1,361 research outputs found
Maintaining Parameter Invariance in Censored Micro-Level Data
Research Methods/ Statistical Methods,
The Evolution of Dust in the Early Universe with Applications to the Galaxy SDSS J1148+5251
Dusty hyperluminous galaxies in the early universe provide unique
environments for studying the role of massive stars in the formation and
destruction of dust. At redshifts above ~ 6, when the universe was less than ~
1 Gyr old, dust could have only condensed in the explosive ejecta of Type II
supernovae (SNe), since most of the progenitors of the AGB stars, the major
alternative source of interstellar dust, did not have time to evolve off the
main sequence since the onset of star formation. In this paper we present
analytical models for the evolution of the gas, dust, and metals in high
redshift galaxies, with a special application to SDSS J1148+5251, a
hyperluminous quasar at z = 6.4. We find that an average supernova must
condense at least 1 Msun of dust to account for the observed dust mass in this
quasar. Observationally, it is in excess of the largest dust yield of ~0.02
Msun found thus far in the ejecta of any SN. If future observations find this
to be a typical supernova dust yield, then additional processes, such as
accretion onto preexisting grains, or condensation around the AGN will need to
be invoked to account for the large amount of dust in this and similar objects.
The galaxy's star formation history is still uncertain, and current
observations of the gas, metal, and dust contents of J1148 can be reproduced by
either an intensive and short burst of star formation (~ 1000 Msun/yr) with a
duration of ~ 100 Myr, or a much lower star formation rate (~ 100 Msun/yr)
occurring over the lifetime of the galaxy.Comment: 35 pages, 11 figures, accepted for publication in the Astrophysical
Journa
Mapping the interstellar medium in galaxies with Herschel/SPIRE
The standard method of mapping the interstellar medium in a galaxy, by observing the molecular gas in the CO 1-0 line and the atomic gas in the 21-cm line, is largely limited with current telescopes to galaxies in the nearby universe. In this letter, we use SPIRE observations of the galaxies
M99 and M100 to explore the alternative approach of mapping the interstellar medium using the continuum emission from the dust. We have compared the methods by measuring the relationship between the star-formation rate and the surface density of gas in the galaxies using both
methods. We find the two methods give relationships with a similar dispersion, confirming that observing the continuum emission from the dust is a promising method of mapping the interstellar medium in galaxies
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 Subillimeter Properties of Extremely Red Objects in the CUDSS Fields
We discuss the submillimeter properties of Extremely Red Objects (EROs) in
the two Canada-UK Deep Submillimeter Survey (CUDSS) Fields. We measure the mean
submillimeter flux of the ERO population (to K < 20.7) and find 0.4 +/- 0.07
mJy for EROs selected by (I-K) > 4.0 and 0.56 +/- 0.09 mJy for EROs selected by
(R-K) > 5.3 but, these measurements are dominated by discrete, bright
submillimeter sources. We estimate that EROs produce 7-11% of the far-infrared
background at 850um. This is substantially less than a previous measurement by
Wehner, Barger & Kneib (2002) and we discuss possible reasons for this
discrepancy. We show that ERO counterparts to bright submillimeter sources lie
within the starburst region of the near-infrared color-color plot of Pozzetti &
Mannucci (2000). Finally, we claim that pairs or small groups of EROs with
separations of < 10 arcseconds often mark regions of strong submillimeter flux.Comment: 9 pages, 8 figures, accepted for publication in Ap
No evidence for a `redshift cut-off' for the most powerful classical double radio sources
We use three samples (3CRR, 6CE and 6C*) to investigate the radio luminosity
function (RLF) for the `most powerful' low-frequency selected radio sources. We
find that the data are well fitted by a model with a constant co-moving space
density at high redshift as well as by one with a declining co-moving space
density above some particular redshift. This behaviour is very similar to that
inferred for steep-spectrum radio quasars by Willott et al (1998) in line with
the expectations of Unified Schemes. We conclude that there is as yet no
evidence for a `redshift cut-off' in the co-moving space densities of powerful
classical double radio sources, and rule out a cut-off at z < 2.5.Comment: To appear in `The Hy-redshift universe: Galaxy formation and
evolution at high redshift' eds. A.J. Bunker and W.J.M. van Breuge
The SCUBA Local Universe Galaxy Survey I: First Measurements of the Submillimetre Luminosity and Dust Mass Functions
We have used SCUBA to observe a complete sample of 104 galaxies selected at
60 microns from the IRAS BGS and we present here the 850 micron measurements.
Fitting the 60,100 and 850 micron fluxes with a single temperature dust model
gives the sample mean temperature T=36 K and beta = 1.3. We do not rule out the
possibility of dust which is colder than this, if a 20 K component was present
then our dust masses would increase by factor 1.5-3. We present the first
measurements of the luminosity and dust mass functions, which were well fitted
by Schechter functions (unlike those 60 microns). We have correlated many
global galaxy properties with the submillimetre and find that there is a
tendancy for less optically luminous galaxies to contain warmer dust and have
greater star formation efficiencies (cf. Young 1999). The average gas-to-dust
ratio for the sample is 581 +/- 43 (using both atomic and molecular hydrogen),
significantly higher than the Galactic value of 160. We believe this
discrepancy is due to a cold dust component at T < 20 K. There is a suprisingly
tight correlation between dust mass and the mass of molecular hydrogen as
estimated from CO measurements, with an intrinsic scatter of ~50%.Comment: 24 pages, 15 figures, 8 tables, accepted for publication in MNRA
Cosmology with redshift surveys of radio sources
We use the K-z relation for radio galaxies to illustrate why it has proved
difficult to obtain definitive cosmological results from studies based entirely
on catalogues of the brightest radio sources, e.g. 3C. To improve on this
situation we have been undertaking redshift surveys of complete samples drawn
from the fainter 6C and 7C radio catalogues. We describe these surveys, and
illustrate the new studies they are allowing. We also discuss our `filtered' 6C
redshift surveys: these have led to the discovery of a radio galaxy at z=4.4,
and are sensitive to similar objects at higher redshift provided the space
density of these objects is not declining too rapidly with z. There is
currently no direct evidence for a sharp decline in the space density of radio
galaxies for z > 4, a result only barely consistent with the observed decline
of flat-spectrum radio quasars.Comment: 8 pages Latex, To appear in the "Cosmology with the New Radio
Surveys" Conference - Tenerife 13-15 January 199
HST and UKIRT imaging observations of z ~ 1 6C radio galaxies - I. The data
The results of Hubble Space Telescope and UKIRT imaging observations are
presented for a sample of 11 6C radio galaxies with redshifts 0.85 < z < 1.5.
The observations of the 6C sources reveal a variety of different features,
similar to those observed around the higher luminosity of the aligned emission
appears less extreme in the case of the 6C radio galaxies. For both samples,
the aligned emission clearly cannot be explained by a single emission
mechanism; line emission and related nebular continuum emission, however, often
provide a significant contribution to the aligned emission.Comment: 17 pages, 11 figures (figs 3,6,11 low resolution - full resolution
images can be obtained from http://www.mrao.cam.ac.uk/~kji/ImagingFigs/).
Accepted for publication in MNRA
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
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