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Register a new userA CO Survey of SpARCS Star-Forming Brightest Cluster Galaxies: Evidence for Uniformity in BCG Molecular Gas Processing Across Cosmic Time
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We present ALMA CO (2-1) detections of 24 star-forming Brightest Cluster Galaxies (BCGs) over $0.2<z<1.2$, constituting the largest and most distant sample of molecular gas measurements in BCGs to date. The BCGs are selected from the Spitzer Adaptation of the Red-Sequence Cluster Survey (SpARCS) to be IR-bright and therefore star-forming. We find that molecular gas is common in star-forming BCGs, detecting CO at a detection rate of 80% in our target sample of 30 objects. We additionally provide measurements of the star formation rate (SFR) and stellar mass, calculated from existing MIPS 24 $mu$m and IRAC 3.6 $mu$m fluxes, respectively. We find these galaxies have molecular gas masses of $0.7-11.0times 10^{10} mathrm{M}_odot$, comparable to other BCGs in this redshift range, and specific star formation rates which trace the Elbaz et al. (2011) Main Sequence. We compare our BCGs to those of the lower-redshift, cooling-flow BCG sample assembled by Edge (2001) and find that at z $lesssim 0.6$ the two samples show very similar correlations between their gas masses and specific SFRs. We suggest that, in this redshift regime, the $sim10%$ (Webb et al., 2015) of BCGs that are star-forming process any accreted molecular gas into stars through means that are agnostic to both their redshift and their cluster mass.
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Brightest cluster galaxies (BCGs) are excellent laboratories to study galaxy evolution in dense Mpc-scale environments. We have observed in CO(1-0), CO(2-1), CO(3-2), or CO(4-3), with the IRAM-30m, 18 BCGs at $zsim0.2-0.9$ that are drawn from the CLASH survey. Our sample includes RX1532, which is our primary target, being among the BCGs with the highest star formation rate (SFR$gtrsim100~M_odot$/yr) in the CLASH sample. We unambiguously detected both CO(1-0) and CO(3-2) in RX1532, yielding a large reservoir of molecular gas, $M_{H_2}=(8.7pm1.1)times10^{10}~M_odot$, and a high level of excitation $r_{31}=0.75pm0.12$. A morphological analysis of the HST I-band image of RX1532 reveals the presence of clumpy substructures both within and outside the half-light radius $r_e=(11.6pm0.3)$ kpc, similarly to those found independently both in ultraviolet and in H$_alpha$ in previous work. We tentatively detected CO(1-0) or CO(2-1) in four other BCGs, with molecular gas reservoirs in the range $M_{H_2}=2times10^{10-11} M_odot$. For the remaining 13 BCGs we set robust upper limits of $M_{H_2}/M_starlesssim0.1$, which are among the lowest molecular gas to stellar mass ratios found for distant ellipticals and BCGs. By comparison with distant cluster galaxies observed in CO our study shows that RX1532 ($M_{H_2}/M_star = 0.40pm0.05$) belongs to the rare population of star forming and gas-rich BCGs in the distant universe. By using available X-ray based estimates of the central intra-cluster medium entropy, we show that the detection of large reservoirs of molecular gas $M_{H_2}gtrsim10^{10}~M_odot$ in distant BCGs is possible when the two conditions are met: i) high SFR and ii) low central entropy, which favors the condensation and the inflow of gas onto the BCGs themselves, similarly to what has been previously found for some local BCGs.
We present the results of a Spitzer/Herschel infrared photometric analysis of the largest (716) and highest-redshift (z=1.8) sample of Brightest Cluster Galaxies (BCGs), those from the Spitzer Adaptation of the Red-Sequence Cluster Survey (SpARCS). Given the tension that exists between model predictions and recent observations of BCGs at z<2, we aim to uncover the dominant physical mechanism(s) guiding the stellar-mass buildup of this special class of galaxies, the most massive in the Universe uniquely residing at the centres of galaxy clusters. Through a comparison of their stacked, broadband, infrared spectral energy distributions (SEDs) to a variety of SED model templates in the literature, we identify the major sources of their infrared energy output, in multiple redshift bins between 0 < z < 1.8. We derive estimates of various BCG physical parameters from the stacked { u}L{ u} SEDs, from which we infer a star-forming, as opposed to a red and dead population of galaxies, producing tens to hundreds of solar masses per year down to z=0.5. This discovery challenges the accepted belief that BCGs should only passively evolve through a series of gas-poor, minor mergers since z~4 (De Lucia & Blaizot 2007), but agrees with the improved semi-analytic model of hierarchical structure formation of Tonini et al. (2012), which predicts star-forming BCGs throughout the epoch considered. We attribute the star formation inferred from the stacked infrared SEDs to both major and minor wet (gas-rich) mergers, based on a lack of key signatures (to date) of the cluster cooling flows to which BCG star formation is typically attributed, as well as a number of observational and simulation-based studies that support this scenario.
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.
We present a multiwavelength morphological analysis of star forming clouds and filaments in the central ($< 50$ kpc) regions of 16 low redshift ($z<0.3$) cool core brightest cluster galaxies (BCGs). New Hubble Space Telescope (HST) imaging of far ultraviolet continuum emission from young ($sim 10$ Myr), massive ($> 5$ Msol) stars reveals filamentary and clumpy morphologies, which we quantify by means of structural indices. The FUV data are compared with X-ray, Ly$alpha$, narrowband H$alpha$, broadband optical/IR, and radio maps, providing a high spatial resolution atlas of star formation locales relative to the ambient hot ($sim10^{7-8}$ K) and warm ionised ($sim 10^4$ K) gas phases, as well as the old stellar population and radio-bright AGN outflows. Nearly half of the sample possesses kpc-scale filaments that, in projection, extend toward and around radio lobes and/or X-ray cavities. These filaments may have been uplifted by the propagating jet or buoyant X-ray bubble, or may have formed {it in situ} by cloud collapse at the interface of a radio lobe or rapid cooling in a cavitys compressed shell. The morphological diversity of nearly the entire FUV sample is reproduced by recent hydrodynamical simulations in which the AGN powers a self-regulating rain of thermally unstable star forming clouds that precipitate from the hot atmosphere. In this model, precipitation triggers where the cooling-to- freefall time ratio is $t_{mathrm{cool}}/t_{mathrm{ff}}sim 10$. This condition is roughly met at the maxmial projected FUV radius for more than half of our sample, and clustering about this ratio is stronger for sources with higher star formation rates.
Over the past decade increasingly robust estimates of the dense molecular gas content in galaxy populations between redshift 0 and the peak of cosmic galaxy/star formation from redshift 1-3 have become available. This rapid progress has been possible due to the advent of powerful ground-based, and space telescopes for combined study of several millimeter to far-IR, line or continuum tracers of the molecular gas and dust components. The main conclusions of this review are: 1. Star forming galaxies contained much more molecular gas at earlier cosmic epochs than at the present time. 2. The galaxy integrated depletion time scale for converting the gas into stars depends primarily on z or Hubble time, and at a given z, on the vertical location of a galaxy along the star-formation rate versus stellar mass main-sequence (MS) correlation. 3. Global rates of galaxy gas accretion primarily control the evolution of the cold molecular gas content and star formation rates of the dominant MS galaxy population, which in turn vary with the cosmological expansion. A second key driver may be global disk fragmentation in high-z, gas rich galaxies, which ties local free-fall time scales to galactic orbital times, and leads to rapid radial matter transport and bulge growth. Third, the low star formation efficiency inside molecular clouds is plausibly set by super-sonic streaming motions, and internal turbulence, which in turn may be driven by conversion of gravitational energy at high-z, and/or by local feedback from massive stars at low-z. 4. A simple gas regulator model is remarkably successful in predicting the combined evolution of molecular gas fractions, star formation rates, galactic winds, and gas phase metallicities.
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