arXiv subjects

Benjamin D. Oppenheimer

Publications and source records attributed to Benjamin D. Oppenheimer.

At least 73 records · Page 4Linked to original sources

arXiv2015

Baryon Cycling in the Low-Redshift Circumgalactic Medium: A Comparison of Simulations to the COS-Halos Survey

We analyze the low-redshift (z~0.2) circumgalactic medium 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 extract for comparison direct observables including equivalent widths, covering factors, ion ratios, and kinematics. Both wind models are generally in good agreement with these observations for HI and certain low ionization metal lines, but show poorer agreement with higher ionization metal lines including SiIII and OVI that are well-observed by COS-Halos. These discrepancies suggest that both wind models predict too much cool, metal-enriched gas and not enough hot gas, and/or that the metals are not sufficiently well-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 super-heating its ISM gas, which we show yields sufficient low ionisation absorption to be broadly consistent with observations, but also substantial OVI absorption that is 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 HI, while OVI almost always arises from metals ejected longer than 1 Gyr ago. The similarity between the wind models is surprising, since we show that they differ substantially in their predicted amount and phase distribution of halo gas. We show that this similarity owes mainly to our comparison here at fixed stellar mass rather than at fixed halo mass in our previous works, which suggests that CGM properties are more closely tied to the stellar mass of galaxies rather than halo mass.

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arXiv2015

Torque-Limited Growth of Massive Black Holes in Galaxies Across Cosmic Time

We combine cosmological hydrodynamic simulations with analytic models to evaluate the role of galaxy-scale gravitational torques on the evolution of massive black holes at the centers of star-forming galaxies. We confirm and extend our earlier results to show that torque-limited growth yields black holes and host galaxies evolving on average along the Mbh-Mbulge relation from early times down to z = 0 and that convergence onto the scaling relation occurs independent of the initial conditions and with no need for mass averaging through mergers or additional self-regulation processes. Smooth accretion dominates the long-term evolution, with black hole mergers with mass ratios >1:5 representing typically a small fraction of the total growth. Winds from the accretion disk are required to eject significant mass to suppress black hole growth, but there is no need for coupling this wind to galactic-scale gas to regulate black holes in a non-linear feedback loop. Torque-limited growth yields a close-to-linear relation for the star formation rate and the black hole accretion rate averaged over galaxy evolution time scales. However, the SFR-AGN connection has significant scatter owing to strong variability of black hole accretion at all resolved time scales. Eddington ratios can be described by a broad lognormal distribution with median value evolving roughly as (1 + z)^1.9, suggesting a main sequence for black hole growth similar to the cosmic evolution of specific SFRs. Our results offer an attractive scenario consistent with available observations in which cosmological gas infall and transport of angular momentum in the galaxy by gravitational instabilities regulate the long-term co-evolution of black holes and star-forming galaxies.

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arXiv2014

The COS-Dwarfs Survey: The Carbon Reservoir Around sub-L* Galaxies

We report new observations of circumgalactic gas from the COS-Dwarfs survey, a systematic investigation of the gaseous halos around 43 low-mass z $\leq$ 0.1 galaxies using background QSOs observed with the Cosmic Origins Spectrograph. From the projected 1D and 2D distribution of C IV absorption, we find that C IV absorption is detected out to ~ 0.5 R$_{vir}$ of the host galaxies. The C IV absorption strength falls off radially as a power law and beyond 0.5 R$_{vir}$, no C IV absorption is detected above our sensitivity limit of ~ 50-100 m$Å$. We find a tentative correlation between detected C IV absorption strength and star formation, paralleling the strong correlation seen in highly ionized oxygen for L~L* galaxies by the COS-Halos survey. The data imply a large carbon reservoir in the CGM of these galaxies, corresponding to a minimum carbon mass of $\gtrsim$ 1.2$\times 10^6$ $M_\odot$ out to ~ 110 kpc. This mass is comparable to the carbon mass in the ISM and more than the carbon mass currently in stars of these galaxies. The C IV absorption seen around these sub-L* galaxies can account for almost two-thirds of all $W_r$> 100 m$Å$ C IV absorption detected at low z. Comparing the C IV covering fraction with hydrodynamical simulations, we find that an energy-driven wind model is consistent with the observations whereas a wind model of constant velocity fails to reproduce the CGM or the galaxy properties.

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arXiv2014

The COS-Halos Survey: Physical Conditions and Baryonic Mass in the Low-Redshift Circumgalactic Medium

We analyze the physical conditions of the cool, photoionized (T $\sim 10^4$ K) circumgalactic medium (CGM) using the COS-Halos suite of gas column density measurements for 44 gaseous halos within 160 kpc of $L \sim L^*$ galaxies at $z \sim 0.2$. These data are well described by simple photoionization models, with the gas highly ionized (n$_{\rm HII}$/n$_{\rm H} \gtrsim 99\%$) by the extragalactic ultraviolet background (EUVB). Scaling by estimates for the virial radius, R$_{\rm vir}$, we show that the ionization state (tracked by the dimensionless ionization parameter, U) increases with distance from the host galaxy. The ionization parameters imply a decreasing volume density profile n$_{\rm H}$ = (10$^{-4.2 \pm 0.25}$)(R/R$_{\rm vir})^{-0.8\pm0.3}$. Our derived gas volume densities are several orders of magnitude lower than predictions from standard two-phase models with a cool medium in pressure equilibrium with a hot, coronal medium expected in virialized halos at this mass scale. Applying the ionization corrections to the HI column densities, we estimate a lower limit to the cool gas mass M$_{\rm CGM}^{\rm cool} > 6.5 \times 10^{10}$ M$_{\odot}$ for the volume within R $<$ R$_{\rm vir}$. Allowing for an additional warm-hot, OVI-traced phase, the CGM accounts for at least half of the baryons purported to be missing from dark matter halos at the 10$^{12}$ M$_{\odot}$ scale.

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arXiv2014

Non-equilibrium chemistry and cooling in the diffuse interstellar medium - II. Shielded gas

We extend the non-equilibrium model for the chemical and thermal evolution of diffuse interstellar gas presented in Richings et al. (2014) to account for shielding from the UV radiation field. We attenuate the photochemical rates by dust and by gas, including absorption by HI, H2, HeI, HeII and CO where appropriate. We then use this model to investigate the dominant cooling and heating processes in interstellar gas as it becomes shielded from the UV radiation. We consider a one-dimensional plane-parallel slab of gas irradiated by the interstellar radiation field, either at constant density and temperature or in thermal and pressure equilibrium. The dominant thermal processes tend to form three distinct regions in the clouds. At low column densities cooling is dominated by ionised metals such as SiII, FeII, FeIII and CII, which are balanced by photoheating, primarily from HI. Once the hydrogen-ionising radiation becomes attenuated by neutral hydrogen, photoelectric dust heating dominates, while CII becomes dominant for cooling. Finally, dust shielding triggers the formation of CO and suppresses photoelectric heating. The dominant coolants in this fully shielded region are H2 and CO. The column density of the HI-H2 transition predicted by our model is lower at higher density (or at higher pressure for gas clouds in pressure equilibrium) and at higher metallicity, in agreement with previous PDR models. We also compare the HI-H2 transition in our model to two prescriptions for molecular hydrogen formation that have been implemented in hydrodynamic simulations.

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arXiv2014

The Photon Underproduction Crisis

We examine the statistics of the low-redshift Lyman-alpha forest from smoothed particle hydrodynamic simulations in light of recent improvements in the estimated evolution of the cosmic ultraviolet background (UVB) and recent observations from the Cosmic Origins Spectrograph (COS). We find that the value of the metagalactic photoionization rate required by our simulations to match the observed properties of the low-redshift Lyman-alpha forest is a factor of 5 larger than the value predicted by state-of-the art models for the evolution of this quantity. This mismatch results in the mean flux decrement of the Lyman-alpha forest being underpredicted by at least a factor of 2 (a 10-sigma discrepancy with observations) and a column density distribution of Lyman-alpha forest absorbers systematically and significantly elevated compared to observations over nearly two decades in column density. We examine potential resolutions to this mismatch and find that either conventional sources of ionizing photons (galaxies and quasars) must be significantly elevated relative to current observational estimates or our theoretical understanding of the low-redshift universe is in need of substantial revision.

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arXiv2014

Parametrising Star Formation Histories

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.

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arXiv2014

Non-equilibrium chemistry and cooling in the diffuse interstellar medium - I. Optically thin regime

An accurate treatment of the multiphase interstellar medium (ISM) in hydrodynamic galaxy simulations requires that we follow not only the thermal evolution of the gas, but also the evolution of its chemical state, including its molecular chemistry, without assuming chemical (including ionisation) equilibrium. We present a reaction network that can be used to solve for this thermo-chemical evolution. Our model follows the evolution of all ionisation states of the 11 elements that dominate the cooling rate, along with important molecules such as H2 and CO, and the intermediate molecular species that are involved in their formation (20 molecules in total). We include chemical reactions on dust grains, thermal processes involving dust, cosmic ray ionisation and heating and photochemical reactions. We focus on conditions typical for the diffuse ISM, with densities of 10^-2 cm^-3 < nH < 10^4 cm^-3 and temperatures of 10^2 K < T < 10^4 K, and we consider a range of radiation fields, including no UV radiation. In this paper we consider only gas that is optically thin, while paper II considers gas that becomes shielded from the radiation field. We verify the accuracy of our model by comparing chemical abundances and cooling functions in chemical equilibrium with the photoionisation code Cloudy. We identify the major coolants in diffuse interstellar gas to be CII, SiII and FeII, along with OI and H2 at densities nH > 10^2 cm^-3. Finally, we investigate the impact of non-equilibrium chemistry on the cooling functions of isochorically or isobarically cooling gas. We find that, at T < 10^4 K, recombination lags increase the electron abundance above its equilibrium value at a given temperature, which can enhance the cooling rate by up to two orders of magnitude. The cooling gas also shows lower H2 abundances than in equilibrium, by up to an order of magnitude.

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arXiv2014

A Budget and Accounting of Metals at z~0: Results from the COS-Halos Survey

We present a budget and accounting of metals in and around star-forming galaxies at $z\sim 0$. We combine empirically derived star formation histories with updated supernova and AGB yields and rates to estimate the total mass of metals produced by galaxies with present-day stellar mass of $10^{9.3}$--$10^{11.6} M_{\odot}$. On the accounting side of the ledger, we show that a surprisingly constant 20--25% mass fraction of produced metals remain in galaxies' stars, interstellar gas and interstellar dust, with little dependence of this fraction on the galaxy stellar mass (omitting those metals immediately locked up in remnants). Thus, the bulk of metals are outside of galaxies, produced in the progenitors of today's $L^*$ galaxies. The COS-Halos survey is uniquely able to measure the mass of metals in the circumgalactic medium (to impact parameters of $< 150$ kpc) of low-redshift $\sim L^*$ galaxies. Using these data, we map the distribution of CGM metals as traced by both the highly ionized OVI ion and a suite of low-ionization species; combined with constraints on circumgalactic dust and hotter X-ray emitting gas out to similar impact parameters, we show that $\sim 40$% of metals produced by $M_{\star}\sim 10^{10}M_{\odot}$ galaxies can be easily accounted for out to 150 kpc. With the current data, we cannot rule out a constant mass of metals within this fixed physical radius. This census provides a crucial boundary condition for the eventual fate of metals in galaxy evolution models.

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arXiv2014

Cosmological Zoom Simulations of z = 2 Galaxies: The Impact of Galactic Outflows

We use high-resolution cosmological zoom simulations with ~200 pc resolution at z = 2 and various prescriptions for galactic outflows in order to explore the impact of winds on the morphological, dynamical, and structural properties of eight individual galaxies with halo masses ~ 10^11--2x10^12 Msun at z = 2. We present a detailed comparison to spatially and spectrally resolved Hα and other observations of z ~ 2 galaxies. We find that simulations without winds produce massive, compact galaxies with low gas fractions, super-solar metallicities, high bulge fractions, and much of the star formation concentrated within the inner kpc. Strong winds are required to maintain high gas fractions, redistribute star-forming gas over larger scales, and increase the velocity dispersion of simulated galaxies, more in agreement with the large, extended, turbulent disks typical of high-redshift star-forming galaxies. Winds also suppress early star formation to produce high-redshift cosmic star formation efficiencies in better agreement with observations. Sizes, rotation velocities, and velocity dispersions all scale with stellar mass in accord with observations. Our simulations produce a diversity of morphological characteristics - among our three most massive galaxies, we find a quiescent grand-design spiral, a very compact star-forming galaxy, and a clumpy disk undergoing a minor merger; the clumps are evident in Hα but not in the stars. Rotation curves are generally slowly rising, particularly when calculated using azimuthal velocities rather than enclosed mass. Our results are broadly resolution-converged. These results show that cosmological simulations including outflows can produce disk galaxies similar to those observed during the peak epoch of cosmic galaxy growth.

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arXiv2013

The High-Ion Content and Kinematics of Low-Redshift Lyman Limit Systems

We study the high-ionization phase and kinematics of the circumgalactic medium around low-redshift galaxies using a sample of 23 Lyman Limit Systems (LLSs) at 0.08<z<0.93 observed with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope. In Lehner et al. (2013), we recently showed that low-z LLSs have a bimodal metallicity distribution. Here we extend that analysis to search for differences between the high-ion and kinematic properties of the metal-poor and metal-rich branches. We find that metal-rich LLSs tend to show higher O VI columns and broader O VI profiles than metal-poor LLSs. The total H I line width (dv90 statistic) in LLSs is not correlated with metallicity, indicating that the H I kinematics alone cannot be used to distinguish inflow from outflow and gas recycling. Among the 17 LLSs with O VI detections, all but two show evidence of kinematic sub-structure, in the form of O VI-H I centroid offsets, multiple components, or both. Using various scenarios for how the metallicity in the high-ion and low-ion phases of each LLS compare, we constrain the ionized hydrogen column in the O VI phase to lie in the range log N(H II)~17.6-20. The O VI phase of LLSs is a substantial baryon reservoir, with M(high-ion)~10^{8.5-10.9}(r/150 kpc)^2 solar masses, similar to the mass in the low-ion phase. Accounting for the O VI phase approximately doubles the contribution of low-z LLSs to the cosmic baryon budget.

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arXiv2013

Hydrogen and Metal Line Absorption Around Low-Redshift Galaxies in Cosmological Hydrodynamic Simulations

We study the physical conditions of the circum-galactic medium (CGM) around z=0.25 galaxies as traced by HI and metal line absorption, using cosmological hydrodynamic simulations that include galactic outflows. Using lines of sight targeted at impact parameters from 10 kpc to 1 Mpc around galaxies with halo masses from 10^11-10^13 M_solar, we study the physical conditions and their variation with impact parameter b and line-of-sight velocity delta v in the CGM as traced by HI, MgII, SiIV, CIV, OVI, and NeVIII absorbers. All ions show a strong excess of absorption near galaxies compared to random lines of sight. The excess continues beyond 1 Mpc, reflecting the correlation of metal absorption with large-scale structure. Absorption is particularly enhanced within about v<300 km/sec and roughly 300 kpc of galaxies (with distances somewhat larger for the highest ion), approximately delineating the CGM; this range contains the majority of global metal absorption. Low ions like MgII and SiIV predominantly arise in denser gas closer to galaxies and drop more rapidly with b, while high ions OVI and NeVIII trace more diffusely distributed gas with a comparatively flat radial profile; CIV is intermediate. All ions predominantly trace T~10^4-4.5 K photo-ionised gas at all b, but when hot CGM gas is present (mostly in larger halos), we see strong collisionally-ionised OVI and NeVIII at b <= 100 kpc. Larger halo masses generally produce more absorption, though overall the trends are not as strong as that with impact parameter. These findings arise using our favoured outflow scalings as expected for momentum-driven winds; with no winds, the CGM gas remains mostly unenriched, while our outflow model with a constant velocity and mass loading factor produce hotter, more widely dispersed metals.

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arXiv2013

Tracing Inflows and Outflows with Absorption Lines in Circumgalactic Gas

We examine how HI and metal absorption lines within low-redshift galaxy halos trace the dynamical state of circumgalactic gas, using cosmological hydrodynamic simulations that include a well-vetted heuristic model for galactic outflows. We categorize inflowing, outflowing, and ambient gas based on its history and fate as tracked in our simulation. Following our earlier work showing that the ionisation level of absorbers was a primary factor in determining the physical conditions of absorbing gas, we show here that it is also a governing factor for its dynamical state. Low-ionisation metal absorbers (e.g. MgII) tend to arise in gas that will fall onto galaxies within several Gyr, while high-ionisation metal absorbers (e.g. OVI) generally trace material that was deposited by outflows many Gyr ago. Inflowing gas is dominated by enriched material that was previously ejected in an outflow, hence accretion at low redshifts is typically substantially enriched. Recycling wind material is preferentially found closer to galaxies, and is more dominant in lower-mass halos since high-mass halos have more hot gas that is able to support itself against infall. Low-mass halos also tend to re-eject more of their accreted material, owing to our outflow prescription that employs higher mass loading factors for lower-mass galaxies. Typical HI absorbers trace unenriched ambient material that is not participating in the baryon cycle, but stronger HI absorbers arise in cool, enriched inflowing gas. Instantaneous radial velocity measures of absorbers are generally poor at distinguishing between inflowing and outflowing gas, except in the case of very recent outflows. These results suggest that probing halo gas using a range of absorbers can provide detailed information about the amount and physical conditions of material that is participating in the baryon cycle.

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arXiv2013

The COS-Halos Survey: Rationale, Design, and A Census of Circumgalactic Neutral Hydrogen

We present the design and methods of the COS-Halos survey, a systematic investigation of the gaseous halos of 44 z = 0.15-0.35 galaxies using background QSOs observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. This survey has yielded 39 spectra of z_em ~ 0.5 QSOs with S/N ~ 10-15 per resolution element. The QSO sightlines pass within 150 physical kpc of the galaxies, which span early and late types over stellar mass log M* / Msun= 9.5 - 11.5. We find that the CGM exhibits strong HI, averaging 1 Ang in Lya equivalent width out to 150 kpc, with 100% covering fraction for star-forming galaxies and 75% covering for passive galaxies. We find good agreement in column densities between this survey and previous studies over similar range of impact parameter. There is weak evidence for a difference between early- and late-type galaxies in the strength and distribution of HI. Kinematics indicate that the detected material is bound to the host galaxy, such that >~90% of the detected column density is confined within +/-200 km s^-1 of the galaxies. This material generally exists well below the halo virial temperatures at T<~ 10^5 K. We evaluate a number of possible origin scenarios for the detected material, and in the end favor a simple model in which the bulk of the detected HI arises in a bound, cool, low-density photoionized diffuse medium that is generic to all L* galaxies and may harbor a total gaseous mass comparable to galactic stellar masses.

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arXiv2013

The effect of metal enrichment and galactic winds on galaxy formation in cosmological zoom simulations

We investigate the differential effects of metal cooling and galactic stellar winds on the cosmological formation of individual galaxies with three sets of cosmological, hydrodynamical zoom simulations of 45 halos in the mass range 10^11 1 and predict reasonable star formation histories, (ii) produce galaxies with high cold gas fractions (30-60 per cent) at high redshift, (iii) significantly reduce the galaxy formation efficiencies for halos (M_halo<10^12M_sun) at all redshifts in agreement with observational and abundance matching constraints, (iv) result in high-redshift galaxies with reduced circular velocities matching the observed Tully-Fisher relation at z~2, and (v) significantly increase the sizes of low-mass galaxies (M_stellar<3x10^10M_sun) at high redshift resulting in a weak size evolution - a trend in agreement with observations. However, the low redshift (z<0.5) star formation rates of massive galaxies are higher than observed (up to ten times). No tested model predicts the observed size evolution for low-mass and high-mass galaxies simultaneously. Due to the delayed onset of star formation in the wind models, the metal enrichment of gas and stars is delayed and agrees well with observational constraints. Metal cooling and stellar winds are both found to increase the ratio of in situ formed to accreted stars - the relative importance of dissipative vs. dissipationless assembly. For halo masses below ~10^12M_sun, this is mainly caused by less stellar accretion and compares well to predictions from semi-analytical models but still differs from abundance matching models. For higher masses, the fraction of in situ stars is over-predicted due to the unrealistically high star formation rates at low redshifts.

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arXiv2013

Non-Equilibirum ionisation and cooling of metal-enriched gas in the presence of a photo-ionisation background

Simulations of the formation of galaxies, as well as ionisation models used to interpret observations of quasar absorption lines, generally either assume ionisation equilibrium or ignore the presence of the extra-galactic background (EGB) radiation. We introduce a method to compute the non-equilibrium ionisation and cooling of diffuse gas exposed to the EGB. Our method iterates the ionisation states of the 11 elements that dominate the cooling and uses tabulated ion-by-ion cooling and photo-heating efficiencies to update the temperature of the gas. Our reaction network includes radiative and di-electric recombination, collisional ionisation, photo-ionisation, Auger ionisation, and charge transfer. We verify that our method reproduces published results for collisional equilibrium, collisional non-equilibrium, and photo-ionised equilibrium. Non-equilibrium effects can become very important in cooling gas, particularly below 10^6 K. Photo-ionisation and non-equilibrium effects both tend to boost the degree of ionisation and to reduce cooling efficiencies. The effect of the EGB is larger for lower densities. Hence, photo-ionisation affects cooling more under isochoric than under isobaric conditions. Non-equilibrium effects are smaller in the presence of the EGB and are thus overestimated when using collisional-only processes. The inclusion of the EGB alters the observational diagnostics of diffuse, metal-enriched gas (e.g. metal absorption lines) even more significantly than the cooling efficiencies. We argue that the cooling efficiency should be considered if ionisation models are used to infer physical conditions from observed line ratios, as the a priori probability of observing gas is lower if its cooling time is shorter. We provide on-line tables of ionisation fractions and cooling efficiencies for equilibrium and non-equilibrium scenarios. (abridged)

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arXiv2013

AGN proximity zone fossils and the delayed recombination of metal lines

We model the time-dependent evolution of metal-enriched intergalactic and circumgalactic gas exposed to the fluctuating radiation field from an active galactic nucleus (AGN). We consider diffuse gas densities (n_H=10^-5-10^-3 cm^-3) exposed to the extra-galactic background (EGB) and initially in thermal equilibrium (T \sim 10^4-10^4.5 K). Once the proximate AGN field turns on, additional photo-ionisation rapidly ionises the HI and metals. The enhanced AGN radiation field turns off after a typical AGN lifetime (tau_AGN=1-20 Myr) and the field returns to the EGB intensity, but the metals remain out of ionisation equilibrium for timescales that can significantly exceed tau_AGN. We define this phase as the AGN proximity zone "fossil" phase and show that high ionisation stages (e.g. OVI, NeVIII, MgX) are in general enhanced, while the abundances of low ions are reduced. In contrast, HI re-equilibrates rapidly (<<tau_AGN) owing to its low neutral fraction at diffuse densities. We demonstrate that metal column densities of intervening gas observed in absorption in quasar sight lines are significantly affected by delayed recombination for a wide range of densities, metallicities, and AGN strengths, lifetimes, and duty cycles. We model the exceptionally strong z=0.9 NeVIII absorbers observed by Tripp et al. (2011) as arising in a possible fossil zone or near a recently turned-on AGN and we demonstrate that at low redshift even moderate strength AGN could significantly enhance the high-ion metal columns in the circumgalactic media of galaxies observed without active AGN. Fossil proximity zones may be very important during the quasar era, z \sim 2-5. AGN proximity zone fossils allow a whole new class of non-equilibrium solutions that may be applicable to a large fraction of observed metal absorbers and which could potentially change the inferred physical conditions and masses of diffuse gases.

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arXiv2013

The Neutral Hydrogen Content of Galaxies in Cosmological Hydrodynamic Simulations

We examine the global HI properties of galaxies in quarter-billion particle cosmological simulations using Gadget-2, focusing on how galactic outflows impact HI content. We consider four outflow models, including a new one (ezw) motivated by recent interstellar medium simulations in which the wind speed and mass loading factor scale as expected for momentum-driven outflows for larger galaxies and energy-driven outflows for dwarfs (sigma<75 km/s). To obtain predicted HI masses, we employ a simple but effective local correction for particle self-shielding, and an observationally-constrained transition from neutral to molecular hydrogen. Our ezw simulation produces an HI mass function whose faint-end slope of -1.3 agrees well with observations from the ALFALFA survey; other models agree less well. Satellite galaxies have a bimodal distribution in HI fraction versus halo mass, with smaller satellites and/or those in larger halos more often being HI-deficient. At a given stellar mass, HI content correlates with star formation rate and inversely correlates with metallicity, as expected if driven by stochasticity in the accretion rate. To higher redshifts, massive HI galaxies disappear and the mass function steepens. The global cosmic HI density conspires to remain fairly constant from z~5-0, but the relative contribution from smaller galaxies increases with redshift.

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