1,551 research outputs found
Volume-limited SDSS/FIRST quasars and the radio dichotomy
Much evidence has been presented in favor of and against the existence of two
distinct populations of quasars, radio-loud and radio-quiet. The SDSS differs
from earlier optically selected quasar surveys in the large number of quasars
and the targeting of FIRST radio source counterparts as quasar candidates. This
allows a qualitatively different approach of constructing a series of samples
at different redshifts which are volume-limited with respect to both radio and
optical luminosity. This technique avoids any biases from the strong evolution
of quasar counts with redshift and potential redshift-dependent selection
effects. We find that optical and radio luminosities of quasars detected in
both SDSS and FIRST are not well correlated within each redshift shell,
although the fraction of radio detections among optically selected quasars
remains roughly constant at 10% for z <= 3.2. The distribution in the
luminosity-luminosity plane does not appear to be strongly bimodal. The optical
luminosity function is marginally flatter at higher radio luminosities.Comment: 4 pages, to appear in ASP proceedings of "AGN physics with the SDSS",
Princeton 200
A Search for X-Ray Bright Distant Clusters of Galaxies
We present the results of a search for X--ray luminous distant clusters of
galaxies. We found extended X--ray emission characteristic of a cluster towards
two of our candidate clusters of galaxies. They both have a luminosity in the
ROSAT bandpass of and a redshift of ;
thus making them two of the most distant X--ray clusters ever observed.
Furthermore, we show that both clusters are optically rich and have a known
radio source associated with them. We compare our result with other recent
searches for distant X--ray luminous clusters and present a lower limit of
for the number density of such high redshift
clusters. This limit is consistent with the expected abundance of such clusters
in a standard (b=2) Cold Dark Matter Universe. Finally, our clusters provide
important high redshift targets for further study into the origin and evolution
of massive clusters of galaxies. Accepted for publication in the 10th September
1994 issue of ApJ.Comment: 20 pages Latex file + 1 postscript figure file appende
The Global Star Formation Rate from the 1.4 GHz Luminosity Function
The decimetric luminosity of many galaxies appears to be dominated by
synchrotron emission excited by supernova explosions. Simple models suggest
that the luminosity is directly proportional to the rate of supernova
explosions of massive stars averaged over the past 30 Myr. The proportionality
may be used together with models of the evolving 1.4 GHz luminosity function to
estimate the global star formation rate density in the era z < 1. The local
value is estimated to be 0.026 solar masses per year per cubic megaparsec, some
50% larger than the value inferred from the Halpha luminosity density. The
value at z ~ 1 is found to be 0.30 solar masses per year per cubic megaparsec.
The 10-fold increase in star formation rate density is consistent with the
increase inferred from mm-wave, far-infrared, ultra-violet and Halpha
observations.Comment: 10 pages, 2 figures, Astrophysical Journal Letters (in press); new PS
version has improved figure placemen
Optimising Spectroscopic and Photometric Galaxy Surveys: Efficient Target Selection and Survey Strategy
The next generation of spectroscopic surveys will have a wealth of
photometric data available for use in target selection. Selecting the best
targets is likely to be one of the most important hurdles in making these
spectroscopic campaigns as successful as possible. Our ability to measure dark
energy depends strongly on the types of targets that we are able to select with
a given photometric data set. We show in this paper that we will be able to
successfully select the targets needed for the next generation of spectroscopic
surveys. We also investigate the details of this selection, including
optimisation of instrument design and survey strategy in order to measure dark
energy. We use color-color selection as well as neural networks to select the
best possible emission line galaxies and luminous red galaxies for a
cosmological survey. Using the Fisher matrix formalism we forecast the
efficiency of each target selection scenario. We show how the dark energy
figures of merit change in each target selection regime as a function of target
type, survey time, survey density and other survey parameters. We outline the
optimal target selection scenarios and survey strategy choices which will be
available to the next generation of spectroscopic surveys.Comment: 16 pages, 22 figures, accepted to MNRAS in dec 201
The Luminosity Function for L>L* Galaxies at z > 3
Through use of multiband (U, B, R, I) photometry we have isolated high
redshift (3.0<z<3.5) galaxy candidates in a survey of 1.27 deg^2 to R = 21.25
and a survey of 0.02 deg^2 to R = 23.5. Our pool of candidates constrains the
nature of the 3.0 < z < 3.5 luminosity function over the range L* < L < 100 L*,
if we grant a similar level of completeness to these data as for very faint
samples (to R = 25.5) selected in a similar fashion. Our constraints agree with
the high redshift sky density at R = 20.5 estimated from Yee et al.'s (1996)
serendipitous discovery of a bright, z = 2.7 galaxy, as well as the density at
R ~ 23 by Steidel et al. (1996b). We strongly rule out -- by more than two
orders of magnitude at M(R) = -25 -- the L > L* luminosity function for z = 3-5
galaxies obtained by a photometric redshift analysis of the Hubble Deep Field
(HDF) by Gwyn & Hartwick (1996). Our results at R ~ 23 are more consistent with
the photometric redshift analysis of the faint HDF galaxies by Sawicki & Yee
(1996), but our present upper limits at the brightest magnitudes (R < 21.5,
M(R) < -24) allow more generous volume densities of these super-L* galaxies.Comment: Accepted for publication in ApJ Letters; 14 pages Latex, including 3
figure
Survey incompleteness and the evolution of the QSO luminosity function
We concentrate on a type of QSO survey which depends on selecting QSO candidates based on combinations of colors. Since QSO's have emission lines and power-law continua, they are expected to yield broadband colors unlike those of stellar photospheres. Previously, the fraction of QSO's expected to be hiding (unselected) within the locus of stellar (U-J, J-F) colors was estimated at about 15 percent. We have now verified that the KK88 survey is at least 11 percent incomplete, but have determined that it may be as much as 34 percent incomplete. The 'missing' QSO's are expected to be predominantly at z less than or = 2.2. We have studied the proper motion and variability properties of all stellar objects with J less than or = 22.5 or F less than or = 21.5 in the SA 57 field which has previously been surveyed with a multicolor QSO search by KK88
Optically Faint Microjansky Radio Sources
We report on the identifications of radio sources from our survey of the
Hubble Deep Field and the SSA13 fields, both of which comprise the deepest
radio surveys to date at 1.4 GHz and 8.5 GHz respectively. About 80% of the
microjansky radio sources are associated with moderate redshift starburst
galaxies or AGNs within the I magnitude range of 17 to 24 with a median of I =
22 mag. Thirty-one (20%) of the radio sources are: 1) fainter than 25 mag,
with two objects in the HDF 28.5, 2) often identified with very red
objects 4, and 3) not significantly different in radio properties than
the brighter objects. We suggest that most of these objects are associated with
heavily obscured starburst galaxies with redshifts between 1 and 3. However,
other mechanisms are discussed and cannot be ruled out with the present
observations.Comment: to appear in Astrophysical Journal Letters, 3 figures, 1 tabl
Onset of collective and cohesive motion
We study the onset of collective motion, with and without cohesion, of groups
of noisy self-propelled particles interacting locally. We find that this phase
transition, in two space dimensions, is always discontinuous, including for the
minimal model of Vicsek et al. [Phys. Rev. Lett. {\bf 75},1226 (1995)] for
which a non-trivial critical point was previously advocated. We also show that
cohesion is always lost near onset, as a result of the interplay of density,
velocity, and shape fluctuations.Comment: accepted for publication in Phys. Rev. Let
Radio Wavelength Constraints on the Sources of the Far Infrared Background
The cosmic far infrared background detected recently by the COBE-DIRBE team
is presumably due, in large part, to the far infrared (FIR) emission from all
galaxies. We take the well-established correlation between FIR and radio
luminosity for individual galaxies and apply it to the FIR background. We find
that these sources make up about half of the extragalactic radio background,
the other half being due to AGN. This is in agreement with other radio
observations, which leads us to conclude that the FIR-radio correlation holds
well for the very faint sources making up the FIR background, and that the FIR
background is indeed due to star-formation activity (not AGN or other possible
sources). If these star-forming galaxies have a radio spectral index between
0.4 and 0.8, and make up 40 to 60% of the extragalactic radio background, we
find that they have redshifts between roughly 1 and 2, in agreement with recent
estimates by Madau et al. of the redshift of peak star-formation activity. We
compare the observed extragalactic radio background to the integral over the
logN-logS curve for star-forming radio sources, and find that the slope of the
curve must change significantly below about 1 microjansky. At 1 microjansky,
the faint radio source counts predict about 25 sources per square arcminute,
and these will cause SIRTF to be confusion limited at 160micron.Comment: 10 pages including 1 figure, AASTeX, accepted by Ap
The Ensemble Variability Properties of Faint QSOs
A refined sample of 64 variable objects with stellar image structure has been
identified in SA 57 to , over a time baseline of 15 years, sampled
at 11 distinct epochs. The photometric data typically have a root-mean-square
error at of only 0.05 mag. Thirty-five quasars in this field have
already been spectroscopically confirmed, 34 of which are among the sample of
variables. Of the other variables, 6 are known spectroscopically to be stars,
10 additional objects are stars based on reliable detection of proper motion,
and 1 is spectroscopically a narrow-emission-line galaxy. Of the 13 remaining
variables, it is argued that they are a mixture of distant halo subdwarfs and
quasars with star-like colors. We compute the ensemble average structure
function and autocorrelation function from the light curves in the respective
quasar rest-frames, which are used to investigate the general dependences on
apparent magnitude, absolute magnitude, and redshift.Comment: 40 pages, uuencoded compressed postscipt with 8 figures, ApJ in
press, CRoNA 940
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