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Register a new userFrom haloes to galaxies -- II. The fundamental relations in star formation and quenching
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Star formation and quenching are two of the most important processes in galaxy formation and evolution. We explore in the local Universe the interrelationships among key integrated galaxy properties, including stellar mass $M_*$, star formation rate (SFR), specific SFR (sSFR), molecular gas mass $M_{rm H_2}$, star formation efficiency (SFE) of the molecular gas and molecular gas to stellar mass ratio $mu$. We aim to identify the most fundamental scaling relations among these key galaxy properties and their interrelationships. We show the integrated $M_{rm H_2}$-SFR, SFR-$M_*$ and $M_{rm H_2}$-$M_*$ relation can be simply transformed from the $mu$-sSFR, SFE-$mu$ and SFE-sSFR relation, respectively. The transformation, in principle, can increase or decrease the scatter of each relation. Interestingly, we find the latter three relations all have significantly smaller scatter than the former three corresponding relations. We show the probability to achieve the observed small scatter by accident is extremely close to zero. This suggests that the smaller scatters of the latter three relations are driven by a more fundamental physical connection among these quantities. We then show the large scatters in the former relations are due to their systematic dependence on other galaxy properties, and on star formation and quenching process. We propose the sSFR-$mu$-SFE relation as the Fundamental Formation Relation (FFR), which governs the star formation and quenching process, and provides a simple framework to study galaxy evolution. Other scaling relations, including integrated Kennicutt-Schmidt law, star-forming main sequence and molecular gas main sequence, can all be derived from the FFR.
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We quantitatively investigate the dependence of central galaxy HI mass ($M_{rm HI}$) on the stellar mass ($M_ast$), halo mass ($M_{rm h}$), star formation rate (SFR), and central stellar surface density within 1 kpc ($Sigma_1$), taking advantage of the HI spectra stacking technique using both the Arecibo Fast Legacy ALFA Survey and the Sloan Digital Sky Survey. We find that the shapes of $M_{rm HI}$-$M_{rm h}$ and $M_{rm HI}$-$M_ast$ relations are remarkably similar for both star-forming and quenched galaxies, with massive quenched galaxies having constantly lower HI masses of around 0.6 dex. This similarity strongly suggests that neither halo mass nor stellar mass is the direct cause of quenching, but rather the depletion of HI reservoir. While the HI reservoir for low-mass galaxies of $M_ast<10^{10.5}M_odot$ strongly increases with $M_{rm h}$, more massive galaxies show no significant dependence of $M_{rm HI}$ on $M_{rm h}$, indicating the effect of halo to determine the smooth cold gas accretion. We find that the star formation and quenching of central galaxies are directly regulated by the available HI reservoir, with an average relation of ${rm SFR}propto M_{rm HI}^{2.75}/M_ast^{0.40}$, implying a quasi-steady state of star formation. We further confirm that galaxies are depleted of their HI reservoir once they drop off the star-formation main sequence and there is a very tight and consistent correlation between $M_{rm HI}$ and $Sigma_1$ in this phase, with $M_{rm HI}proptoSigma_1^{-2}$. This result is in consistent with the compaction-triggered quenching scenario, with galaxies going through three evolutionary phases of cold gas accretion, compaction and post-compaction, and quenching.
Detecting galaxies when their star-formation is being quenched is crucial to understand the mechanisms driving their evolution. We identify for the first time a sample of quenching galaxies selected just after the interruption of their star formation by exploiting the [O III]5007/Halpha ratio and searching for galaxies with undetected [O III]. Using a sample of ~174000 star-forming galaxies extracted from the SDSS-DR8 at 0.04 < z < 0.21,we identify the ~300 quenching galaxy best candidates with low [O III]/Halpha, out of ~26000 galaxies without [O III] emission. They have masses between 10^9.7 and 10^10.8 Mo, consistently with the corresponding growth of the quiescent population at these redshifts. Their main properties (i.e. star-formation rate, colours and metallicities) are comparable to those of the star-forming population, coherently with the hypothesis of recent quenching, but preferably reside in higher-density environments.Most candidates have morphologies similar to star-forming galaxies, suggesting that no morphological transformation has occurred yet. From a survival analysis we find a low fraction of candidates (~0.58% of the star-forming population), leading to a short quenching timescale of tQ~50Myr and an e-folding time for the quenching history of tauQ~90Myr, and their upper limits of tQ<0.76 Gyr and tauQ<1.5Gyr, assuming as quenching galaxies 50% of objects without [O III] (~7.5%).Our results are compatible with a rapid quenching scenario of satellites galaxies due to the final phase of strangulation or ram-pressure stripping. This approach represents a robust alternative to methods used so far to select quenched galaxies (e.g. colours, specific star-formation rate, or post-starburst spectra).
High resolution 2D hydrodynamical simulations describing the evolution of the hot ISM in axisymmetric two-component models of early-type galaxies well reproduced the observed trends of the X-ray luminosity ($L_mathrm{x}$) and temperature ($T_mathrm{x}$) with galaxy shape and rotation, however they also revealed the formation of an exceedingly massive cooled gas disc in rotating systems. In a follow-up of this study, here we investigate the effects of star formation in the disc, including the consequent injection of mass, momentum and energy in the pre-existing interstellar medium. It is found that subsequent generations of stars originate one after the other in the equatorial region; the mean age of the new stars is $> 5$ Gyr, and the adopted recipe for star formation can reproduce the empirical Kennicutt-Schmidt relation. The results of the previous investigation without star formation, concerning $L_mathrm{x}$ and $T_mathrm{x}$ of the hot gas, and their trends with galactic shape and rotation, are confirmed. At the same time, the consumption of most of the cold gas disc into new stars leads to more realistic final systems, whose cold gas mass and star formation rate agree well with those observed in the local universe. In particular, our models could explain the observation of kinematically aligned gas in massive, fast-rotating early-type galaxies.
We study the star formation quenching mechanism in cluster galaxies by fitting the SED of the Herschel Reference Survey, a complete volume-limited K-band-selected sample of nearby galaxies including objects in different density regions, from the core of the Virgo cluster to the general field. The SED are fitted using the CIGALE SED modelling code. The truncated activity of cluster galaxies is parametrised using a specific SFH with 2 free parameters, the quenching age QA and the quenching factor QF. These 2 parameters are crucial for the identification of the quenching mechanism which acts on long timescales if starvation while rapid and efficient if ram pressure. To be sensitive to an abrupt and recent variation of the star formation activity, we combine in a new way 20 UV to FIR photometric bands with 3 age-sensitive Balmer line absorption indices extracted from available medium-resolution integrated spectroscopy and with Halpha narrow band imaging data. The use of a truncated SFH significantly increases the quality of the fit in those objects whose atomic gas content has been removed during the interaction with the hostile cluster environment. The typical QA of the perturbed late-type galaxies is QA < 300 Myr whenever the activity of star formation is reduced by 50% < QF <= 80% and QA < 500 Myr for QF > 80%, while that of the quiescent early-types is QA ~ 1-3 Gyr. The fraction of late-types with a star formation activity reduced by QF > 80% and with an HI-deficiency parameter HI-def > 0.4 drops by a factor of ~ 5 from the inner half virial radius of the Virgo cluster, where the hot diffuse X-ray emitting gas of the cluster is located, to the outer regions. The efficient quenching of the star formation activity observed in Virgo suggests that the dominant stripping process is ram pressure. We discuss the implication of this result in the cosmological context of galaxy evolution.
Assessments of the cold-gas reservoir in galaxies are a cornerstone for understanding star-formation processes and the role of feedback and baryonic cycling in galaxy evolution. Here we exploit a sample of 392 galaxies (dubbed MAGMA, Metallicity and Gas for Mass Assembly), presented in a recent paper, to quantify molecular and atomic gas properties across a broad range in stellar mass, Mstar, from $sim 10^7 - 10^{11}$ Msun. First, we find the metallicity ($Z$) dependence of alpha_CO to be shallower than previous estimates, with alpha_CO$propto (Z/Z_odot)^{-1.55}$. Second, molecular gas mass MH2 is found to be strongly correlated with Mstar and star-formation rate (SFR), enabling predictions of MH2 good to within $sim$0.2 dex. The behavior of atomic gas mass MHI in MAGMA scaling relations suggests that it may be a third, independent variable that encapsulates information about the circumgalactic environment and gas accretion. If Mgas is considered to depend on MHI, together with Mstar and SFR, we obtain a relation that predicts Mgas to within $sim$0.05 dex. Finally, the analysis of depletion times and the scaling of MHI/Mstar and MH2/Mstar over three different mass bins suggests that the partition of gas and the regulation of star formation through gas content depends on the mass regime. Dwarf galaxies tend to be overwhelmed by (HI) accretion, while for galaxies in the intermediate Mstar gas-equilibrium bin, star formation proceeds apace with gas availability. In the most massive gas-poor, bimodality galaxies, HI does not apparently participate in star formation, although it generally dominates in mass over H2. Our results confirm that atomic gas plays a key role in baryonic cycling, and is a fundamental ingredient for current and future star formation, especially in dwarf galaxies. (abridged for arXiv)
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