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Register a new userEnvironmental impacts on molecular gas in protocluster galaxies at z~2
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We present the results from ALMA CO(3-2) observations of 66 Halpha-selected galaxies in three protoclusters around radio galaxies, PKS1138-262 (z=2.16) and USS1558-003 (z=2.53), and 4C23.56 (z=2.49). The pointing areas have an overdensity of ~100 compared to a mean surface number density of galaxies in field environments. We detect CO emission line in 16 star-forming galaxies, including previously published six galaxies, to measure the molecular gas mass. In the stellar mass range of 10.5<log(Mstar/Msolar)<11.0, the protocluster galaxies have larger gas mass fractions and longer gas depletion timescales compared to the scaling relations established by field galaxies. On the other hand, the amounts of molecular gas in more massive galaxies with log(Mstar/Msolar)>11.0 are comparable in mass to the scaling relation, or smaller. Our results suggest that the environmental effects on gas properties are mass-dependent: in high-density environments, gas accretion through cosmic filaments is accelerated in less massive galaxies while this is suppressed in the most massive system.
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We have used the Atacama Large Millimeter/submillimeter Array (ALMA) to carry out a search for CO (3$-$2) or (4$-$3) emission from the fields of 12 high-metallicity ([M/H]~$geq -0.72$,dex) damped Lyman-$alpha$ absorbers (DLAs) at $z approx 1.7-2.6$. We detected CO emission from galaxies in the fields of five DLAs (two of which have been reported earlier), obtaining high molecular gas masses, $rm M_{mol} approx (1.3 - 20.7) times (alpha_{rm CO}/4.36) times 10^{10} ; M_odot$. The impact parameters of the CO emitters to the QSO sightline lie in the range $b approx 5.6-100$~kpc, with the three new CO detections having $b lesssim 15$~kpc. The highest CO line luminosities and inferred molecular gas masses are associated with the highest-metallicity DLAs, with [M/H]~$gtrsim -0.3$,dex. The high inferred molecular gas masses may be explained by a combination of a stellar mass-metallicity relation and a high molecular gas-to-stars mass ratio in high-redshift galaxies; the DLA galaxies identified by our CO searches have properties consistent with those of emission-selected samples. None of the DLA galaxies detected in CO emission were identified in earlier optical or near-IR searches and vice-versa; DLA galaxies earlier identified in optical/near-IR searches were not detected in CO emission. The high ALMA CO and C[{sc ii}]~158$mu$m detection rate in high-$z$, high-metallicity DLA galaxies has revolutionized the field, allowing the identification of dusty, massive galaxies associated with high-$z$ DLAs. The H{sc i}-absorption criterion identifying DLAs selects the entire high-$z$ galaxy population, including dusty and UV-bright galaxies, in a wide range of environments.
Based on ALMA Band 3 observations of the CO(2-1) line transition, we report the discovery of three new gas-rich (M_H2 ~ 1.5-4.8 x 10^10 M_sun, SFRs in the range ~5-100 M_sun/yr) galaxies in an overdense region at z=1.7, that already contains eight spectroscopically confirmed members. This leads to a total of 11 confirmed overdensity members, within a projected distance of ~ 1.15 Mpc and in a redshift range of Dz = 0.012. Under simple assumptions, we estimate that the system has a total mass of >= 3-6 x 10^13 M_sun, and show that it will likely evolve into a >~ 10^14 M_sun cluster at z = 0. The overdensity includes a powerful Compton-thick Fanaroff-Riley type II (FRII) radio-galaxy, around which we discovered a large molecular gas reservoir (M_H2 ~ 2 x 10^11 M_sun). We fitted the FRII resolved CO emission with a 2-D Gaussian model with major (minor) axis of ~ 27 (~ 17) kpc, that is a factor of ~3 larger than the optical rest-frame emission. Under the assumption of a simple edge-on disk morphology, we find that the galaxy interstellar medium produces a column density towards the nucleus of ~ 5.5 x 10^23 cm^-2. Such a dense ISM may then contribute significantly to the total nuclear obscuration measured in the X-rays (N_(H,X) ~ 1.5 x 10^24 cm^-2) in addition to a small, pc-scale absorber around the central engine. The velocity map of this source unveils a rotational motion of the gas that is perpendicular to the radio-jets. The FRII is located at the center of the projected spatial distribution of the structure members, and its velocity offset from the peak of the redshift distribution is well within the structures velocity dispersion. All this, coupled with the large amount of gas around the FRII, its stellar mass of ~ 3 x 10^11 M_sun, SFR of ~ 200-600 M_sun/yr, and powerful radio-to-X-ray emission, suggests that this source is the likely progenitor of the future brightest cluster galaxy.
We study the mass-metallicity relation for 19 members of a spectroscopically-confirmed protocluster in the COSMOS field at $z=2.2$ (CC2.2), and compare it with that of 24 similarly selected field galaxies at the same redshift. Both samples are $rm Halpha$ emitting sources, chosen from the HiZELS narrow-band survey, with metallicities derived from $rm N2 (frac{rm [NII] lambda 6584}{rm H alpha})$ line ratio. For the mass-matched samples of protocluster and field galaxies, we find that protocluster galaxies with $10^{9.9} rm M_odot leq M_* leq 10^{10.9} rm M_odot$ are metal deficient by $0.10 pm 0.04$ dex ($2.5sigma$ significance) compared to their coeval field galaxies. This metal deficiency is absent for low mass galaxies, $rm M_* < 10^{9.9} rm M_odot$. Moreover, relying on both SED-derived and $rm {Halpha}$ (corrected for dust extinction based on $rm {M_*}$) SFRs, we find no strong environmental dependence of SFR-$rm {M_*}$ relation, however, we are not able to rule out the existence of small dependence due to inherent uncertainties in both SFR estimators. The existence of $2.5sigma$ significant metal deficiency for massive protocluster galaxies favors a model in which funneling of the primordial cold gas through filaments dilutes the metal content of protoclusters at high redshifts ($z gtrsim 2$). At these redshifts, gas reservoirs in filaments are dense enough to cool down rapidly and fall into the potential well of the protocluster to lower the gas-phase metallicity of galaxies. Moreover, part of this metal deficiency could be originated from galaxy interactions which are more prevalent in dense environments.
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 $z=1.2$, i.e., $sim2$ 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 reported clear evidence of AGN. We exploit observations taken with the NOEMA interferometer at $sim3$ mm to detect CO(2-1) line emission from the two selected galaxies, unresolved by our observations. Based on the CO(2-1) spectrum we estimate a total molecular gas mass $M({rm H_2})=(2.2^{+0.5}_{-0.4})times10^{10}$ $M_odot$ and dust mass $M_{rm dust}<4.2times10^8~M_odot$ for the two blended sources. The two galaxies have similar stellar masses and a large relative velocity of $sim$400 km/s estimated from the CO(2-1) line width. These findings tend to privilege a scenario where both sources contribute to the observed CO(2-1). By using the archival Spitzer MIPS flux at 24$mu$m we estimate an ${rm SFR(24mu m)}=(28^{+12}_{-8})~M_odot$/yr for each of the two galaxies. Assuming that the two sources equally contribute to the observe CO(2-1) our analysis yields a depletion time scale $tau_{rm dep}=(3.9^{+1.4}_{-1.8})times10^8$ yr, and a molecular gas to stellar mass ratio $0.17pm0.13$ for each of two sources, separately. Our results are in overall agreement with those of other distant cluster galaxies. The two target galaxies have molecular gas mass and depletion time that are marginally compatible with, but smaller than those of main sequence field galaxies, suggesting that the molecular gas has not been refueled enough. Higher resolution and higher frequency observations will enable us to spatially resolve the two sources and possibly distinguish between different gas processing mechanisms.
Using the Australia Telescope Compact Array (ATCA), we conducted a survey of CO J=1-0 and J=2-1 line emission towards strongly lensed high-redshift dusty star forming galaxies (DSFGs) previously discovered with the South Pole Telescope (SPT). Our sample comprises 17 sources that had CO-based spectroscopic redshifts obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX). We detect all sources with known redshifts in either CO J=1-0 or J=2-1. Twelve sources are detected in the 7-mm continuum. The derived CO luminosities imply gas masses in the range (0.5-11)x10^{10} M_sun and gas depletion timescales <200 Myr, using a CO to gas mass conversion factor alpha_CO=0.8 M_sun (K km/s pc^2)^{-1}. Combining the CO luminosities and dust masses, along with a fixed gas-to-dust ratio, we derive alpha_CO factors in the range 0.4-1.8, similar to what is found in other starbursting systems. We find small scatter in alpha_CO values within the sample, even though inherent variations in the spatial distribution of dust and gas in individual cases could bias the dust-based alpha_CO estimates. We find that lensing magnification factors based on the CO linewidth to luminosity relation (mu_CO) are highly unreliable, but particularly when mu<5. Finally, comparison of the gas and dynamical masses suggest that the average molecular gas fraction stays relatively constant at z=2-5 in the SPT DSFG sample.
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