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Register a new userThe STAGES view of red spirals and dusty red galaxies: Mass-dependent quenching of star-formation in cluster infall
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We investigate the properties of optically passive spirals and dusty red galaxies in the A901/2 cluster complex at redshift ~0.17 using restframe near-UV-optical SEDs, 24 micron IR data and HST morphologies from the STAGES dataset. The cluster sample is based on COMBO-17 redshifts with an rms precision of sigma_cz~2000 km/sec. We find that dusty red galaxies and optically passive spirals in A901/2 are largely the same phenomenon, and that they form stars at a substantial rate, which is only 4x lower than that in blue spirals at fixed mass. This star formation is more obscured than in blue galaxies and its optical signatures are weak. They appear predominantly in the stellar mass range of log M*/Msol=[10,11] where they constitute over half of the star-forming galaxies in the cluster; they are thus a vital ingredient for understanding the overall picture of star formation quenching in clusters. We find that the mean specific SFR of star-forming galaxies in the cluster is clearly lower than in the field, in contrast to the specific SFR properties of blue galaxies alone, which appear similar in cluster and field. Such a rich red spiral population is best explained if quenching is a slow process and morphological transformation is delayed even more. At log M*/Msol<10, such galaxies are rare, suggesting that their quenching is fast and accompanied by morphological change. We note, that edge-on spirals play a minor role; despite being dust-reddened they form only a small fraction of spirals independent of environment.
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We point out a natural mechanism for quenching of star formation in early-type galaxies. It automatically links the color of a galaxy with its morphology and does not require gas consumption, removal or termination of gas supply. Given that star formation takes place in gravitationally unstable gas disks, it can be quenched when a disk becomes stable against fragmentation to bound clumps. This can result from the growth of a stellar spheroid, for instance by mergers. We present the concept of morphological quenching (MQ) using standard disk instability analysis, and demonstrate its natural occurrence in a cosmological simulation using an efficient zoom-in technique. We show that the transition from a stellar disk to a spheroid can be sufficient to stabilize the gas disk, quench star formation, and turn an early-type galaxy red and dead while gas accretion continues. The turbulence necessary for disk stability can be stirred up by sheared perturbations within the disk in the absence of bound star-forming clumps. While gas stripping processes are limited to dense groups and clusters, and other quenching mechanisms like AGN feedback, virial shock heating and gravitational heating, are limited to halos more massive than 10^12 Mo, the MQ can explain the appearance of red ellipticals even in less massive halos and in the field. The dense gas disks observed in some of todays red ellipticals may be the relics of this mechanism, whereas red galaxies with quenched gas disks are expected to be more frequent at high redshift.
We introduce a simple model to explore the star formation histories of disk galaxies. We assume that the disk origins and grows by continuous gas infall. The gas infall rate is parametrized by the Gaussian formula with one free parameter: infall-peak time $t_p$. The Kennicutt star formation law is adopted to describe how much cold gas turns into stars. The gas outflow process is also considered in our model. We find that, at given galactic stellar mass $M_*$, model adopting late infall-peak time $t_p$ results in blue colors, low metallicity, high specific star formation rate and high gas fraction, while gas outflow rate mainly influences the gas-phase metallicity and star formation efficiency mainly influences the gas fraction. Motivated by the local observed scaling relations, we construct a mass-dependent model by assuming low mass galaxy has later infall-peak time $t_p$ and larger gas outflow rate than massive systems. It is shown that this model can be in agreement with not only the local observations, but also the observed correlations between specific star formation rate and galactic stellar mass $SFR/M_* sim M_*$ at intermediate redshift $z<1$. Comparison between the Gaussian-infall model and exponential-infall model is also presented. It shows that the exponential-infall model predicts higher star formation rate at early stage and lower star formation rate later than that of Gaussian-infall. Our results suggest that the Gaussian infall rate may be more reasonable to describe the gas cooling process than the exponential infall rate, especially for low-mass systems.
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.
Galaxies migrate from the blue cloud to the red sequence when their star formation is quenched. Here, we report on galaxies quenched by environmental effects and not by mergers or strong AGN as often invoked: They form stars at a reduced rate which is optically even less conspicuous, and manifest a transition population of blue spirals evolving into S0 galaxies. These optically passive or red spirals are found in large numbers in the STAGES project (and by Galaxy Zoo) in the infall region of clusters and groups.
We present an analysis of the mid-infrared (MIR) colours of 165 70um-detected galaxies in the Shapley supercluster core (SSC) at z=0.048 using panoramic Spitzer/MIPS 24 and 70um imaging. While the bulk of galaxies show f70/f24 colours typical of local star-forming galaxies, we identify a significant sub-population of 23 70micron-excess galaxies, whose MIR colours (f70/f24>25) are much redder and cannot be reproduced by any of the standard model infrared SEDs. These galaxies are found to be strongly concentrated towards the cores of the five clusters that make up the SSC, and also appear rare among local field galaxies, confirming them as a cluster-specific phenomenon. Their optical spectra and lack of significant UV emission imply little or no ongoing star formation, while fits to their panchromatic SEDs require the far-IR emission to come mostly from a diffuse dust component heated by the general interstellar radiation field rather than ongoing star formation. Most of these 70micron-excess galaxies are identified as ~L* S0s with smooth profiles. We find that almost every cluster galaxy in the process of star-formation quenching is already either an S0 or Sa, while we find no passive galaxies of class Sb or later. Hence the formation of passive early-type galaxies in cluster cores must involve the prior morphological transformation of late-type spirals into Sa/S0s, perhaps via pre-processing or the impact of cluster tidal fields, before a subsequent quenching of star formation once the lenticular encounters the dense environment of the cluster core. In the cases of many cluster S0s, this phase of star-formation quenching is characterised by an excess of 70um emission, indicating that the cold dust content is declining at a slower rate than star formation.
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