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Register a new userMAGICC haloes: confronting simulations with observations of the circumgalactic medium at z=0
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Greg Stinson
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We explore the circumgalactic medium (CGM) of two simulated star-forming galaxies with luminosities L ~ 0.1 and 1 L* generated using the smooth particle hydrodynamic code GASOLINE. These simulations are part of the Making Galaxies In a Cosmological Context (MAGICC) program in which the stellar feedback is tuned to match the stellar mass-halo mass relationship. For comparison, each galaxy was also simulated using a lower feedback (LF) model which has strength comparable to other implementations in the literature. The MAGICC feedback (MF) model has a higher incidence of massive stars and an approximately two times higher energy input per supernova. Apart from the low-mass halo using LF, each galaxy exhibits a metal-enriched CGM that extends to approximately the virial radius. A significant fraction of this gas has been heated in supernova explosions in the disc and subsequently ejected into the CGM where it is predicted to give rise to substantial O VI absorption. The simulations do not yet address the question of what happens to the O VI when the galaxies stop forming stars. Our models also predict a reservoir of cool H I clouds that show strong Lyalpha absorption to several hundred kpc. Comparing these models to recent surveys with the Hubble Space Telescope, we find that only the MF models have sufficient O VI and H I gas in the CGM to reproduce the observed distributions. In separate analyses, these same MF models also show better agreement with other galaxy observables (e.g. rotation curves, surface brightness profiles and H I gas distribution). We infer that the CGM is the dominant reservoir of baryons for galaxy haloes.
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Over the past decade, Lyman-alpha and metal line absorption observations have established the ubiquity of a gas-rich circumgalactic medium (CGM) around star-forming galaxies at z~0.2 potentially tracing half of the missing baryonic mass within galaxy halos. Unfortunately, these observations only provide a statistical measure of the gas in the CGM and do not constrain the spatial distribution and kinematics of the gas. Furthermore, we have limited sensitivity to Lyman-alpha at z~0 with existing instruments. As such, we remain ignorant of how this gas may flow from the CGM onto the disks of galaxies where it can fuel ongoing star-formation in the present day. Fortunately, 21-cm HI observations with radio telescopes can map HI emission providing both spatial and kinematic information for the CGM in galaxies at z=0. Observations with phased array feeds, radio cameras, on single-dish telescopes yield unmatched surface brightness sensitivity and survey speed. These observations can complete the census of HI in the CGM below N(HI)<10^17 cm^-2 and constrain how gas accretion is proceeding in the local universe, particularly when used in concert with UV absorption line data.
We assess the metal content of the cool (10^4 K) circumgalactic medium (CGM) about galaxies at z<1 using an H I-selected sample of 28 Lyman limit systems (LLS, defined here as absorbers with 16.2<log N(H I)<18.5) observed in absorption against background QSOs by the Cosmic Origins Spectrograph on-board the Hubble Space Telescope. The N(H I) selection avoids metallicity biases inherent in many previous studies of the low-redshift CGM. We compare the column densities of weakly ionized metal species (e.g., O II, Si II, Mg II) to N(H I) in the strongest H I component of each absorber. We find that the metallicity distribution of the LLS (and hence the cool CGM) is bimodal with metal-poor and metal-rich branches peaking at [X/H]=-1.6 and -0.3 (or about 2.5% and 50% solar metallicities). The cool CGM probed by these LLS is predominantly ionized. The metal-rich branch of the population likely traces winds, recycled outflows, and tidally stripped gas; the metal-poor branch has properties consistent with cold accretion streams thought to be a major source of fresh gas for star forming galaxies. Both branches have a nearly equal number of absorbers. Our results thus demonstrate there is a significant mass of previously-undiscovered cold metal-poor gas and confirm the presence of metal enriched gas in the CGM of z<1 galaxies.
Galaxies are surrounded by massive gas reservoirs (i.e. the circumgalactic medium; CGM) which play a key role in their evolution. The properties of the CGM, which are dependent on a variety of internal and environmental factors, are often inferred from absorption line surveys which rely on a limited number of single lines-of-sight. In this work we present an analysis of 28 galaxy haloes selected from the Auriga project, a cosmological magneto-hydrodynamical zoom-in simulation suite of isolated Milky Way-mass galaxies, to understand the impact of CGM diversity on observational studies. Although the Auriga haloes are selected to populate a narrow range in halo mass, our work demonstrates that the CGM of L* galaxies is extremely diverse: column densities of commonly observed species span ~3-4 dex and their covering fractions range from ~5 to 90 per cent. Despite this diversity, we identify the following correlations: 1) the covering fractions (CF) of hydrogen and metals of the Auriga haloes positively correlate with stellar mass, 2) the CF of H I, C IV, and Si II anticorrelate with active galactic nucleus luminosity due to ionization effects, and 3) the CF of H I, C IV, and Si II positively correlate with galaxy disc fraction due to outflows populating the CGM with cool and dense gas. The Auriga sample demonstrates striking diversity within the CGM of L* galaxies, which poses a challenge for observations reconstructing CGM characteristics from limited samples, and also indicates that long-term merger assembly history and recent star formation are not the dominant sculptors of the CGM.
Single-phase photoionization equilibrium (PIE) models are often used to infer the underlying physical properties of galaxy halos probed in absorption with ions at different ionization potentials. To incorporate the effects of turbulence, we use the MAIHEM code to model an isotropic turbulent medium exposed to a redshift zero metagalactic UV background, while tracking the ionizations, recombinations, and species-by-species radiative cooling for a wide range of ions. By comparing observations and simulations over a wide range of turbulent velocities, densities, and metallicity with a Markov chain Monte Carlo technique, we find that MAIHEM models provide an equally good fit to the observed low-ionization species compared to PIE models, while reproducing at the same time high-ionization species such as ion{Si}{4} and ion{O}{6}. By including multiple phases, MAIHEM models favor a higher metallicity ($Z/Z_odot approx 40%$) for the circumgalactic medium compared to PIE models. Furthermore, all of the solutions require some amount of turbulence ($sigma_{rm 3D} geqslant 26 {rm km} {rm s}^{-1}$). Correlations between turbulence, metallicity, column density, and impact parameter are discussed alongside mechanisms that drive turbulence within the halo.
We investigate the evolution of the galaxy stellar mass function at high-redshift ($zge 5$) using a pair of large cosmological hydrodynamical simulations: {em MassiveBlack} and {em MassiveBlack-II}. By combining these simulations we can study the properties of galaxies with stellar masses greater than $10^{8},{rm M_{odot}},h^{-1}$ and (co-moving) number densities of $log_{10}(phi, [{rm Mpc^{-3},dex^{-1}},h^{3}])>-8$. Observational determinations of the galaxy stellar mass function at very-high redshift typically assume a relation between the observed UV luminosity and stellar mass-to-light ratio which is applied to high-redshift samples in order to estimate stellar masses. This relation can also be measured from the simulations. We do this, finding two significant differences with the usual observational assumption: it evolves strongly with redshift and has a different shape. Using this relation to make a consistent comparison between galaxy stellar mass functions we find that at $z=6$ and above the simulation predictions are in good agreement with observed data over the whole mass range. Without using the correct UV luminosity and stellar mass-to-light ratio, the discrepancy would be up to two orders of magnitude for large galaxies $>10^{10},{rm M_{odot}},h^{-1}$. At $z=5$, however the stellar mass function for low mass $<10^{9},{rm M_{odot}},h^{-1}$ galaxies is overpredicted by factors of a few, consistent with the behaviour of the UV luminosity function, and perhaps a sign that feedback in the simulation is not efficient enough for these galaxies.
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