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Register a new userHow environment drives galaxy evolution: lessons learnt from satellite galaxies
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A. Pasquali
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It is by now well established that galaxy evolution is driven by intrinsic and environmental processes, both contributing to shape the observed properties of galaxies. A number of early studies, both observational and theoretical, have shown that the star formation activity of galaxies depends on their environmental local density and also on galaxy hierarchy, i.e. centrals vs. satellites. In fact, contrary to their central (most massive) galaxy of a group/cluster, satellite galaxies are stripped of their gas and stars, and have their star formation quenched by their environment. Large galaxy surveys like SDSS now permit us to investigate in detail environment-driven transformation processes by comparing centrals and satellites. In this paper I summarize what we have so far learnt about environmental effects by analysing the observed properties of local central and satellite galaxies in SDSS, as a function of their stellar mass and the dark matter mass of their host group/cluster.
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Using the Sloan Digital Sky Survey, we adopt the sSFR-$Sigma_{1kpc}$ diagram as a diagnostic tool to understand quenching in different environments. sSFR is the specific star formation rate, and $Sigma_{1kpc}$ is the stellar surface density in the inner kpc. Although both the host halo mass and group-centric distance affect the satellite population, we find that these can be characterised by a single number, the quenched fraction, such that key features of the sSFR-$Sigma_{1kpc}$ diagram vary smoothly with this proxy for the environment. Particularly, the sSFR of star-forming galaxies decreases smoothly with this quenched fraction, the sSFR of satellites being 0.1 dex lower than in the field. Furthermore, $Sigma_{1kpc}$ of the transition galaxies (i.e., the green valley or GV) decreases smoothly with the environment, by as much as 0.2 dex for $M_* = 10^{9.75-10} M_{odot}$ from the field, and decreasing for satellites in larger halos and at smaller radial distances within same-mass halos. We interpret this shift as indicating the relative importance of todays field quenching track vs. the cluster quenching track. These environmental effects in the sSFR-$Sigma_{1kpc}$ diagram are most significant in our lowest mass range ($9.75 < log M_{*}/M_{odot} < 10$). One feature that is shared between all environments is that at a given $M_{*}$ quenched galaxies have about 0.2-0.3 dex higher $Sigma_{1kpc}$ than the star-forming population. These results indicate that either $Sigma_{1kpc}$ increases (subsequent to satellite quenching), or $Sigma_{1kpc}$ for individual galaxies remains unchanged, but the original $M_*$ or the time of quenching is significantly different from those now in the GV.
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We investigate how star formation quenching proceeds within central and satellite galaxies using spatially resolved spectroscopy from the SDSS-IV MaNGA DR15. We adopt a complete sample of star formation rate surface densities ($Sigma_{rm SFR}$), derived in Bluck et al. (2020), to compute the distance at which each spaxel resides from the resolved star forming main sequence ($Sigma_{rm SFR} - Sigma_*$ relation): $Delta Sigma_{rm SFR}$. We study galaxy radial profiles in $Delta Sigma_{rm SFR}$, and luminosity weighted stellar age (${rm Age_L}$), split by a variety of intrinsic and environmental parameters. Via several statistical analyses, we establish that the quenching of central galaxies is governed by intrinsic parameters, with central velocity dispersion ($sigma_c$) being the most important single parameter. High mass satellites quench in a very similar manner to centrals. Conversely, low mass satellite quenching is governed primarily by environmental parameters, with local galaxy over-density ($delta_5$) being the most important single parameter. Utilising the empirical $M_{BH}$ - $sigma_c$ relation, we estimate that quenching via AGN feedback must occur at $M_{BH} geq 10^{6.5-7.5} M_{odot}$, and is marked by steeply rising $Delta Sigma_{rm SFR}$ radial profiles in the green valley, indicating `inside-out quenching. On the other hand, environmental quenching occurs at over-densities of 10 - 30 times the average galaxy density at z$sim$0.1, and is marked by steeply declining $Delta Sigma_{rm SFR}$ profiles, indicating `outside-in quenching. Finally, through an analysis of stellar metallicities, we conclude that both intrinsic and environmental quenching must incorporate significant starvation of gas supply.
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