Virgo filaments I. Processing of gas in cosmological filaments around the Virgo cluster

It is now well established that galaxies have different morphologies, gas contents, and star formation rates (SFR) in dense environments like galaxy clusters. The impact of environmental density extends to several virial radii, and galaxies appear to be pre-processed in filaments and groups before falling into the cluster. Our goal is to quantify this pre-processing in terms of gas content and SFR, as a function of density in cosmic filaments. We have observed the two first CO transitions in 163 galaxies with the IRAM-30 m telescope, and added 82 more measurements from the literature, thus forming a sample of 245 galaxies in the filaments around the Virgo cluster. We gathered HI-21cm measurements from the literature and observed 69 galaxies with the Nancay telescope to complete our sample. We compare our filament galaxies with comparable samples from the Virgo cluster and with the isolated galaxies of the AMIGA sample. We find a clear progression from field galaxies to filament and cluster galaxies for decreasing SFR, increasing fraction of galaxies in the quenching phase, an increasing proportion of early-type galaxies, and decreasing gas content. Galaxies in the quenching phase, defined as having a SFR below one-third of that of the main sequence (MS), are only between 0% and 20% in the isolated sample, according to local galaxy density, while they are 20%-60% in the filaments and 30%-80% in the Virgo cluster. Processes that lead to star formation quenching are already at play in filaments; they depend mostly on the local galaxy density, while the distance to the filament spine is a secondary parameter. While the HI-to-stellar-mass ratio decreases with local density by an order of magnitude in the filaments, and two orders of magnitude in the Virgo cluster with respect to the field, the decrease is much less for the H-2-to-stellar-mass ratio. As the environmental density increases, the gas depletion time decreases, because the gas content decreases faster than the SFR. This suggests that gas depletion precedes star formation quenching.

Molecular gas in two companion cluster galaxies at z=1.2

Gianluca Castignani, Anand Stéphane Raichoor

Context. Probing both star formation history and evolution of distant cluster galaxies is essential to evaluate the effect of dense environment on shaping the galaxy properties we observe today. Aims. We investigate the effect of cluster environment on the processing of the molecular gas in distant cluster galaxies. We study the molecular gas properties of two star-forming galaxies separated by 6 kpc in the projected space and belonging to a galaxy cluster selected from the frac Shallow Cluster Survey, at a redshift z = 1.2, that is,similar to 2 Gyr after the cosmic star formation density peak. This work describes the first CO detection from 1 < z < 1.4 star-forming cluster galaxies with no clear reported evidence of active galactic nuclei. Methods. We exploit observations taken with the NOEMA interferometer at similar to 3 mm to detect CO(2-1) line emission from the two selected galaxies, unresolved by our observations. Results. Based on the CO(2-1) spectrum, we estimate a total molecular gas mass M(H-2) = (2.2(-0.4)(+0.5)) x 10(10) M-circle dot, where fully excited gas is assumed, and a dust mass M-dust < 4.2 x 10(8) M(circle dot )for the two blended sources. The two galaxies have similar stellar masses and H alpha-based star formation rates (SFRs) found in previous work, as well as a large relative velocity of similar to 400 km s(-1) estimated from the CO(2-1) line width. These findings tend to privilege a scenario where both sources contribute to the observed CO(2-1). Using the archival Spitzer MIPS flux at 24 mu m we estimate an SFR (24 mu m) = (24(-8)(+12)) M-circle dot/yr for each of the two galaxies. Assuming that the two sources contribute equally to the observed CO(2-1), our analysis yields a depletion timescale of tau(dep) = (3.9(-1.8)(+1.4)) x 10(8) yr, and a molecular gas to stellar mass ratio of 0.17 +/- 0.13 for each of two sources, separately. We also provide a new, more precise measurement of an unknown weighted mean of the redshifts of the two galaxies, z = 1.163 +/- 0.001. Conclusions. Our results are in overall agreement with those of other distant cluster galaxies and with model predictions for main sequence (MS) field galaxies at similar redshifts. The two target galaxies have molecular gas mass and depletion times that are marginally compatible with, but smaller than those of MS field galaxies, suggesting that the molecular gas has not been sufficiently refueled. We speculate that the cluster environment might have played a role in preventing the refueling via environmental mechanisms such as galaxy harassment, strangulation, ram-pressure, or tidal stripping. Higher-resolution and higher-frequency observations will enable us to spatially resolve the two sources and possibly distinguish between different gas processing mechanisms.

EDP SCIENCES S A2018

Molecular gas and star formation activity in luminous infrared galaxies in clusters at intermediate redshifts

Gianluca Castignani, Pascale Jablonka, Mathilde Jauzac, Damien Louis Alexandre Spérone-Longin

We investigate the role of dense megaparsec-scale environments in processing molecular gas of cluster galaxies as they fall into the cluster cores. We selected a sample of similar to 20 luminous infrared galaxies (LIRGs) belonging to intermediate-redshift clusters, mainly from the Herschel Lensing Survey and the Local Cluster Substructure Survey. These galaxies include MACS J0717.5+3745 at z=0.546 and Abell 697, 963, 1763, and 2219 at z=0.2-0.3. We performed spectral energy distribution modeling from the far-infrared to ultraviolet of the LIRGs, which span cluster-centric distances within r/r(200)similar or equal to 0.2-1.6. We observed the LIRGs in CO(1 -> 0) or CO(2 -> 1) with the Plateau de Bure interferometer and its successor NOEMA, as part of five observational programs carried out between 2012 and 2017. We compared the molecular gas to stellar mass ratio M(H-2)/M-star, star formation rate (SFR), and depletion time (tau (dep)) of the LIRGs with those of a compilation of cluster and field star-forming galaxies from the literature. The targeted LIRGs have SFR, M(H-2)/M-star, and tau (dep) that are consistent with those of both main-sequence (MS) field galaxies and star-forming galaxies from the comparison sample. However we find that the depletion time, normalized to the MS value, tentatively increases with increasing r/r(200), with a significance of 2.8 sigma, which is ultimately due to a deficit of cluster-core LIRGs with tau (dep)greater than or similar to tau (dep, MS). We suggest that a rapid exhaustion of the molecular gas reservoirs occurs in the cluster LIRGs and is indeed effective in suppressing their star formation and ultimately quenching them. This mechanism may explain the exponential decrease of the fraction of cluster LIRGs with cosmic time. The compression of the gas in LIRGs, possibly induced by intra-cluster medium shocks, may be responsible for the short timescales that are observed in a large fraction of cluster-core LIRGs. Some of our LIRGs may also belong to a population of infalling filament galaxies.

EDP SCIENCES S A2020

The colour-magnitude relation of elliptical and lenticular galaxies in the ESO Distant Cluster Survey

Pascale Jablonka

In this paper we study the colour-magnitude relation (CMR) for a sample of 172 morphologically classified elliptical and S0 cluster galaxies from the ESO Distant Cluster Survey (EDisCS) at 0.4 less than or similar to z less than or similar to 0.8. The intrinsic colour scatter about the CMR is very small ( = 0.076) in rest-frame U - V. However, there is a small minority of faint early-type galaxies (7 per cent) that are significantly bluer than the CMR. We observe no significant dependence of Sigma(int) with redshift or cluster velocity dispersion. Because our sample is strictly morphologically selected, this implies that by the time cluster elliptical and S0 galaxies achieve their morphology, the vast majority have already joined the red sequence. The only exception seems to be the very small fraction of faint blue early types. Assuming that the intrinsic colour scatter is due to differences in stellar population ages, we estimate the galaxy formation redshift z(F) of each cluster and find that z(F) does not depend on the cluster velocity dispersion. However, z(F) increases weakly with cluster redshift within the EDisCS sample. This trend becomes very clear when higher redshift clusters from the literature are included. This suggests that, at any given redshift, in order to have a population of fully formed ellipticals and S0s they needed to have formed most of their stars similar or equal to 2-4 Gyr prior to observation. That does not mean that all early-type galaxies in all clusters formed at these high redshifts. It means that the ones we see already having early-type morphologies also have reasonably old stellar populations. This is partly a manifestation of the 'progenitor bias', but also a consequence of the fact that the vast majority of the early-type galaxies in clusters (in particular the massive galaxies) were already red (i.e. already had old stellar populations) by the time they achieved their morphology. Elliptical and S0 galaxies exhibit very similar colour scatter, implying similar stellar population ages. The scarcity of blue S0s indicates that, if they are the descendants of spirals whose star formation has ceased, the parent galaxies were already red when they became S0s. This suggests the red spirals found preferentially in dense environments could be the progenitors of these S0s. We also find that fainter early-type galaxies finished forming their stars later (i.e. have smaller z(F)), consistent with the cluster red sequence being built over time and the brightest galaxies reaching the red sequence earlier than fainter ones. Combining the CMR scatter analysis with the observed evolution in the CMR zero-point we find that the early-type cluster galaxy population must have had their star formation truncated/stopped over an extended period delta t greater than or similar to 1 Gyr.

Blackwell Publishing Ltd, Oxford2011