and (2) HST/WFPC-2 images to study the absorbing galaxy morphologies. We have searched for correlations between quantified gas absorption properties, and host galaxy impact parameters, inclinations, position angles, and quantified morphological parameters. We report a 3.2-sigma correlation between asymmetric perturbations in the host galaxy morphology and the MgII absorption equivalent width. We suggest that this correlation may indicate a connection between past merging and/or interaction events in MgII absorption-selected galaxies and the velocity dispersion and quantity of gas surrounding these galaxies.

The Local Lyα Forest. III. Relationship between Lyα Absorbers and Galaxies, Voids, and Superclusters

In this paper we use large-angle, nearby galaxy redshift surveys to investigate the relationship between the 81 low-redshift Lyα absorbers in our Hubble Space Telescope GHRS survey and galaxies, superclusters, and voids. In a subsample of 46 Lyα absorbers located in regions where the 2000 February 8 CfA catalog is complete down to at least L* galaxies, the nearest galaxy neighbors range from 100 h kpc to greater than 10 h Mpc. Of these 46 absorbers, eight are found in galaxy voids. After correcting for path length and sensitivity, we find that 22% ± 8% of the Lyα absorbers lie in voids, which requires that at least some low column density absorbers are not extended halos of individual bright galaxies. The number density of these clouds yields a baryon fraction of 4.5% ± 1.5% in voids. The stronger Lyα absorbers (1013.2-1015.4 cm-2) cluster with galaxies more weakly than galaxies cluster with each other, while the weaker absorbers (1012.4-1013.2 cm-2) are more randomly distributed. The median distance from a low-z Lyα absorber in our sample to its nearest galaxy neighbor (~500 h kpc) is twice the median distance between bright galaxies in the same survey volume. This makes any purported association between these Lyα absorbers and individual galaxies problematic. The suggested correlation between Lyα absorber equivalent width () and nearest galaxy impact parameter does not extend to ≤ 200 mA or to impact parameters greater than 200 h kpc. Instead, we find statistical support for the contention that absorbers align with large-scale filaments of galaxies. The pair of sight lines, 3C 273 and Q1230+0115, separated by 09 on the sky, provides an example of eight absorbers and seven galaxies aligned along a possible filamentary structure at least 20 h Mpc long. While some strong ( > 400 mA) Lyα absorbers may be gas in the extended gaseous halos of individual galaxies, much of the local Lyα forest appears to be associated with the large-scale structures of galaxies and some with voids.

The Gaseous Extent of Galaxies and the Origin of Lyα Absorption Systems. V. Optical and Near‐Infrared Photometry of Lyα‐absorbing Galaxies atz &lt; 1

We present results of a program to obtain and analyze HST WFPC2 images and ground-based images of galaxies identified in an imaging and spectroscopic survey of faint galaxies in fields of HST spectroscopic target QSOs. Considering a sample of physically correlated galaxy and absorber pairs with galaxy-absorber cross-correlation amplitude ξga(v,ρ) > 1 and with galaxy impact parameter ρ < 200 h-1 kpc, we confirm and improve the results presented by Lanzetta et al. and Chen et al. that (1) extended gaseous envelopes are a common and generic feature of galaxies of a wide range of luminosity and morphological type, (2) the extent of tenuous gas [N(H ) 1014 cm-2] around galaxies scales with galaxy B-band luminosity as r ∝ L, and (3) galaxy interactions do not play an important role in distributing tenuous gas around galaxies in most cases. We further demonstrate that (4) the gaseous extent of galaxies scales with galaxy K-band luminosity as r ∝ L, and (5) tenuous gas around typical L* galaxies is likely to be distributed in spherical halos of radius ≈180 h-1 kpc of covering factor of nearly unity. The sample consists of 34 galaxy and absorber pairs and 13 galaxies that do not produce Lyα absorption lines to within sensitive upper limits. Redshifts of the galaxy and absorber pairs range from z = 0.0752 to 0.8920 with a median of z = 0.3567; impact parameter separations of the galaxy and absorber pairs range from ρ = 12.4 to 175.2 h-1 kpc with a median of ρ = 62.2 h-1 kpc. Of the galaxies, 15 (32%) are of B-band luminosity LB < 0.25 L and six (13%) are of low surface brightness. The galaxy sample is therefore representative of the galaxy population over a large fraction of the Hubble time. Because galaxies of all morphological types possess extended gaseous halos and because the extent of tenuous gas around galaxies scales with galaxy K-band luminosity, we argue that galaxy mass—rather than recent star formation activity—is likely to be the dominant factor that determines the extent of tenuous gas around galaxies. Nevertheless, applying the scaling relationship between the extent of Lyα-absorbing gas around galaxies and galaxy B-band luminosity, the results of our analysis also suggest that the number density evolution of Lyα absorption systems may serve to constrain the evolution of the comoving galaxy B-band luminosity density (at least for the redshift interval between z ~ 0 and z ~ 1 that has been studied in our survey).

The Low‐Redshift Lyα Forest in Cold Dark Matter Cosmologies

We study the physical origin of the low-redshift Lyα forest in hydrodynamic simulations of four cosmological models, all variants of the cold dark matter scenario. Our most important conclusions are insensitive to the cosmological model, but they depend on our assumption that the UV background declines at low redshift in concert with the declining population of quasar sources. We find that the expansion of the universe drives rapid evolution of dN/dz (the number of absorbers per unit redshift above a specified equivalent width threshold) at z1.7, but that at lower redshift the fading of the UV background counters the influence of expansion, leading to slow evolution of dN/dz. The draining of gas from low-density regions into collapsed structures has a mild but not negligible effect on the evolution of dN/dz, especially for high equivalent-width thresholds. At every redshift, weaker lines come primarily from moderate fluctuations of the diffuse, unshocked intergalactic medium (IGM) and stronger lines originate in shocked or radiatively cooled gas of higher overdensity. However, the neutral hydrogen column density associated with structures of fixed overdensity drops as the universe expands, so an absorber at z=0 is dynamically analogous to an absorber that has column density 10-50 times higher at z=2-3. In particular, the mildly overdense IGM fluctuations that dominate the Lyα forest opacity at z>2 produce optically thin lines at z<1, while the marginally saturated (NH I~1014.5 cm−2) lines at z<1 typically arise in gas that is overdense by a factor of 20-100. We find no clear distinction between lines arising in galaxy halos and lines arising in larger scale structures; however, galaxies tend to lie near the dense regions of the IGM that are responsible for strong Lyα lines. The simulations provide a unified physical picture that accounts for the most distinctive observed properties of the low-redshift Lyα forest: (1) a sharp transition in the evolution of dN/dz at z~1.7, (2) stronger evolution for absorbers of higher equivalent width, (3) a correlation of increasing Lyα equivalent width with decreasing galaxy impact parameter that extends to rp~500 h−1 kpc, and (4) a tendency for stronger lines to arise in close proximity to galaxies while weaker lines trace more diffuse large-scale structure.

The Gaseous Extent of Galaxies and the Origin of Lyα Absorption Systems. III.HubbleSpaceTelescopeImaging of Lyα‐absorbing Galaxies atz&lt; 1

We present initial results of a program to obtain and analyze Hubble Space Telescope (HST) Wide Field Planetary Camera 2 images of galaxies identified in an imaging and spectroscopic survey of faint galaxies in fields of HST spectroscopic target QSOs. We measure properties of 87 galaxies, of which 33 are associated with corresponding Lyα absorption systems and 24 do not produce corresponding Lyα absorption lines to within sensitive upper limits. Considering only galaxy and absorber pairs that are likely to be physically associated and excluding galaxy and absorber pairs within 3000 km s-1 of the background QSOs leaves 26 galaxy and absorber pairs and seven galaxies that do not produce corresponding Lyα absorption lines to within sensitive upper limits. Redshifts of the galaxy and absorber pairs range from 0.0750 to 0.8912 with a median of 0.3718, and impact parameter separations of the galaxy and absorber pairs range from 12.4 to 157.4 h-1 kpc with a median of 62.4 h-1 kpc. The primary result of the analysis is that the amount of gas encountered along the line of sight depends on the galaxy impact parameter and B-band luminosity but does not depend strongly on the galaxy average surface brightness, disk-to-bulge ratio, or redshift. This result confirms and improves upon the anticorrelation between Lyα absorption equivalent width and galaxy impact parameter found previously by Lanzetta et al. in 1995. Spherical halos cannot be distinguished from flattened disks on the basis of the current observations, and there is no evidence that galaxy interactions play an important role in distributing tenuous gas around galaxies in most cases. Galaxies might account for all Lyα absorption systems with W > 0.3 A, but this depends on the unknown luminosity function and gaseous cross sections of low-luminosity galaxies as well as on the uncertainties of the observed number density of Lyα absorption systems.

A comparison of metal and Ly-α absorption in galactic haloes

The metallicity structure of galactic haloes is one of the major unknowns in modern galactic astrophysics and observational cosmology. Hereby we outline a very simple method for obtaining at least a crude picture of gas abundances, based on recent important statistical determination of the relationship between neutral hydrogen absorbing column density and galaxy impact parameter. It is a fine illustration of the great theoretical and observational power of low-redshift Ly? absorption studies. Future detailed analyses of large samples of absorption-selected galaxies will certainly provide us with more data on the abundance structure of extended gaseous component of galaxies.