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Register a new userThe Effect of Galactic Feedback on Gas Accretion and Wind Recycling
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Freeke van de Voort
Publication date
2016
fields
Physics
and research's language is
English
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In the absence of galactic winds, the rate at which gas accretes onto galaxies is determined by the gravitational potential and by radiative cooling. However, outflows driven by supernovae and active galactic nuclei not only eject gas from galaxies, but also prevent gas from accreting in the first place. Furthermore, gas previously ejected from a galaxy can re-accrete onto (the same or a different) galaxy. Because this gas has a high metallicity, its cooling rate is relatively high, which will increase its chances to re-accrete. This complex interplay between gas inflows and outflows is discussed in this chapter. Wind recycling is found to be an important process that fuels galaxies at late times and the recycled gas has different properties than gas accreting for the first time. Quantitative conclusions, however, vary between studies, because the amount of wind recycling is dependent on the details of the feedback model. We discuss these differences, known caveats, and ways to make progress in understanding how galaxies are fed at low redshift.
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The role of galactic wind recycling represents one of the largest unknowns in galaxy evolution, as any contribution of recycling to galaxy growth is largely degenerate with the inflow rates of first-time infalling material, and the rates with which outflowing gas and metals are driven from galaxies. We present measurements of the efficiency of wind recycling from the EAGLE cosmological simulation project, leveraging the statistical power of large-volume simulations that reproduce a realistic galaxy population. We study wind recycling at the halo scale, i.e. gas that has been ejected beyond the halo virial radius, and at the galaxy scale, i.e. gas that has been ejected from the ISM to at least $approx 10 , %$ of the virial radius (thus excluding smaller-scale galactic fountains). Galaxy-scale wind recycling is generally inefficient, with a characteristic return timescale that is comparable or longer than a Hubble time, and with an efficiency that clearly peaks at the characteristic halo mass of $M_{200} = 10^{12} , mathrm{M_odot}$. Correspondingly, the majority of gas being accreted onto galaxies in EAGLE is infalling for the first time. At the halo scale, the efficiency of recycling onto haloes differs by orders of magnitude from values assumed by semi-analytic galaxy formation models. Differences in the efficiency of wind recycling with other hydrodynamical simulations are currently difficult to assess, but are likely smaller. We are able to show that the fractional contribution of wind recycling to galaxy growth is smaller in EAGLE than in some other simulations. We find that cumulative first-time gas accretion rates at the virial radius are reduced relative to the expectation from dark matter accretion for haloes with mass, $M_{200} < 10^{12} , mathrm{M_odot}$, indicating efficient preventative feedback on halo scales.
We study the effect of the gas accretion rate ($dot M_{rm accr}$) on the radial gas metallicity profile (RMP) of galaxies using the EAGLE cosmological hydrodynamic simulations, focusing on central galaxies of stellar mass $M_star gtrsim 10^9 , {rm M_odot}$ at $z le 1$. We find clear relations between $dot M_{rm accr}$ and the slope of the RMP (measured within an effective radius), where higher $dot M_{rm accr}$ are associated with more negative slopes. The slope of the RMPs depends more strongly on $dot M_{rm accr}$ than on stellar mass, star formation rate or gas fraction, suggesting $dot M_{rm accr}$ to be a more fundamental driver of the RMP slope of galaxies. We find that eliminating the dependence on stellar mass is essential for pinning down the properties that shape the slope of the RMP. Although $dot M_{rm accr}$ is the main property modulating the slope of the RMP, we find that it causes other correlations that are more easily testable observationally: at fixed stellar mass, galaxies with more negative RMP slopes tend to have higher gas fractions and SFRs, while galaxies with lower gas fractions and SFRs tend to have flatter metallicity profiles within an effective radius.
We generalize the analytic solutions presented in Pantoni et al. (2019) by including a simple yet effective description of wind recycling and galactic fountains, with the aim of self-consistently investigating the spatially-averaged time evolution of the gas, stellar, metal, and dust content in disc-dominated late-type galaxies (LTGs). Our analytic solutions, when supplemented with specific prescriptions for parameter setting and with halo accretion rates from $N-$body simulations, can be exploited to reproduce the main statistical relationships followed by local LTGs; these involve, as a function of the stellar mass, the star formation efficiency, the gas mass fraction, the gas/stellar metallicity, the dust mass, the star formation rate, the specific angular momentum, and the overall mass/metal budget. Our analytic solutions allow to easily disentangle the diverse role of the main physical processes ruling galaxy formation in LTGs; in particular, we highlight the crucial relevance of wind recycling and galactic fountains in efficiently refurnishing the gas mass, extending the star-formation timescale, and boosting the metal enrichment in gas and stars. All in all, our analytic solutions constitute a transparent, handy, and fast tool that can provide a basis for improving the (subgrid) physical recipes presently implemented in more sophisticated semi-analytic models and numerical simulations, and can offer a benchmark for interpreting and forecasting current and future spatially-averaged observations of local and higher redshift LTGs.
We present results from our on-going MusE GAs FLOw and Wind (MEGAFLOW) survey, which consists of 22 quasar lines-of-sight, each observed with the integral field unit (IFU) MUSE and the UVES spectrograph at the ESO Very Large Telescopes (VLT). The goals of this survey are to study the properties of the circum-galactic medium around $zsim1$ star-forming galaxies. The absorption-line selected survey consists of 79 strong MgII absorbers (with rest-frame equivalent width (REW)$gtrsim$0.3AA) and, currently, 86 associated galaxies within 100 projected~kpc of the quasar with stellar masses ($M_star$) from $10^9$ to $10^{11}$ msun. We find that the cool halo gas traced by MgII is not isotropically distributed around these galaxies, as we show the strong bi-modal distribution in the azimuthal angle of the apparent location of the quasar with respect to the galaxy major-axis. This supports a scenario in which outflows are bi-conical in nature and co-exist with a coplanar gaseous structure extending at least up to 60 to 80 kpc. Assuming that absorbers near the minor axis probe outflows, the current MEGAFLOW sample allowed us to select 26 galaxy-quasar pairs suitable for studying winds. From this sample, using a simple geometrical model, we find that the outflow velocity only exceeds the escape velocity when $M_{star}lesssim 4times10^9$~msun, implying the cool material is likely to fall back except in the smallest halos. Finally, we find that the mass loading factor $eta$, the ratio between the ejected mass rate and the star formation rate (SFR), appears to be roughly constant with respect to the galaxy mass.
Galactic outflows are thought to eject baryons back out to the circum-galactic medium (CGM). Studies based on metal absorption lines (MgII in particular) in the spectra of background quasars indicate that the gas is ejected anisotropically, with galactic winds likely leaving the host in a bi-conical flow perpendicular to the galaxy disk. In this paper, we present a detailed analysis of an outflow from a z = 0.7 green-valley galaxy (log($M_*$/$mathrm{M}_odot$) = 9.9; SFR = 0.5 $mathrm{M}_odot,mathrm{yr}^{-1}$) probed by two background sources part of the MUSE Gas Flow and Wind (MEGAFLOW) survey. Thanks to a fortuitous configuration with a background quasar (SDSSJ1358+1145) and a bright background galaxy at $z = 1.4$, both at impact parameters of $approx 15,mathrm{kpc}$, we can - for the first time - probe both the receding and approaching components of a putative galactic outflow around a distant galaxy. We measure a significant velocity shift between the MgII absorption from the two sightlines ($84pm17,mathrm{km},mathrm{s}^{-1}$), which is consistent with the expectation from our simple fiducial wind model, possibly combined with an extended disk contribution.
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