Molecular gas in distant brightest cluster galaxies

The mechanisms governing the stellar mass assembly and star formation history of brightest cluster galaxies (BCGs) are still being debated. By means of new and archival molecular gas observations we investigate the role of dense megaparsec-scale environments in regulating the fueling of star formation in distant BCGs, through cosmic time. We observed in CO with the IRAM 30 m telescope two star-forming BCGs belonging to SpARCS clusters, namely, 3C 244.1 (z=0.4) and SDSS J161112.65+550823.5 (z=0.9), and compared their molecular gas and star formation properties with those of a compilation of similar to 100 distant cluster galaxies from the literature, including nine additional distant BCGs at z similar to 0.4-3.5. We set robust upper limits of M-H2< 1.0x10(10) M-circle dot and < 2.8x10(10) M-circle dot to their molecular gas content, respectively, and to the ratio of molecular gas to stellar mass M(H-2)/M-star less than or similar to 0.2 and depletion time tau(dep)less than or similar to 40 Myr of the two targeted BCGs. They are thus among the distant cluster galaxies with the lowest gas fractions and shortest depletion times. The majority (64%+/- 15% and 73%+/- 18%) of the 11 BCGs with observations in CO have lower M(H-2)/M-star values and tau(dep), respectively, than those estimated for main sequence galaxies. Statistical analysis also tentatively suggests that the values of M(H-2)/M-star and tau(dep) for the 11 BCGs deviates, with a significance of similar to 2 sigma, from those of the comparison sample of cluster galaxies. A morphological analysis for a subsample of seven BCGs with archival HST observations reveals that 71%+/- 17% of the BCGs are compact or show star-forming components or substructures. Our results suggest a scenario where distant star-forming BCGs assemble a significant fraction similar to 16% of their stellar mass on the relatively short timescale similar to tau(dep), while environmental mechanisms might prevent the replenishment of gas feeding the star formation. We speculate that compact components also favor the rapid exhaustion of molecular gas and ultimately help to quench the BCGs. Distant star-forming BCGs are excellent targets for ALMA and for next-generation telescopes such as the James Webb Space Telescope.

Molecular gas in radio galaxies in dense megaparsec-scale environments at z=0.4-2.6

Gianluca Castignani

Context. Low luminosity radio galaxies (LLRGs) typically reside in dense megaparsec-scale environments and are often associated with brightest cluster galaxies (BCGs). They are an excellent tool to study the evolution of molecular gas reservoirs in giant ellipticals, even close to the active galactic nucleus. Aims. We investigate the role of dense megaparsec-scale environment in processing molecular gas in LLRGs in the cores of galaxy (proto-)clusters. To this aim we selected within the COSMOS and DES surveys a sample of five LLRGs at z = 0.4-2.6 that show evidence of ongoing star formation on the basis of their far-infrared (FIR) emission. Methods. We assembled and modeled the FIR-to-UV spectral energy distributions (SEDs) of the five radio sources to characterize their host galaxies in terms of stellar mass and star formation rate. We observed the LLRGs with the IRAM-30m telescope to search for CO emission. We then searched for dense megaparsec-scale overdensities associated with the LLRGs using photometric redshifts of galaxies and the Poisson Probability Method, which we have upgraded using an approach based on the wavelet-transform (wPPM), to ultimately characterize the overdensity in the projected space and estimate the radio galaxy miscentering. Color-color and colormagnitude plots were then derived for the fiducial cluster members, selected using photometric redshifts. Results. Our IRAM-30m observations yielded upper limits to the CO emission of the LLRGs, at z = 0 : 39, 0.61, 0.91, 0.97, and 2.6. For the most distant radio source, COSMOS-FRI 70 at z = 2 : 6, a hint of CO(7 -> 6) emission is found at 2.2 sigma. The upper limits found for the molecular gas content M(H-2) = M-star< 0 : 11, 0.09, 1.8, 1.5, and 0.29, respectively, and depletion time tau(dep) less than or similar to (0.2-7) Gyr of the five LLRGs are overall consistent with the corresponding values of main sequence field galaxies. Our SED modeling implies large stellar-mass estimates in the range log(M-star/M-Theta) = 10.9-11.5, typical for giant ellipticals. Both our wPPM analysis and the cross-matching of the LLRGs with existing cluster/group catalogs suggest that the megaparsec-scale overdensities around our LLRGs are rich (less than or similar to 10(14) M-Theta) groups and show a complex morphology. The color-color and color-magnitude plots suggest that the LLRGs are consistent with being star forming and on the high-luminosity tail of the red sequence. The present study thus increases the still limited statistics of distant cluster core galaxies with CO observations. Conclusions. The radio galaxies of this work are excellent targets for ALMA as well as next-generation telescopes such as the James Webb Space Telescope.

EDP SCIENCES S A2019

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

LESS THAN 10 PERCENT OF STAR FORMATION IN z similar to 0.6 MASSIVE GALAXIES IS TRIGGERED BY MAJOR INTERACTIONS

Both observations and simulations show that major tidal interactions or mergers between gas-rich galaxies can lead to intense bursts of star formation. Yet, the average enhancement in star formation rate (SFR) in major mergers and the contribution of such events to the cosmic SFR are not well estimated. Here we use photometric redshifts, stellar masses, and UV SFRs from COMBO-17, 24 mu m SFRs from Spitzer, and morphologies from two deep Hubble Space Telescope (HST) cosmological survey fields (ECDFS/GEMS and A901/STAGES) to study the enhancement in SFR as a function of projected galaxy separation. We apply two-point projected correlation function techniques, which we augment with morphologically selected very close pairs (separation = 10(10) M-circle dot) star-forming galaxies at 0.4 < z < 0.8, we find that the SFRs of galaxies undergoing a major interaction (mass ratios