154 research outputs found
Non-steller light from high-redshift radiogalaxies
With the aid of a new IRCAM image of 3C356, researchers question the common assumption that radiosource-stimulated starbursts are responsible for the extended optical emission aligned with radio structures in high-redshift radiogalaxies. They propose an alternative model in which the radiation from a hidden luminous quasar is beamed along the radio axis and illuminates dense clumps of cool gas to produce both extended narrow emission line regions and, by Thomson scattering, extended optical continua. Simple observational tests of this model are possible and necessary if we are to continue to accept that the color, magnitude and shape evolution of radiogalaxies are controlled by the active evolution of stellar populations
Evidence against a simple two-component model for the far-infrared emission from galaxies
Two of the first Infrared Astronomy Satellite (IRAS) results were that galaxies have a wide range of values for the ratio of 60 micron to 100 micron flux density (0.2 less than or equal to S sub 60/S sub 100 less than or equal to 1.0) and that this ratio is correlated with L sub fir, L sub b, L sub fir being the total far-infrared luminosity and L sub b being the luminosity at visible wavelengths (de Jong et al. 1984; Soifer et al. 1984). From these results arose the following simple model for the far-infrared emission from galaxies (de Jong et al. 1984), which has remained the standard model ever since. In this model, the far-infrared emission comes from two dust components: warm dust (T approx. equals 50 K) intermingled with, and heated by, young massive OB stars in molecular clouds and HII regions, and colder dust (T approx. equals 20 K) associated with the diffuse atomic hydrogen in the interstellar medium and heated by the general interstellar radiation field. As the number of young stars in a galaxy increases, S sub 60/S sub 100 increases, because there is a greater proportion of warm dust, and so does L sub fir/L sub b, because most of the radiation from the young stars is absorbed by the dust, leading to a swifter increase in far-infrared emission than in visible light. Although this model explains the basic IRAS results, it is inelegant. It uses two free parameters to fit two data (the 60 and 100 micron flux densities)-and there are now several observations that contradict it. Despite these major problems with the two-component model, it is not clear what should be put in its place. When considering possible models for the far-infrared emission from galaxies, the observational evidence for our own galaxy must be considered. Researchers suspect that the study by Boulanger and Perault (1988) of the far-infrared properties of the local interstellar medium may be particularly relevant. They showed that molecular clouds are leaky - that most of the light from OB stars in molecular clouds does not heat the dust in the clouds, but instead leaks out. The consequence of this is that that while most of the far-infrared emission from the solar neighborhood is from dust associated with diffuse HI, this dust is mostly heated by young stars
Upper limits on K-band polarization in three high-redshift radio galaxies: LBDS 53W091, 3C 441 and MRC 0156-252
We present the results of K-band imaging polarimetry of three radio galaxies,
including the very red and apparently old z=1.55 galaxy 53W091. We find weak
evidence for polarization in components of 3C 441 and in the south-east
companion of 53W091, but no evidence of significant polarization in 53W091
itself. We also find strong evidence that MRC 0156-252 is unpolarised. We
present upper limits for the K-band polarization of all three sources. For
53W091, the lack of significant K-band polarization provides further confidence
that its red R-K colour can be attributed to a mature stellar population,
consistent with the detailed analyses of its ultraviolet spectral-energy
distribution which indicate a minimum age of 2-3.5 Gyr.Comment: 7 pages, 3 postscript figures. In press at MNRA
The new galaxy evolution paradigm revealed by the Herschel surveys
The Herschel Space Observatory has revealed a very different galaxyscape from that shown by optical surveys which presents a challenge for galaxy-evolution models. The Herschel surveys reveal (1) that there was rapid galaxy evolution in the very recent past and (2) that galaxies lie on a single Galaxy Sequence (GS) rather than a star-forming ‘main sequence’ and a separate region of ‘passive’ or ‘red-and-dead’ galaxies. The form of the GS is now clearer because far-infrared surveys such as the Herschel ATLAS pick up a population of optically red starforming galaxies that would have been classified as passive using most optical criteria. The space-density of this population is at least as high as the traditional star-forming population. By stacking spectra of H-ATLAS galaxies over the redshift range 0.001 < z < 0.4, we show that the galaxies responsible for the rapid low-redshift evolution have high stellar masses, high star-formation rates but, even several billion years in the past, old stellar populations – they are thus likely to be relatively recent ancestors of early-type galaxies in the Universe today. The form of the GS is inconsistent with rapid quenching models and neither the analytic bathtub model nor the hydrodynamical EAGLE simulation can reproduce the rapid cosmic evolution. We propose a new gentler model of galaxy evolution that can explain the new Herschel results and other key properties of the galaxy population
The Linear-Size Evolution of Classical Double Radio Sources
Recent investigations of how the median size of extragalactic radio sources
change with redshift have produced inconsistent results. Eales compared the
radio and optical properties of a bright 3C and faint 6C sample and concluded
that (), with being the median
size of the radio sources at a given epoch and z the redshift. Oort, Katgert,
and Windhorst, on the other hand, from a comparison of the properties of a
number of radio samples, found much stronger evolution, with
. In this paper we attempt to resolve the
difference. We have repeated the analysis of Eales using the virtually complete
redshift information that now exists for the 6C sample. Confining our analysis
to FR2 sources, which we argue is the best-understood class of radio sources
and the least likely to be affected by selection effects, we find
() and
(). Our complete redshift information allows us to gain insight
into our result by plotting a radio luminosity-size (P-D) diagram for the 6C
sample. The most obvious difference between the 3C and 6C P-D diagrams is the
clump of sources in the 6C diagram at . These clump sources have similar sizes to the emission-line
regions found around high-redshift radio galaxies, suggesting that the presence
of dense line-emitting gas around high-redshift radio galaxies is responsible
for the size evolution. We show that this explanation can quantitatively
explain the observed size evolution, as long as there is either little X-ray
emitting gas around these objects or, if there is, it is distributed in a
similar way to the emission-line gas: highly anisotropic and inhomogeneous.Comment: compressed and uuencoded postscript file. 33 pages including 5
figures (441951 bytes). Accepted for publication in September Ap
The Canada-UK Deep Submillimetre Survey: First Submillimetre Images, the Source Counts, and Resolution of the Background
We present the first results of a deep unbiased submillimetre survey carried
out at 450 and 850 microns. We detected 12 sources at 850 microns, giving a
surface density of sources with 850-micron flux densities > 2.8mJy of of
0.49+-0.16 per square arcmin. The sources constitute 20-30% of the background
radiation at 850 microns and thus a significant fraction of the entire
background radiation produced by stars. This implies, through the connection
between metallicity and background radiation, that a significant fraction of
all the stars that have ever been formed were formed in objects like those
detected here. The combination of their large contribution to the background
radiation and their extreme bolometric luminosities make these objects
excellent candidates for being proto-ellipticals. Optical astronomers have
recently shown that the UV-luminosity density of the universe increases by a
factor of about 10 between z=0 and z=1 and then decreases again at higher
redshifts. Using the results of a parallel submillimetre survey of the local
universe, we show that both the submillimetre source density and background can
be explained if the submillimetre luminosity density evolves in a similar way
to the UV-luminosity density. Thus, if these sources are ellipticals in the
process of formation, they may be forming at relatively modest redshifts.Comment: 8 pages (LATEX), 6 postscript figures, submitted to ApJ Letter
The Canada-UK Deep Sub-Millimeter Survey II: First identifications, redshifts and implications for galaxy evolution
Identifications are sought for 12 sub-mm sources detected by Eales et al
(1998). Six are securely identified, two have probable identifications and four
remain unidentified with I_AB > 25. Spectroscopic and estimated photometric
redshifts indicate that four of the sources have z < 1, and four have 1 < z <
3, with the remaining four empty field sources probably lying at z > 3. The
spectral energy distributions of the identifications are consistent with those
of high extinction starbursts such as Arp 220. The far-IR luminosities of the
sources at z > 0.5 are of order 3 x 10^12 h_50^-2 L_sun, i.e. slightly larger
than that of Arp 220. Based on this small sample, the cumulative bolometric
luminosity function shows strong evolution to z ~ 1, but weaker or possibly
even negative evolution beyond. The redshift dependence of the far-IR
luminosity density does not appear, at this early stage, to be inconsistent
with that seen in the ultraviolet luminosity density. Assuming that the energy
source in the far-IR is massive stars, the total luminous output from
star-formation in the Universe is probably dominated by the far-IR emission.
The detected systems have individual star-formation rates (exceeding 300
h_50^-2 M_O yr^-1) that are much higher than seen in the ultraviolet selected
samples, and which are sufficient to form substantial stellar populations on
dynamical timescales of 10^8 yr. The association with merger-like morphologies
and the obvious presence of dust makes it attractive to identify these systems
as forming the metal-rich spheroid population, in which case we would infer
that much of this activity has occurred relatively recently, at z ~ 2.Comment: 17 pages text + 14 figures. Accepted for publication in the
Astrophysical Journal. Gzipped tar file contains one text.ps file for text
and tables, one Fig2.jpg file for Fig 2, and 13 Fig*.ps files for the
remaining figure
The Canada-UK Deep Submillimetre Survey: The Survey of the 14-hour field
We have used SCUBA to survey an area of 50 square arcmin, detecting 19
sources down to a 3sigma sensitivity limit of 3.5 mJy at 850 microns. We have
used Monte-Carlo simulations to assess the effect of source confusion and noise
on the SCUBA fluxes and positions, finding that the fluxes of sources in the
SCUBA surveys are significantly biased upwards and that the fraction of the 850
micron background that has been resolved by SCUBA has been overestimated. The
radio/submillmetre flux ratios imply that the dust in these galaxies is being
heated by young stars rather than AGN. We have used simple evolution models
based on our parallel SCUBA survey of the local universe to address the major
questions about the SCUBA sources: (1) what fraction of the star formation at
high redshift is hidden by dust? (2) Does the submillimetre luminosity density
reach a maximum at some redshift? (3) If the SCUBA sources are
proto-ellipticals, when exactly did ellipticals form? However, we show that the
observations are not yet good enough for definitive answers to these questions.
There are, for example, acceptable models in which 10 times as much
high-redshift star formation is hidden by dust as is seen at optical
wavelengths, but also acceptable ones in which the amount of hidden star
formation is less than that seen optically. There are acceptable models in
which very little star formation occurred before a redshift of three (as might
be expected in models of hierarchical galaxy formation), but also ones in which
30% of the stars have formed by this redshift. The key to answering these
questions are measurements of the dust temperatures and redshifts of the SCUBA
sources.Comment: 41 pages (latex), 17 postscript figures, to appear in the November
issue of the Astronomical Journa
The radio luminosity function from the low-frequency 3CRR, 6CE & 7CRS complete samples
We measure the radio luminosity function (RLF) of steep-spectrum radio
sources using three redshift surveys of flux-limited samples selected at low
(151 & 178 MHz) radio frequency, low-frequency source counts and the local RLF.
The redshift surveys used are the new 7C Redshift Survey (7CRS) and the
brighter 3CRR and 6CE surveys totalling 356 sources with virtually complete
redshift information. This yields unprecedented coverage of the radio
luminosity versus z plane for steep-spectrum sources, and hence the most
accurate measurements of the steep-spectrum RLF yet made. We find that a simple
dual-population model for the RLF fits the data well, requiring differential
density evolution (with z) for the two populations. The low-luminosity
population can be associated with radio galaxies with weak emission lines, and
includes sources with both FRI and FRII radio structures; its comoving space
density rises by about one dex between z~0 and z~1 but cannot yet be
meaningfully constrained at higher redshifts. The high-luminosity population
can be associated with FRII radio galaxies and quasars with strong emission
lines; its rises by nearly three dex between z~0 and z~2. These results
mirror the situation seen in X-ray and optically-selected AGN. The integrated
radio luminosity density of the combination of the two populations is
controlled by the value of at the low-luminosity end of the RLF of the
high-luminosity population, a quantity which has been directly measured at z~1
by the 7CRS. We argue that robust determination of this quantity at higher
redshifts requires a new redshift survey based on a large (~1000 source) sample
about five times fainter than the 7CRS.Comment: 20 pages, 16 figures, accepted for publication in MNRA
A sample of 6C radio sources designed to find objects at redshift > 4: the radio data
We describe the selection of a sample of 34 radio sources from the 6C survey
(Hales, Baldwin & Warner 1993) from a region of sky covering 0.133 sr. The
selection criteria for this sample, hereafter called 6C*, were chosen to
optimise the chances of finding radio galaxies at redshift z > 4. Optical
follow-up observations have already led to the discovery of the most distant
known radio galaxy at z = 4.41 (Rawlings et al. 1996). We present VLA radio
maps and derive radio spectra for all the 6C* objects.Comment: 18 pages, LaTeX; also available at
http://www-astro.physics.ox.ac.uk/research/preprints/ To appear in MNRA
- …