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Register a new userThe environmental dependence of HI in galaxies in the EAGLE simulations
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We use the EAGLE suite of cosmological hydrodynamical simulations to study how the HI content of present-day galaxies depends on their environment. We show that EAGLE reproduces observed HI mass-environment trends very well, while semi-analytic models typically overpredict the average HI masses in dense environments. The environmental processes act primarily as an on/off switch for the HI content of satellites with stellar mass Mstar>10^9 Msun. At a fixed Mstar, the fraction of HI-depleted satellites increases with increasing host halo mass M200 in response to stronger environmental effects, while at a fixed M200 it decreases with increasing satellite Mstar as the gas is confined by deeper gravitational potentials. HI-depleted satellites reside mostly, but not exclusively, within the virial radius r200 of their host halo. We investigate the origin of these trends by focussing on three environmental mechanisms: ram pressure stripping by the intra-group medium, tidal stripping by the host halo, and satellite-satellite encounters. By tracking back in time the evolution of the HI-depleted satellites, we find that the most common cause of HI removal is satellite encounters. The timescale for HI removal is typically less than 0.5 Gyr. Tidal stripping occurs in halos of M200<10^{14} Msun within half r200, while the other processes act also in more massive halos, generally within r200. Conversely, we find that ram pressure stripping is the most common mechanism that disturbs the HI morphology of galaxies at redshift z=0. This implies that HI removal due to satellite-satellite interactions occurs on shorter timescales than the other processes.
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We use the eagle simulations to study the connection between the quenching timescale, $tau_{rm Q}$, and the physical mechanisms that transform star-forming galaxies into passive galaxies. By quantifying $tau_{rm Q}$ in two complementary ways - as the time over which (i) galaxies traverse the green valley on the colour-mass diagram, or (ii) leave the main sequence of star formation and subsequently arrive on the passive cloud in specific star formation rate (SSFR)-mass space - we find that the $tau_{rm Q}$ distribution of high-mass centrals, low-mass centrals and satellites are divergent. In the low stellar mass regime where $M_{star}<10^{9.6}M_{odot}$, centrals exhibit systematically longer quenching timescales than satellites ($approx 4$~Gyr compared to $approx 2$~Gyr). Satellites with low stellar mass relative to their halo mass cause this disparity, with ram pressure stripping quenching these galaxies rapidly. Low mass centrals are quenched as a result of stellar feedback, associated with long $tau_{rm Q}gtrsim 3$~Gyr. At intermediate stellar masses where $10^{9.7},rm M_{odot}<M_{star}<10^{10.3},rm M_{odot}$, $tau_{rm Q}$ are the longest for both centrals and satellites, particularly for galaxies with higher gas fractions. At $M_{star}gtrsim 10^{10.3},rm M_{odot}$, galaxy merger counts and black hole activity increase steeply for all galaxies. Quenching timescales for centrals and satellites decrease with stellar mass in this regime to $tau_{rm Q}lesssim2$~Gyr. In anticipation of new intermediate redshift observational galaxy surveys, we analyse the passive and star-forming fractions of galaxies across redshift, and find that the $tau_{rm Q}$ peak at intermediate stellar masses is responsible for a peak (inflection point) in the fraction of green valley central (satellite) galaxies at $zapprox 0.5-0.7$.
We investigate the abundance of galactic molecular hydrogen (H$_2$) in the Evolution and Assembly of GaLaxies and their Environments (EAGLE) cosmological hydrodynamic simulations. We assign H$_2$ masses to gas particles in the simulations in post-processing using two different prescriptions that depend on the local dust-to-gas ratio and the interstellar radiation field. Both result in H$_2$ galaxy mass functions that agree well with observations in the local and high-redshift Universe. The simulations reproduce the observed scaling relations between the mass of H$_2$ and the stellar mass, star formation rate and stellar surface density. Towards high edshifts, galaxies in the simulations display larger H$_2$ mass fractions, and correspondingly lower H$_2$ depletion timescales, also in good agreement with observations. The comoving mass density of H$_2$ in units of the critical density, $Omega_{rm H_2}$, peaks at $zapprox 1.2-1.5$, later than the predicted peak of the cosmic star formation rate activity, at $zapprox 2$. This difference stems from the decrease in gas metallicity and increase in interstellar radiation field with redshift, both of which hamper H$_2$ formation. We find that the cosmic H$_2$ budget is dominated by galaxies with $M_{rm H_2}>10^9,rm M_{odot}$, star formation rates $>10,rm M_{odot},rm yr^{-1}$ and stellar masses $M_{rm stellar}>10^{10},rm M_{odot}$, which are readily observable in the optical and near-IR. The match between the H$_2$ properties of galaxies that emerge in the simulations and observations is remarkable, particularly since H$_2$ observations were not used to adjust parameters in EAGLE.
We present and explore the resolved atomic hydrogen (HI) content of 13 HI-rich and late-type dominated groups denoted `Choirs. We quantify the HI content of the Choir galaxies with respect to the median of the HI-mass fraction ($f_{textrm{HI}}$) of their grandparent HIPASS sample. We find that the HI mass fraction of the Choir galaxies is dispersed around the HIPASS median in the range $-1.4 leq Delta f_{textrm{HI}}textrm{[dex]}leq 0.7$, from HI-excess to HI-deficient galaxy regime. The HI-excess/HI-deficient galaxies contain more/less than 2.5 times their expected HI content with respect to the HIPASS median. We show and discuss that the environmental processing in Choirs occurs via tidal stripping and galaxy mergers. Our analysis suggests that tidal stripping contributes to the loss of the HI, while galaxy mergers contribute to the enhancement of the HI. Exploring the mid-infrared properties of Choir galaxies we find possible environmental processing in only nine Choir galaxies, which indicates that environmental processing is more perceptible in the HI content than the mid-infrared properties. Moreover, we find that environmental processing occurs in Choir groups regardless of their global environment, whether they are in isolation or in proximity to the denser structures, such as cosmic web filaments. We explore possible scenarios of the Choirs evolution, taking into account their HI content, velocity dispersion, crossing time and their global environment. We conclude that the most likely evolution for the majority of Choir groups is that they will become more compact as their members undergo multiple HI-rich mergers.
The neutral hydrogen~(HI) gas is an important barometer of recent star formation and metal enrichment activities in galaxies. I develop a novel statistical method for predicting the HI-to-stellar mass ratio $f_{gas}$ of galaxies from their stellar mass and optical colour, and apply it to a volume-limited galaxy sample jointly observed by the Sloan Digital Sky Survey and the Arecibo Legacy Fast ALFA survey. I eliminate the impact of the Malmquist bias against HI-deficient systems on the $f_{gas}$ predictor by properly accounting for the HI detection probability of each galaxy in the analysis. The best-fitting $f_{gas}$ predictor, with an estimated scatter of $0.272$ dex, provides excellent description to the observed HI mass function. After defining an HI excess parameter as the deviation of the observed $f_{gas}$ from the expected value, I confirm that there exists a strong secondary dependence of the mass-metallicity relation on HI excess. By further examining the 2D metallicity distribution on the specific star formation rate vs. HI excess plane, I show that the metallicity dependence on HI is likely more fundamental than that on specific star formation rate. In addition, I find that the environmental dependence of HI in the local Universe can be effectively described by the cross-correlation coefficient between HI excess and the red galaxy overdensity $rho_{cc}{=}-0.18$. This weak anti-correlation also successfully explains the observed dependence of HI clustering on $f_{gas}$. My method provides a useful framework for learning HI gas evolution from the synergy between future HI and optical galaxy surveys.
We calculate the colours and luminosities of redshift z = 0.1 galaxies from the EAGLE simulation suite using the GALAXEV population synthesis models. We take into account obscuration by dust in birth clouds and diffuse ISM using a two-component screen model, following the prescription of Charlot and Fall. We compare models in which the dust optical depth is constant to models where it depends on gas metallicity, gas fraction and orientation. The colours of EAGLE galaxies for the more sophisticated models are in broad agreement with those of observed galaxies. In particular, EAGLE produces a red sequence of passive galaxies and a blue cloud of star forming galaxies, with approximately the correct fraction of galaxies in each population and with g-r colours within 0.1 magnitudes of those observed. Luminosity functions from UV to NIR wavelengths differ from observations at a level comparable to systematic shifts resulting from a choice between Petrosian and Kron photometric apertures. Despite the generally good agreement there are clear discrepancies with observations. The blue cloud of EAGLE galaxies extends to somewhat higher luminosities than in the data, consistent with the modest underestimate of the passive fraction in massive EAGLE galaxies. There is also a moderate excess of bright blue galaxies compared to observations. The overall level of agreement with the observed colour distribution suggests that EAGLE galaxies at z = 0.1 have ages, metallicities and levels of obscuration that are comparable to those of observed galaxies.
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