54 research outputs found
The Likelihood Ratio as a tool for Radio Continuum Surveys with SKA precursor telescopes
In this paper we investigate the performance of the likelihood ratio method
as a tool for identifying optical and infrared counterparts to proposed radio
continuum surveys with SKA precursor and pathfinder telescopes. We present a
comparison of the infrared counterparts identified by the likelihood ratio in
the VISTA Deep Extragalactic Observations (VIDEO) survey to radio observations
with 6, 10 and 15 arcsec resolution. We cross-match a deep radio catalogue
consisting of radio sources with peak flux density 60 Jy with deep
near-infrared data limited to 22.6. Comparing the
infrared counterparts from this procedure to those obtained when cross-matching
a set of simulated lower resolution radio catalogues indicates that degrading
the resolution from 6 arcsec to 10 and 15 arcsec decreases the completeness of
the cross-matched catalogue by approximately 3 and 7 percent respectively. When
matching against shallower infrared data, comparable to that achieved by the
VISTA Hemisphere Survey, the fraction of radio sources with reliably identified
counterparts drops from 89%, at 22.6, to 47% with
20.0. Decreasing the resolution at this shallower
infrared limit does not result in any further decrease in the completeness
produced by the likelihood ratio matching procedure. However, we note that
radio continuum surveys with the MeerKAT and eventually the SKA, will require
long baselines in order to ensure that the resulting maps are not limited by
instrumental confusion noise.Comment: 10 pages, 7 figures, accepted for publication in mnra
Evidence for a large fraction of Compton-thick quasars at high redshift
Using mid-infrared and radio selection criteria, we pre-select a sample of
candidate high-redshift type-2 quasars in the Subaru XMM-Newton Deep Field
(SXDF). To filter out starburst contaminants, we use a bayesian method to fit
the spectral energy distributions (SEDs) between 24-microns and B-band, obtain
photometric redshifts, and identify the best candidates for high-z type-2
quasars. This leaves us with 12 z_phot >= 1.7 type-2 quasar candidates in an
area ~0.8 deg^2, of which only two have secure X-ray detections. The two
detected sources have estimated column densities N_H~2 & 3x10^27 m^-2, i.e.
heavily obscured but Compton-thin quasars. Given the large bolometric
luminosities and redshifts of the undetected objects, the lack of X-ray
detections suggests extreme absorbing columns N_H >= 10^28 m^-2 are typical. We
have found evidence for a population of ``Compton-thick'' high-redshift type-2
quasars, at least comparable to, and probably larger than the type-1 quasar
population, although spectroscopic confirmation of their AGN nature is
important.Comment: 6 pages, 2 colour figures. Accepted by MNRAS. Full resolution version
and supplementary figures can be found at:
http://www.mpia.de/homes/martinez/publications.htm
Nonparametric Reconstruction of the Dark Energy Equation of State
A basic aim of ongoing and upcoming cosmological surveys is to unravel the
mystery of dark energy. In the absence of a compelling theory to test, a
natural approach is to better characterize the properties of dark energy in
search of clues that can lead to a more fundamental understanding. One way to
view this characterization is the improved determination of the
redshift-dependence of the dark energy equation of state parameter, w(z). To do
this requires a robust and bias-free method for reconstructing w(z) from data
that does not rely on restrictive expansion schemes or assumed functional forms
for w(z). We present a new nonparametric reconstruction method that solves for
w(z) as a statistical inverse problem, based on a Gaussian Process
representation. This method reliably captures nontrivial behavior of w(z) and
provides controlled error bounds. We demonstrate the power of the method on
different sets of simulated supernova data; the approach can be easily extended
to include diverse cosmological probes.Comment: 16 pages, 11 figures, accepted for publication in Physical Review
PDS 144: the first confirmed Herbig Ae-Herbig Ae wide binary
PDS 144 is a pair of Herbig Ae stars that are separated by 5.'' 35 on the sky. It has previously been shown to have an A2Ve Herbig Ae star viewed at 83 degrees inclination as its northern member and an A5Ve Herbig Ae star as its southern member. Direct imagery revealed a disk occulting PDS 144 N-the first edge-on disk observed around a Herbig Ae star. The lack of an obvious disk in direct imagery suggested PDS 144 S might be viewed face-on or not physically associated with PDS 144 N. Multi-epoch Hubble Space Telescope imagery of PDS 144 with a 5 year baseline demonstrates PDS 144 N & S are comoving and have a common proper motion with TYC 6782-878-1. TYC 6782-878-1 has previously been identified as a member of Upper Sco sub-association A at d = 145 +/- 2 pc with an age of 5-10 Myr. Ground-based imagery reveals jets and a string of Herbig-Haro knots extending 13' (possibly further) which are aligned to within 7 degrees +/- 6 degrees on the sky. By combining proper motion data and the absence of a dark mid-plane with radial velocity data, we measure the inclination of PDS 144 S to be i = 73 degrees +/- 7 degrees. The radial velocity of the jets from PDS 144 N & S indicates they, and therefore their disks, are misaligned by 25 degrees +/- 9 degrees. This degree of misalignment is similar to that seen in T Tauri wide binaries.Peer reviewe
Impact of Redshift Information on Cosmological Applications with Next-Generation Radio Surveys
In this paper, we explore how the forthcoming generation of large-scale radio
continuum surveys, with the inclusion of some degree of redshift information,
can constrain cosmological parameters. By cross-matching these radio surveys
with shallow optical to near-infrared surveys, we can essentially separate the
source distribution into a low- and a high-redshift sample, thus providing a
constraint on the evolution of cosmological parameters such as those related to
dark energy. We examine two radio surveys, the Evolutionary Map of the Universe
(EMU) and the Westerbork Observations of the Deep APERTIF Northern sky (WODAN).
A crucial advantage is their combined potential to provide a deep, full-sky
survey. The surveys used for the cross-identifications are SkyMapper and SDSS,
for the southern and northern skies, respectively. We concentrate on the galaxy
clustering angular power spectrum as our benchmark observable, and find that
the possibility of including such low redshift information yields major
improvements in the determination of cosmological parameters. With this
approach, and provided a good knowledge of the galaxy bias evolution, we are
able to put strict constraints on the dark energy parameters, i.e.
w_0=-0.9+/-0.041 and w_a=-0.24+/-0.13, with type Ia supernovae and CMB priors
(with a one-parameter bias in this case); this corresponds to a Figure of Merit
(FoM) > 600, which is twice better than what is obtained by using only the
cross-identified sources and greater than four time better than the case
without any redshift information at all.Comment: 12 pages, 6 figures, 6 tables; accepted for publication in MNRA
An exponential decline at the bright end of the z=6 galaxy luminosity function
We present the results of a search for the most luminous star-forming
galaxies at redshifts z~6 based on CFHT Legacy Survey data. We identify a
sample of 40 Lyman break galaxies brighter than magnitude z'=25.3 across an
area of almost 4 square degrees. Sensitive spectroscopic observations of seven
galaxies provide redshifts for four, of which only two have moderate to strong
Lyman alpha emission lines. All four have clear continuum breaks in their
spectra. Approximately half of the Lyman break galaxies are spatially resolved
in 0.7 arcsec seeing images, indicating larger sizes than lower luminosity
galaxies discovered with the Hubble Space Telescope, possibly due to on-going
mergers. The stacked optical and infrared photometry is consistent with a
galaxy model with stellar mass ~ 10^{10} solar masses. There is strong evidence
for substantial dust reddening with a best-fit A_V=0.7 and A_V>0.48 at 2 sigma
confidence, in contrast to the typical dust-free galaxies of lower luminosity
at this epoch. The spatial extent and spectral energy distribution suggest that
the most luminous z~6 galaxies are undergoing merger-induced starbursts. The
luminosity function of z=5.9 star-forming galaxies is derived. This agrees well
with previous work and shows strong evidence for an exponential decline at the
bright end, indicating that the feedback processes which govern the shape of
the bright end are occurring effectively at this epoch.Comment: 14 pages, 11 figures, AJ in press, revised to address referee
comment
Herschel -ATLAS: Extragalactic number counts from 250 to 500 microns
Aims. The Herschel-ATLAS survey (H-ATLAS) will be the largest area survey to be undertaken by the Herschel Space Observatory. It will cover 550 sq. deg. of extragalactic sky at wavelengths of 100, 160, 250, 350 and 500 μm when completed, reaching flux limits (5σ) from 32 to 145 mJy. We here present galaxy number counts obtained for SPIRE observations of the first ~14 sq. deg. observed at 250, 350 and 500 μm.
Methods. Number counts are a fundamental tool in constraining models of galaxy evolution. We use source catalogs extracted from the H-ATLAS maps as the basis for such an analysis. Correction factors for completeness and flux boosting are derived by applying our extraction method to model catalogs and then applied to the raw observational counts.
Results. We find a steep rise in the number counts at flux levels of 100–200 mJy in all three SPIRE bands, consistent with results from BLAST. The counts are compared to a range of galaxy evolution models. None of the current models is an ideal fit to the data but all ascribe the steep rise to a population of luminous, rapidly evolving dusty galaxies at moderate to high redshift
Evolution of star formation in the UKIDSS Ultra Deep Survey Field - I. Luminosity functions and cosmic star formation rate out to z = 1.6 (vol 433, pg 796, 2013)
We present new results on the cosmic star formation history in the Subaru/XMM–Newton Deep Survey (SXDS)–Ultra Deep Survey (UDS) field out to z = 1.6. We compile narrowband
data from the Subaru Telescope and the Visible and Infrared Survey Telescope for Astronomy (VISTA) in conjunction with broad-band data from the SXDS and UDS, to make a selection of 5725 emission-line galaxies in 12 redshift slices, spanning 10 Gyr of cosmic time. We determine photometric redshifts for the sample using 11-band photometry, and use
a spectroscopically confirmed subset to fine tune the resultant redshift distribution. We use the maximum-likelihood technique to determine luminosity functions in each redshift slice and model the selection effects inherent in any narrow-band selection statistically, to obviate the retrospective corrections ordinarily required. The deep narrow-band data are sensitive to very low star formation rates (SFRs), and allow an accurate evaluation of the faint end slope of the Schechter function, α. We find that α is particularly sensitive to the assumed faintest
broad-band magnitude of a galaxy capable of hosting an emission line, and propose that this limit should be empirically motivated. For this analysis, we base our threshold on the limiting observed equivalent widths of emission lines in the local Universe. We compute the
characteristic SFR of galaxies in each redshift slice, and the integrated SFR density, ρSFR. We find our results to be in good agreement with the literature and parametrize the evolution of the SFR density as ρSFR ∝ (1 + z)4.58 confirming a steep decline in star formation activity
since z ∼ 1.6.
Key words: surveys – galaxies: evolution – galaxies: formation – galaxies: high-redshift –
galaxies: star formation – cosmology: observations
The Subaru/XMM-Newton Deep Field - II. The 37 brightest radio sources
We study the 37 brightest radio sources in the Subaru/XMM-Newton Deep Field
(SXDF). We have spectroscopic redshifts for 24 of 37 objects and photometric
redshifts for the remainder, yielding a median redshift z_med for the whole
sample of z_med ~= 1.1 and a median radio luminosity close to the `FRI/FRII'
luminosity divide. Using mid-IR (Spitzer MIPS 24 um) data we expect to trace
nuclear accretion activity, even if it is obscured at optical wavelengths,
unless the obscuring column is extreme. Our results suggest that above the
FRI/FRII radio luminosity break most of the radio sources are associated with
objects that have excess mid-IR emission, only some of which are broad-line
objects, although there is one clear low-accretion-rate object with an FRI
radio structure. For extended steep-spectrum radio sources, the fraction of
objects with mid-IR excess drops dramatically below the FRI/FRII luminosity
break, although there exists at least one high-accretion-rate `radio-quiet'
QSO. We have therefore shown that the strong link between radio luminosity (or
radio structure) and accretion properties, well known at z ~ 0.1, persists to z
~ 1. Investigation of mid-IR and blue excesses shows that they are correlated
as predicted by a model in which, when significant accretion exists, a torus of
dust absorbs ~30% of the light, and the dust above and below the torus scatters
>~1% of the light.Comment: Accepted for publication by MNRAS; 39 pages, 7 figures, 4 table
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