26 research outputs found
Obscured Starburst Activity in High Redshift Clusters and Groups
Using Spitzer-MIPS 24um imaging and Keck spectroscopy we examine the nature
of the obscured star forming population in three clusters and three groups at
z~0.9. These six systems are components of the Cl1604 supercluster, the largest
structure imaged by Spitzer at redshifts near unity. We find that the average
density of 24um-detected galaxies within the Cl1604 clusters is nearly twice
that of the surrounding field and that this overdensity scales with the
cluster's dynamical state. The 24um-bright members often appear optically
unremarkable and exhibit only moderate [OII] line emission due to severe
obscuration. Their spatial distribution suggests they are an infalling
population, but an examination of their spectral properties, morphologies and
optical colors indicate they are not simply analogs of the field population
that have yet to be quenched. Using stacked composite spectra, we find the
24um-detected cluster and group galaxies exhibit elevated levels of Balmer
absorption compared to galaxies undergoing normal, continuous star formation. A
similar excess is not observed in field galaxies with equivalent infrared
luminosities, indicating a greater fraction of the detected cluster and group
members have experienced a burst of star formation in the recent past compared
to their counterparts in the field. Our results suggest that gas-rich galaxies
at high redshift experience a temporary increase in their star formation
activity as they assemble into denser environments. Using HST-ACS imaging we
find that disturbed morphologies are common among the 24um-detected cluster and
group members and become more prevalent in regions of higher galaxy density. We
conclude that mergers are the dominant triggering mechanism responsible for the
enhanced star formation found in the Cl1604 groups, while a mix of harassment
and mergers are likely driving the activity of the cluster galaxies.Comment: 18 pages, 19 figures, submitted to Ap
Galaxy Clusters in the IRAC Dark Field II: Mid-IR Sources
We present infrared luminosities, star formation rates, colors, morphologies,
locations, and AGN properties of 24 micron-detected sources in photometrically
detected high-redshift clusters in order to understand the impact of
environment on star formation and AGN evolution in cluster galaxies. We use
three newly-identified z=1 clusters selected from the IRAC dark field; the
deepest ever mid-IR survey with accompanying, 14 band multiwavelength data
including deep HST imaging and deep wide-area Spitzer MIPS 24 micron imaging.
We find 90 cluster members with MIPS detections within two virial radii of the
cluster centers, of which 17 appear to have spectral energy distributions
dominated by AGN and the rest dominated by star formation. We find that 43 of
the star forming are luminous infrared galaxies (LIRGs). The majority of
sources (81%) are spirals or irregulars. A large fraction (at least 25%) show
obvious signs of interactions. The MIPS -detected member galaxies have varied
spatial distributions as compared to the MIPS-undetected members with one of
the three clusters showing SF galaxies being preferentially located on the
cluster outskirts, while the other 2 clusters show no such trend. Both the AGN
fraction and the summed SFR of cluster galaxies increases from z=0 to 1, at a
rate that is a few times faster in clusters than over the same redshift range
in the field. Cluster environment does have an effect on the evolution of both
AGN fraction and SFR from redshift one to the present, but does not effect the
infrared luminosities or morphologies of the MIPS sample. Star formation
happens in the same way regardless of environment making MIPS sources look the
same in the cluster and field, however the cluster environment does encourage a
more rapid evolution with time as compared to the field.Comment: 18 pages, 9 figures, ApJ accepte
CO excitation of normal star forming galaxies out to z=1.5 as regulated by the properties of their interstellar medium
We investigate the CO excitation of normal star forming galaxies at z=1.5 using IRAM PdBI observations of the CO[2-1], CO[3-2] and CO[5-4] transitions for 4 galaxies, and VLA observations of CO[1-0] for 3 of them, measuring reliable line fluxes with S/N>4-7 for individual transitions. While the average CO Spectral Line Energy Distribution (SLED) has a sub-thermal excitation similar to the Milky Way (MW) up to CO[3-2], we show that the average CO[5-4] emission is 4 times stronger than assuming MW excitation. This demonstrates the presence of an additional component of more excited, denser and possibly warmer molecular gas. The ratio of CO[5-4] to lower-J CO emission is however lower than in local (U)LIRGs and high-redshift starbursting SMGs, and appears to correlate closely with the average intensity of the radiation field and with the star formation surface density, but not with SF efficiency (SFE). This suggests that the overall CO excitation is at least indirectly affected by the metallicity of the ISM. The luminosity of the CO[5-4] transition is found to correlate linearly with the bolometric infrared luminosity over 4 orders of magnitudes, with BzK galaxies following the same linear trend as local spirals and (U)LIRGs and high redshift star bursting sub-millimeter galaxies. The CO[5-4] luminosity is thus related to the dense gas, and might be a more convenient way to probe it than standard high--density tracers. We see excitation variations among our sample galaxies, linked to their evolutionary state and clumpiness in optical rest frame images. In one galaxy we see spatially resolved excitation variations, the more highly excited part corresponds to the location of massive SF clumps. This provides support to models that suggest that giant clumps are the main source of the high excitation CO emission in high redshift disk-like galaxies
The evolving far-IR galaxy luminosity function and dust-obscured star-formation rate density out to z~5
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. ©: 2017 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We present a new measurement of the evolving galaxy far-IR luminosity function (LF) extending out to redshifts z~5, with resulting implications for the level of dust-obscured star-formation density in the young Universe. To achieve this we have exploited recent advances in sub-mm/mm imaging with SCUBA-2 on the James Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter/Submillimeter Array (ALMA), which together provide unconfused imaging with sufficient dynamic range to provide meaningful coverage of the luminosity-redshift plane out to z>4. Our results support previous indications that the faint-end slope of the far-IR LF is sufficiently flat that comoving luminosity-density is dominated by bright objects (~L*). However, we find that the number-density/luminosity of such sources at high redshifts has been severely over-estimated by studies that have attempted to push the highly-confused Herschel SPIRE surveys beyond z~2. Consequently we confirm recent reports that cosmic star-formation density is dominated by UV-visible star formation at z>4. Using both direct (1/Vmax) and maximum likelihood determinations of the LF, we find that its high-redshift evolution is well characterized by continued positive luminosity evolution coupled with negative density evolution (with increasing redshift). This explains why bright sub-mm sources continue to be found at z>5, even though their integrated contribution to cosmic star-formation density at such early times is very small. The evolution of the far-IR galaxy LF thus appears similar in form to that already established for active galactic nuclei, possibly reflecting a similar dependence on the growth of galaxy mass.Peer reviewe
STAGES: the Space Telescope A901/2 Galaxy Evolution Survey
We present an overview of the Space Telescope A901/2 Galaxy Evolution Survey
(STAGES). STAGES is a multiwavelength project designed to probe physical
drivers of galaxy evolution across a wide range of environments and luminosity.
A complex multi-cluster system at z~0.165 has been the subject of an 80-orbit
F606W HST/ACS mosaic covering the full 0.5x0.5 (~5x5 Mpc^2) span of the
supercluster. Extensive multiwavelength observations with XMM-Newton, GALEX,
Spitzer, 2dF, GMRT, and the 17-band COMBO-17 photometric redshift survey
complement the HST imaging. Our survey goals include simultaneously linking
galaxy morphology with other observables such as age, star-formation rate,
nuclear activity, and stellar mass. In addition, with the multiwavelength
dataset and new high resolution mass maps from gravitational lensing, we are
able to disentangle the large-scale structure of the system. By examining all
aspects of environment we will be able to evaluate the relative importance of
the dark matter halos, the local galaxy density, and the hot X-ray gas in
driving galaxy transformation. This paper describes the HST imaging, data
reduction, and creation of a master catalogue. We perform Sersic fitting on the
HST images and conduct associated simulations to quantify completeness. In
addition, we present the COMBO-17 photometric redshift catalogue and estimates
of stellar masses and star-formation rates for this field. We define galaxy and
cluster sample selection criteria which will be the basis for forthcoming
science analyses, and present a compilation of notable objects in the field.
Finally, we describe the further multiwavelength observations and announce
public access to the data and catalogues.Comment: 29 pages, 22 figures; accepted to MNRAS. Full data release available
at http://www.nottingham.ac.uk/astronomy/stage
CANDELS: The progenitors of compact quiescent galaxies at z~2
We combine high-resolution HST/WFC3 images with multi-wavelength photometry
to track the evolution of structure and activity of massive (log(M*) > 10)
galaxies at redshifts z = 1.4 - 3 in two fields of the Cosmic Assembly
Near-infrared Deep Extragalactic Legacy Survey (CANDELS). We detect compact,
star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star
formation rates qualify them as likely progenitors of compact, quiescent,
massive galaxies (cQGs) at z = 1.5 - 3. At z > 2 most cSFGs have specific
star-formation rates (sSFR = 10^-9 yr^-1) half that of typical, massive SFGs at
the same epoch, and host X-ray luminous AGN 30 times (~30%) more frequently.
These properties suggest that cSFGs are formed by gas-rich processes (mergers
or disk-instabilities) that induce a compact starburst and feed an AGN, which,
in turn, quench the star formation on dynamical timescales (few 10^8 yr). The
cSFGs are continuously being formed at z = 2 - 3 and fade to cQGs by z = 1.5.
After this epoch, cSFGs are rare, thereby truncating the formation of new cQGs.
Meanwhile, down to z = 1, existing cQGs continue to enlarge to match local QGs
in size, while less-gas-rich mergers and other secular mechanisms shepherd
(larger) SFGs as later arrivals to the red sequence. In summary, we propose two
evolutionary scenarios of QG formation: an early (z > 2), fast-formation path
of rapidly-quenched cSFGs that evolve into cQGs that later enlarge within the
quiescent phase, and a slow, late-arrival (z < 2) path for SFGs to form QGs
without passing through a compact state.Comment: Submitted to the Astrophysical Journal Letters, 6 pages, 4 figure
Molecular Characterization of a Novel Intracellular ADP-Ribosyl Cyclase
Background. ADP-ribosyl cyclases are remarkable enzymes capable of catalyzing multiple reactions including the synthesis of the novel and potent intracellular calcium mobilizing messengers, cyclic ADP-ribose and NAADP. Not all ADP-ribosyl cyclases however have been characterized at the molecular level. Moreover, those that have are located predominately at the outer cell surface and thus away from their cytosolic substrates. Methodology/Principal Findings. Here we report the molecular cloning of a novel expanded family of ADP-ribosyl cyclases from the sea urchin, an extensively used model organism for the study of inositol trisphosphate-independent calcium mobilization. We provide evidence that one of the isoforms (SpARC1) is a soluble protein that is targeted exclusively to the endoplasmic reticulum lumen when heterologously expressed. Catalytic activity of the recombinant protein was readily demonstrable in crude cell homogenates, even under conditions where luminal continuity was maintained. Conclusions/Significance. Our data reveal a new intracellular location for ADP-ribosyl cyclases and suggest that production of calcium mobilizing messengers may be compartmentalized
ALMA constraints on star-forming gas in a prototypical z=1.5 clumpy galaxy: the dearth of CO(5-4) emission from UV-bright clumps
We present deep ALMA CO(5-4) observations of a main sequence, clumpy galaxy at z = 1.5 in the HUDF. Thanks to the ∼0.500 resolution of the ALMA data, we can link stellar population properties to the CO(5-4) emission on scales of a few kpc. We detect strong CO(5-4) emission from the nuclear region of the galaxy, consistent with the observed LIR–L 0 CO(5−4) correlation and indicating on-going nuclear star formation. The CO(5-4) gas component appears more concentrated than other star formation tracers or the dust distribution in this galaxy. We discuss possible implications of this difference in terms of star formation efficiency and mass build-up at the galaxy centre. Conversely, we do not detect any CO(5-4) emission from the UV-bright clumps. This might imply that clumps have a high star formation efficiency (although they do not display unusually high specific star formation rates) and are not entirely gas dominated, with gas fractions no larger than that of their host galaxy (∼50%). Stellar feedback and disk instability torques funnelling gas towards the galaxy centre could contribute to the relatively low gas content. Alternatively, clumps could fall in a more standard star formation efficiency regime if their actual star-formation rates are lower than generally assumed. We find that clump star-formation rates derived with several different, plausible methods can vary by up to an order of magnitude. The lowest estimates would be compatible with a CO(5-4) non-detection even for main-sequence like values of star formation efficiency and gas content