32 research outputs found
Starburst or AGN Dominance in Submillimetre-Luminous Candidate AGN?
It is widely believed that ultraluminous infrared (IR) galaxies and active
galactic nuclei (AGN) activity are triggered by galaxy interactions and
merging, with the peak of activity occurring at z~2, where submillimetre
galaxies are thousands of times more numerous than local ULIRGs. In this
evolutionary picture, submillimetre galaxies (SMGs) would host an AGN, which
would eventually grow a black hole (BH) strong enough to blow off all of the
gas and dust leaving an optically luminous QSO. To probe this evolutionary
sequence we have focussed on the 'missing link' sources, which demonstrate both
strong starburst (SB) and AGN signatures, in order to determine if the SB is
the main power source even in SMGs when we have evidence that an AGN is present
from their IRAC colours. The best way to determine if a dominant AGN is present
is to look for their signatures in the mid-infrared with the Spitzer IRS, since
often even deep X-ray observations miss identifying the presence of AGN in
heavily dust-obscured SMGs. We present the results of our audit of the energy
balance between star-formation and AGN within this special sub-population of
SMGs -- where the BH has grown appreciably to begin heating the dust emission.Comment: 2 pages, 1 figure. To appear in "Hunting for the Dark: The Hidden
Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P. Debattista and
C.C. Popescu, AIP Conf. Ser., in pres
Galaxy pairs in the SDSS - XIII. The connection between enhanced star formation and molecular gas properties in galaxy mergers.
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We investigate the connection between star formation and molecular gas properties in galaxy mergers at low redshift (z ≤ 0.06). The study we present is based on IRAM 30-m CO(1-0) observations of 11 galaxies with a close companion selected from the Sloan Digital Sky Survey (SDSS). The pairs have mass ratios ≤4, projected separations r p ≤ 30 kpc and velocity separations ΔV ≤ 300 km s -1, and have been selected to exhibit enhanced specific star formation rates (sSFRs). We calculate molecular gas (H 2) masses, assigning to each galaxy a physically motivated conversion factor αCO, and we derive molecular gas fractions and depletion times. We compare these quantities with those of isolated galaxies from the extended CO Legacy Data base for the GALEX Arecibo SDSS Survey sample (xCOLDGASS; Saintonge et al.) with gas quantities computed in an identical way. Ours is the first study which directly compares the gas properties of galaxy pairs and those of a control sample of normal galaxies with rigorous control procedures and for which SFR and H 2 masses have been estimated using the same method. We find that the galaxy pairs have shorter depletion times and an average molecular gas fraction enhancement of 0.4 dex compared to the mass matched control sample drawn from xCOLDGASS. However, the gas masses (and fractions) in galaxy pairs and their depletion times are consistent with those of non-mergers whose SFRs are similarly elevated. We conclude that both external interactions and internal processes may lead to molecular gas enhancement and decreased depletion times.Peer reviewe
An 850-micron SCUBA map of the Groth Strip and reliable source extraction
We present an 850-micron map and list of candidate sources in a 70 arcmin^2
sub-area of the Groth Strip observed using SCUBA. We initially detect 7
candidate sources with signal-to-noise ratios (SNRs) between 3.0 and 3.5 and 4
candidate sources with SNR > 3.5. Simulations suggest that on average in a map
this size one expects 1.6 false positive sources for SNR > 3.5 and 4.5 for 3 <
SNR < 3.5. Flux boosting in maps is a well known effect and we have developed a
simple Bayesian prescription for estimating the unboosted flux distribution and
used this method to determine the best flux estimates of our sources. This
method is easily adapted for any other modest signal-to-noise survey in which
there is prior knowledge of the source counts. We performed follow-up
photometry in an attempt to confirm or reject 5 of our source candidates. We
confirm the reality of 2 of the SCUBA sources, although at lower levels than
suggested in the map and we find that the photometry results are consistent
with and confirm the de-boosted map fluxes. Our final candidate source list
contains 3 sources, including the 2 confirmed detections and 1 further
candidate source with SNR > 3.5 which has a reasonable chance of being real.Comment: 8 pages, 4 figures, accepted for publication in MNRAS December 9,
200
The HiZELS/UKIRT large area survey for bright Lyman-alpha emitters at z~9
We present the largest area survey to date (1.4 deg2) for Lyman-alpha
emitters (LAEs) at z~9, as part of the Hi-z Emission Line Survey (HiZELS). The
survey, which primarily targets H-alpha emitters at z < 3, uses the Wide Field
CAMera on the United Kingdom Infrared Telescope and a custom narrow-band filter
in the J band to reach a Lyman-alpha luminosity limit of ~10^43.8 erg/s over a
co-moving volume of 1.12x10^6 Mpc^3 at z = 8.96+-0.06. Two candidates were
found out of 1517 line emitters, but those were rejected as LAEs after
follow-up observations. This improves the limit on the space density of bright
Lyman-alpha emitters by 3 orders of magnitude and is consistent with
suppression of the bright end of the Lyman-alpha luminosity function beyond
z~6. Combined with upper limits from smaller but deeper surveys, this rules out
some of the most extreme models for high-redshift Lyman-alpha emitters. The
potential contamination of narrow-band Lyman-alpha surveys at z>7 by Galactic
brown dwarf stars is also examined, leading to the conclusion that such
contamination may well be significant for searches at 7.7 < z < 8.0, 9.1 < z <
9.5 and 11.7 < z < 12.2.Comment: To appear in proceedings of "UKIRT at 30: A British Success Story
Origins of the extragalactic background at 1mm from a combined analysis of the AzTEC and MAMBO data in GOODS-N
We present a study of the cosmic infrared background, which is a measure of
the dust obscured activity in all galaxies in the Universe. We venture to
isolate the galaxies responsible for the background at 1mm; with spectroscopic
and photometric redshifts we constrain the redshift distribution of these
galaxies. We create a deep 1.16mm map (sigma ~ 0.5mJy) by combining the AzTEC
1.1mm and MAMBO 1.2mm datasets in GOODS-N. This combined map contains 41 secure
detections, 13 of which are new. By averaging the 1.16mm flux densities of
individually undetected galaxies with 24um flux densities > 25uJy, we resolve
31--45 per cent of the 1.16mm background. Repeating our analysis on the SCUBA
850um map, we resolve a higher percentage (40--64 per cent) of the 850um
background. A majority of the background resolved (attributed to individual
galaxies) at both wavelengths comes from galaxies at z > 1.3. If the ratio of
the resolved submillimeter to millimeter background is applied to a reasonable
scenario for the origins of the unresolved submillimeter background, 60--88 per
cent of the total 1.16mm background comes from galaxies at z > 1.3.Comment: 12 pages, 10 figures. Accepted by MNRAS. The combined map is publicly
available at http://www.astro.umass.edu/~pope/goodsn_mm
An excess of star-forming galaxies in the fields of high-redshift QSOs
We present submillimetre (submm) and mid-infrared (MIR) imaging observations of five fields centred on quasi-stellar objects (QSOs) at 1.7 <z< 2.8. All five QSOs were detected previously at submm wavelengths. At 850 (450) μm, we detect 17 (11) submillimetre galaxies (SMGs) in addition to the QSOs. The total area mapped at 850 μm is ∼28 arcmin2 down to rms noise levels of 1–2 mJy beam−1, depending on the field. Integral number counts are computed from the 850-μm data using the same analytical techniques adopted by ‘blank-field’ submm surveys. We find that the ‘QSO-field’ counts show a clear excess over the blank-field counts at deboosted flux densities of ∼2–4 mJy; at higher flux densities, the counts are consistent with the blank-field counts. Robust MIR counterparts are identified for all four submm detected QSOs and ∼60 per cent of the SMGs. The MIR colours of the QSOs are similar to those of the local ultraluminous infrared galaxy (ULIRG)/active galactic nuclei (AGN) Mrk 231 if placed at 1 <z< 3 whilst most of the SMGs have colours very similar to those of the local ULIRG Arp 220 at 1 <z< 3. MIR diagnostics therefore find no strong evidence that the SMGs host buried AGN although we cannot rule out such a possibility. Taken together our results suggest that the QSOs sit in regions of the early universe which are undergoing an enhanced level of major star formation activity, and should evolve to become similarly dense regions containing massive galaxies at the present epoch. Finally, we find evidence that the level of star formation activity in individual galaxies appears to be lower around the QSOs than it is around more powerful radio-loud AGN at higher redshifts.We thank Ian Smail for extensive comments on the draft manuscript and Mark Thompson for useful discussions. The JCMT is operated by The Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the United Kingdom, the Netherlands Organisation for Scientific Research and the National Research Council of Canada. JCMT data were taken under project IDs M03AU46, M03BU32 and M04BU14. This work is based (in part) on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. JAS, MJP and FJC acknowledge support from the Royal Society. FJC acknowledges further support from the Spanish Ministerio de Educación y Ciencia under project ESP2006-13608
Submillimetre Constraints on Hyper-Extremely Red Objects in the Subaru Deep Field
We have mapped the submillimetre wavelength continuum emission from the
Subaru Deep Field (SDF) at 450 and 850 microns with the Submillimetre
Common-User Bolometer Array (SCUBA) detector on the James Clerk Maxwell
Telescope (JCMT). The near-IR image of the SDF is one of the deepest near-IR
images available and contains four `hyper extremely red objects' (HEROs). These
data allow us to test the connection between `extremely red objects' (EROs)
found in IR surveys and the population of bright submillimetre sources found
with SCUBA. We present a weak measurement of the average flux of the four
K-band selected HEROs of 1.15 (+/-0.46) mJy, which fails to support the
hypothesis that HEROs should be bright SCUBA sources. Our data are consistent
with the HEROs being objects with SEDs like that of Arp220 out to z~1.7,
however, the extinction in the HEROs must be about 1 magnitude greater in the
J-band than is the case for Arp220 and they would need to be 1.7 times as
luminous as Arp220. On the other hand, an evolutionary model of elliptical
galaxies at z~2-3 in a dusty starburst phase is also in agreement with the
submillimetre data, as was originally proposed for the HEROs.Comment: 7 pages, 5 figures, accepted for publication in MNRAS July 5th, 200
An ALMA Survey of Submillimeter Galaxies in the Extended Chandra Deep Field South : The Redshift Distribution and Evolution of Submillimeter Galaxies
Accepted by ApJ. 45 pages, 16 figuresWe present the first photometric redshift distribution for a large unbiased sample of 870um selected submillimeter galaxies (SMGs) with robust identifications based on observations with the Atacama Large Millimeter Array (ALMA). In our analysis we consider 96 SMGs in the Extended Chandra Deep Field South, 77 of which have 4-19 band, optical-near-infrared, photometry. We model the Spectral Energy Distributions (SEDs) for these 77 SMGs, deriving a median photometric redshift of z=2.3+/-0.1. The remaining 19 SMGs have insufficient optical or near-infrared photometry to derive photometric redshifts, but a stacking analysis of IRAC and Herschel observations confirms they are not spurious. Assuming these sources have an absolute H-band magnitude distribution comparable to that of a complete sample of z~1-2 SMGs, we demonstrate that the undetected SMGs lie at higher redshifts, raising the median redshift for SMGs to z=2.5+/-0.2. More critically we show that the proportion of galaxies undergoing an SMG phase at z>3 is 35+/-5% of the total population. We derive a median stellar mass for SMGs of Mstar=(8+/-1)x10^10Mo, but caution that there are significant systematic uncertainties in our stellar mass estimate, up to x5 for individual sources. We compare our sample of SMGs to a volume-limited, morphologically classified sample of ellipticals in the local Universe. Assuming the star formation activity in SMGs has a timescale of ~100Myr we show that their descendants at z~0 would have a space density and M_H distribution which are in good agreement with those of local ellipticals. In addition the inferred mass-weighted ages of the local ellipticals broadly agree with the look-back times of the SMG events. Taken together, these results are consistent with a simple model that identifies SMGs as events that form most of the stars seen in the majority of luminous elliptical galaxies at the present day.Peer reviewe