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Register a new userRed but not dead : Unveiling the Star-forming Far-infrared Spectral Energy Distribution of SpARCS Brightest Cluster Galaxies at 0 < z < 1.8
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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.
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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.
We analyze the infrared (IR) spectral energy distributions (SEDs) for 10micron < lambda(rest) < 100micron for ~600 galaxies at z~0.7 in the extended Chandra Deep Field South by stacking their Spitzer 24, 70 and 160micron images. We place interesting constraints on the average IR SED shape in two bins: the brightest 25% of z~0.7 galaxies detected at 24micron, and the remaining 75% of individually-detected galaxies. Galaxies without individual detections at 24micron were not well-detected at 70micron and 160micron even through stacking. We find that the average IR SEDs of z~0.7 star-forming galaxies fall within the diversity of z~0 templates. While dust obscuration Lir/Luv seems to be only a function of star formation rate (SFR; ~ Lir+Luv), not of redshift, the dust temperature of star-forming galaxies (with SFR ~ 10 solar mass per year) at a given IR luminosity was lower at z~0.7 than today. We suggest an interpretation of this phenomenology in terms of dust geometry: intensely star-forming galaxies at z~0 are typically interacting, and host dense centrally-concentrated bursts of star formation and warm dust temperatures. At z~0.7, the bulk of intensely star-forming galaxies are relatively undisturbed spirals and irregulars, and we postulate that they have large amounts of widespread lower-density star formation, yielding lower dust temperatures for a given IR luminosity. We recommend what IR SEDs are most suitable for modeling intermediate redshift galaxies with different SFRs.
We investigate the intrinsic spectral energy distribution (SED) of active galactic nuclei (AGNs) at infrared (IR) bands with 42 $z < 0.5$ optically luminous Palomar Green survey quasars through SED decomposition. We decompose the SEDs of the 42 quasars by combining an AGN IR template library Siebenmorgen2015 that covers a wide range of the AGN parameter space with three commonly used galaxy template libraries. We determine the median AGN SED from the best-fitting results. The far-IR (FIR) contribution of our median AGN SED is significantly smaller than that of Symeonidis et al. 2016, but roughly consistent with that of Lyu et al. 2017. The AGN IR SED becomes cooler with increasing bolometric luminosity, which might be due to that more luminous AGNs might have stronger radiative feedback to change torus structures and/or their tori might have higher metallicities. Our conclusions do not depend on the choice of galaxy template libraries. However, since the predicted polycyclic aromatic hydrocarbon (PAH) emission line flux is galaxy template-dependent, cautions should be taken on deriving galaxy FIR contribution from PAH fluxes.
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.
We study the far-infrared (IR) and sub-millimeter properties of a sample of ultraviolet (UV) selected galaxies at zsim1.5. Using stacking at 250, 350 and 500 um from Herschel Space Observatory SPIRE imaging of the COSMOS field obtained within the HerMES key program, we derive the mean IR luminosity as a function of both UV luminosity and slope of the UV continuum beta. The IR to UV luminosity ratio is roughly constant over most of the UV luminosity range we explore. We also find that the IR to UV luminosity ratio is correlated with beta. We observe a correlation that underestimates the correlation derived from low-redshift starburst galaxies, but is in good agreement with the correlation derived from local normal star-forming galaxies. Using these results we reconstruct the IR luminosity function of our UV-selected sample. This luminosity function recovers the IR luminosity functions measured from IR selected samples at the faintest luminosities (Lir ~ 10^{11} L_sun), but might underestimate them at the bright-end (Lir > 5.10^{11} L_sun). For galaxies with 10^{11}<Lir/L_sun<10^{13}, the IR luminosity function of a UV selection recovers (given the differences in IR-based estimates) 52-65 to 89-112 per cent of the star-formation rate density derived from an IR selection. The cosmic star-formation rate density derived from this IR luminosity function is 61-76 to 100-133 per cent of the density derived from IR selections at the same epoch. Assuming the latest Herschel results and conservative stacking measurements, we use a toy model to fully reproduce the far IR luminosity function from our UV selection at zsim 1.5. This suggests that a sample around 4 magnitudes deeper (i.e. reaching u sim 30 mag) and a large dispersion of the IR to UV luminosity ratio are required.
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