Molecular gas in two companion cluster galaxies at z = 1.2

Abstract

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 Irac Shallow Cluster Survey, at a redshift z = 1.2, that is, ∼ 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 ∼ 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 +0.5 −0.4 ) × 10 10 M , where fully excited gas is assumed, and a dust mass M dust < 4.2 × 10 8 M for the two blended sources. The two galaxies have similar stellar masses and Hα-based star formation rates (SFRs) found in previous work, as well as a large relative velocity of ∼400 km/s 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 µm we estimate an SFR(24µm) = (28 +12 −8 ) M /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 τ dep = (3.9 +1.4 −1.8 ) × 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.

Using ALMA to resolve the nature of the early star-forming large-scale structure PLCK G073.4−57.5

“…In addition, a comparison with galaxies in the field, in clusters, and in other proto-clusters can provide insights into the role played by the environment. To this end, we collected CO data from the literature for galaxies in clusters (62 galaxies in 11 clusters, Wang et al 2018;Rudnick et al 2017;Stach et al 2017;Aravena et al 2012;Casasola et al 2013;Castignani et al 2018;Coogan et al 2018;Hayashi et al 2017;Webb et al 2017) and in proto-clusters (16 galaxies in four proto-clusters, Dannerbauer et al 2017;Ivison et al 2013;Tadaki et al 2014;Lee et al 2017), and of SMGs and AGN in the z = 1-3 redshift range (31 SMGs, 15 AGN, and 38 obscured AGN, Perna et al 2018). In order to represent normal star-forming galaxies and SB galaxies, we used the SFR-M rela-tion 5 from Speagle et al (2014), and the L CO − L IR relation from MS and SB galaxies as derived by Sargent et al (2014) 6 For estimating molecular gas masses, we converted all high transition CO luminosities to L CO(1−0) , when not available, using the median brightness temperature ratios derived by Bothwell et al (2013).…”

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

“…We have assumed α CO = 4.36 M (K km s −1 pc 2 ) −1 , in agreement with the value adopted in this work, while for each galaxy we have assumed the excitation level adopted in the corresponding work. Similarly to Castignani et al (2018) we stress that by assuming the same α CO we aim at having comparable molecular gas mass estimates for the galaxies considered. According to our SFR< 6 SFR MS criterion the majority of the sources lie around the MS, which justifies the choice for α CO .…”

“…This work is the second reporting the results of a wider search for molecular gas in distant cluster galaxies (see also Castignani et al 2018). The paper is structured as follows.…”

Massive molecular gas reservoir around the central AGN in the CARLA J1103 + 3449 cluster at z = 1.44

“…Current observations of the CO emission line in clusters at these epochs show that cluster galaxies still have cold gas to fuel their star formation. However, these results are not yet statistically significant, and some results point toward higher molecular gas content in cluster galaxies with respect to the field and others to lower (Casasola et al 2013;Rudnick et al 2017;Noble et al 2017;Hayashi et al 2018;Coogan et al 2018;Castignani et al 2018). Molecular gas has also been detected in two protoclusters at z ∼ 2.5 (Chapman et al 2015;Wang et al 2016b).…”

“…At higher redshifts, the results are somewhat conflicting, as it also becomes more difficult to Lee et al 2017). Finally, at z < 1.5, for galaxies of the same mass and at the same redshift, cluster galaxies show lower amounts of molecular gas and thus, lower gas fractions (Jablonka et al 2013;Rudnick et al 2017;Lee et al 2017;Castignani et al 2018;Hayashi et al 2018). Some works have shown that at higher redshifts (z > 2), there is no difference in the gas fraction of cluster and field galaxies (Husband et al 2016;Dannerbauer et al 2017).…”