275 research outputs found
The evolution of classical doubles: clues from complete samples
We describe the inter-dependence of four properties of classical double radio
sources - spectral index, linear size, luminosity and redshift - from an
extensive study based on spectroscopically-identified complete samples. We use
these relationships to discuss aspects of strategies for searching for radio
galaxies at extreme redshifts, in the context of possible capabilities of the
new generation of proposed radio telescopes.Comment: To appear in `Perspectives in Radio Astronomy: scientific imperatives
at cm and m wavelengths.' eds: M.P. van Haarlem and J.M. van der Hulst
Version with colour figures available at
http://www-astro.physics.ox.ac.uk/~km
Star formation and the interstellar medium in z>6 UV-luminous Lyman-break galaxies
We present Atacama Large Millimeter Array (ALMA) detections of atomic carbon
line and dust continuum emission in two UV-luminous galaxies at redshift 6. The
far-infrared (FIR) luminosities of these galaxies are substantially lower than
similar starbursts at later cosmic epochs, indicating an evolution in the dust
properties with redshift, in agreement with the evolution seen in ultraviolet
(UV) attenuation by dust. The [CII] to FIR ratios are found to be higher than
at low redshift showing that [CII] should be readily detectable by ALMA within
the reionization epoch. One of the two galaxies shows a complex merger nature
with the less massive component dominating the UV emission and the more massive
component dominating the FIR line and continuum. Using the interstellar atomic
carbon line to derive the systemic redshifts we investigate the velocity of
Lyman alpha emission emerging from high-z galaxies. In contrast to previous
work, we find no evidence for decreasing Lyman alpha velocity shifts at
high-redshift. We observe an increase in velocity shifts from z2 to
z6, consistent with the effects of increased IGM absorption.Comment: 10 pages, 9 figures, submitted to ApJ, revised after referees
comment
A wide dispersion in star formation rate and dynamical mass of 10^8 solar mass black hole host galaxies at redshift 6
ALMA [CII] line and continuum observations of five redshift z>6 quasars are
presented. This sample was selected to probe lower black hole mass quasars than
most previous studies. We find a wide dispersion in properties with CFHQS
J0216-0455, a low-luminosity quasar with absolute magnitude M_1450=-22.2,
remaining undetected implying a limit on the star formation rate in the host
galaxy of <10 solar masses per year, whereas other host galaxies have star
formation rates up to hundreds of solar masses per year. Two other quasars have
particularly interesting properties. VIMOS2911 is one of the least luminous z>6
quasars known with M_1450=-23.1, yet its host galaxy is experiencing a very
powerful starburst. PSO J167-13 has a broad and luminous [CII] line and a
neighbouring galaxy a projected distance of 5kpc away that is also detected in
the [CII] line and continuum. Combining with similar observations from the
literature, we study the ratio of [CII] line to far-infrared luminosity finding
this ratio increases at high-redshift at a fixed far-infrared luminosity,
likely due to lower dust content, lower metallicity and/or higher gas masses.
We compile a sample of 21 high-redshift quasars with dynamical masses and
investigate the relationship between black hole mass and dynamical mass. The
new observations presented here reveal dynamical masses consistent with the
relationship defined by local galaxies. However, the full sample shows a very
wide scatter across the black hole mass - dynamical mass plane, whereas both
the local relationship and simulations of high-redshift quasars show a much
lower dispersion in dynamical mass.Comment: 10 pages, 11 figures, ApJ in pres
Star formation rate and dynamical mass of 10^8 solar mass black hole host galaxies at redshift 6
We present ALMA observations of two moderate luminosity quasars at redshift
6. These quasars from the Canada-France High-z Quasar Survey (CFHQS) have black
hole masses of ~10^8 M_solar. Both quasars are detected in the [CII] line and
dust continuum. Combining these data with our previous study of two similar
CFHQS quasars we investigate the population properties. We show that z>6
quasars have a significantly lower far-infrared luminosity than
bolometric-luminosity-matched samples at lower redshift, inferring a lower star
formation rate, possibly correlated with the lower black hole masses at z=6.
The ratios of [CII] to far-infrared luminosities in the CFHQS quasars are
comparable with those of starbursts of similar star formation rate in the local
universe. We determine values of velocity dispersion and dynamical mass for the
quasar host galaxies based on the [CII] data. We find that there is no
significant offset from the relations defined by nearby galaxies with similar
black hole masses. There is however a marked increase in the scatter at z=6,
beyond the large observational uncertainties.Comment: 10 pages, 7 figures, accepted for publication in Ap
Redshift 6.4 host galaxies of 10^8 solar mass black holes: low star formation rate and dynamical mass
We present ALMA observations of rest-frame far-infrared continuum and [CII]
line emission in two z=6.4 quasars with black hole masses of ~10^8 M_sun. CFHQS
J0210-0456 is detected in the continuum with a 1.2 mm flux of 120+/-35 microJy,
whereas CFHQS J2329-0301 is undetected at a similar noise level. J2329-0301 has
a star formation rate limit of <40 M_sun/yr, considerably below the typical
value at all redshifts for this bolometric luminosity. By comparison with hydro
simulations, we speculate that this quasar is observed at a relatively rare
phase where quasar feedback has effectively shut down star formation in the
host galaxy. [CII] emission is also detected only in J0210-0456. The ratio of
[CII] to far-infrared luminosity is similar to that of low redshift galaxies of
comparable luminosity, suggesting the previous finding of an offset in the
relationships between this ratio and far-infrared luminosity at low- and
high-redshift may be partially due to a selection effect from the limited
sensitivity of previous observations. The [CII] line of J0210-0456 is
relatively narrow (FWHM=189+/-18 km/s), indicating a dynamical mass
substantially lower than expected from the local black hole - velocity
dispersion correlation. The [CII] line is marginally resolved at 0.7"
resolution with the blue and red wings spatially offset by 0.5" (3 kpc) and a
smooth velocity gradient of 100 km/s across a scale of 6 kpc, possibly due to
rotation of a galaxy-wide disk. These observations are consistent with the idea
that stellar mass growth lags black hole accretion for quasars at this epoch
with respect to more recent times.Comment: 8 pages, 7 figures, ApJ in press, replaced with final versio
No evidence of "gray" dust from composite quasar spectra
Two recent studies based on composite reddened quasar spectra have indicated
the presence of `gray' dust in quasar environments. This gray dust has a
relatively flat extinction law in the UV, consistent with the theoretical
expectation of a lack of small dust grains close to a quasar. In contrast,
individual reddened quasars in the Sloan Digital Sky Survey tend to have steep
extinction laws in the UV, similar to that in the SMC. We analyze the method
used in determining extinction laws from composite quasar spectra in order to
resolve this discrepancy. We show that quasars reddened by SMC-type dust that
are present in quasar samples have a negative correlation between E_{B-V} and
redshift, due to selection effects. The fact that the highest redshift quasars
(which contribute to the UV part of a composite spectrum) are less extincted
leads to shallower extinction in the UV. We construct a composite quasar
spectrum from a simulated sample of quasars reddened by SMC-type dust and show
that the extinction curve derived from the composite does not recover the
intrinsic extinction law. We conclude there is no evidence of gray dust in
quasar environments.Comment: 4 pages, 2 figures, ApJL, in press. Minor typos corrected, title and
some text Americanize
Implications for unified schemes from submillimetre and far-infrared follow-up of radio-selected samples
We extend our previous analysis which used generalized luminosity functions
(GLFs) to predict the number of quasars and galaxies in
low-radio-frequency-selected samples as a function of redshift, radio
luminosity, narrow-emission-line luminosity and type of unified scheme. Our
extended analysis incorporates the observed submillimetre (850 micron) flux
densities of radio sources, employs a new method which allows us to deal with
non detections, and focuses on the high-luminosity population. First, we
conclude that the submillimetre luminosity L_{850} of low-frequency-selected
radio sources is correlated with the bolometric luminosity L_{Bol} of their
quasar nuclei via an approximate scaling relation L_{850} \propto L_{Bol}^{0.7
\pm 0.2}. Second, we conclude that there is quantitative evidence for a
receding-torus-like physical process for the high-luminosity population within
a two-population unified scheme for radio sources; this evidence comes from the
fact that radio quasars are brighter in both narrow emission lines and
submillimetre luminosity than radio galaxies matched in radio luminosity and
redshift. Third, we note that the combination of a receding-torus-like scheme
and the assumption that the observed submillimetre emission is dominated by
quasar-heated dust yields a scaling relation L_{850} \propto L_{Bol}^{0.5}
which is within the errors of that determined here for radio-selected quasars,
and consistent with that inferred for radio-quiet quasars by Willott, Rawlings
& Grimes (2003).Comment: 13 pages (including an appendix), 5 figures, to appear in MNRA
Extremely red galaxy counterparts to 7C radio sources
We present RIJHK imaging of seven radio galaxies from the 7C Redshift Survey
(7CRS) which lack strong emission lines and we use these data to investigate
their spectral energy distributions (SEDs) with models which constrain their
redshifts. Six of these seven galaxies have extremely red colours (R-K>5.5) and
we find that almost all of them lie in the redshift range 1<z<2. We also
present near-infrared spectroscopy of these galaxies which demonstrate that
their SEDs are not dominated by emission lines, although tentative lines,
consistent with H-alpha at z=1.45 and z=1.61, are found in two objects.
Although the red colours of the 7CRS galaxies can formally be explained by
stellar populations which are either very old or young and heavily reddened,
independent evidence favours the former hypothesis. At z~1.5 at least 1/4 of
powerful radio jets are triggered in massive (>L*) galaxies which formed the
bulk of their stars several Gyr earlier, that is at epochs corresponding to
redshifts z>5. If a similar fraction of all z~1.5 radio galaxies are old, then
extrapolation of the radio luminosity function shows that, depending on the
radio source lifetimes, between 10-100% of the near-IR selected extremely red
object (ERO) population undergo a radio outburst at epochs corresponding to
1<z<2. An ERO found serendipitously in the field of one of the 7CRS radio
sources appears to be a radio-quiet analogue of the 7CRS EROs with an emission
line likely to be [OII] at z=1.20. The implication is that some of the most
massive elliptical galaxies formed the bulk of their stars at z>5 and these
objects probably undergo at least two periods of AGN activity: one at high
redshift during which the black hole forms and another one at an epoch
corresponding to z~1.5.Comment: 20 pages, 10 figures, accepted for publication in MNRA
The Nature and Evolution of Classical Double Radio Sources from Complete Samples
We present a study of the trends in luminosity, linear size, spectral index,
and redshift of classical double radio sources from three complete samples
selected at successively fainter low radio-frequency flux-limits. We have been
able to decouple the effects of the tight correlation between redshift and
luminosity (inherent in any single flux-limited sample) which have hitherto
hindered interpretation of the relationships between these four source
properties. The major trends found are that (i) spectral indices increase with
linear size, (ii) rest-frame spectral indices have a stronger dependence on
luminosity than on redshift except at high (GHz) frequencies, and that (iii)
the linear sizes are smaller at higher redshifts. We reproduce the observed
dependences in a model for radio sources (born throughout cosmic time according
to a radio-source birth function) whose lobes are fed with a
synchrotron-emitting population (whose energy distribution is governed by
compact hotspots), and which suffer inverse Compton, synchrotron and adiabatic
expansion losses. In simulating the basic observed dependences, we find that
there is no need to invoke any systematic change in the environments of these
objects with redshift if the consequences of imposing a survey flux-limit on
our simulated datasets are properly included in the model. We present evidence
that for a radio survey there is an unavoidable `youth--redshift degeneracy',
even though radio sources are short-lived relative to the age of the Universe;
it is imperative to take this into account in studies which seemingly reveal
correlations of source properties with redshift such as the `alignment effect'.Comment: 48 pages, 19 figures, uses aas2pp4.sty. To appear in AJ. Also
available at http://www-astro.physics.ox.ac.uk/~kmb References updated and
minor typos correcte
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