952 research outputs found
An Analysis of Private School Closings
We add to the small literature on private school supply by exploring exits of K-12 private schools. We find that the closure of private schools is not an infrequent event, and use national survey data from the National Center for Education Statistics to study closures of private schools. We assume that the probability of an exit is a function of excess supply of private schools over the demand, as well as the school's characteristics such as age, size, and religious affiliation. Our empirical results generally support the implications of the model. Working Paper 07-0
Molecular Gas in Infrared Ultraluminous QSO Hosts
We report CO detections in 17 out of 19 infrared ultraluminous QSO (IR QSO)
hosts observed with the IRAM 30m telescope. The cold molecular gas reservoir in
these objects is in a range of 0.2--2.1 (adopting a
CO-to- conversion factor ). We find that the molecular gas properties of IR QSOs,
such as the molecular gas mass, star formation efficiency () and the CO (1-0) line widths, are indistinguishable
from those of local ultraluminous infrared galaxies (ULIRGs). A comparison of
low- and high-redshift CO detected QSOs reveals a tight correlation between
L and for all QSOs. This suggests that,
similar to ULIRGs, the far-infrared emissions of all QSOs are mainly from dust
heated by star formation rather than by active galactic nuclei (AGNs),
confirming similar findings from mid-infrared spectroscopic observations by
{\it Spitzer}. A correlation between the AGN-associated bolometric luminosities
and the CO line luminosities suggests that star formation and AGNs draw from
the same reservoir of gas and there is a link between star formation on
kpc scale and the central black hole accretion process on much smaller scales.Comment: 30 pages, 9 figures, accepted for publication in The Astrophysical
Journa
Civilian casualties and public support for military action: Experimental evidence
In contrast to the expansive literature on military casualties and support for war, we know very little about public reactions to foreign civilian casualties. This article, based on representative sample surveys in the US and Britain, reports four survey experiments weaving information about civilian casualties into vignettes about Western military action. These produce consistent evidence of civilian casualty aversion: where death tolls were higher, support for force was invariably and significantly lower. Casualty effects were moderate in size but robust across our two cases and across different scenarios. They were also strikingly resistant to moderation by other factors manipulated in the experiments, such as the framing of casualties or their religious affiliation. The importance of numbers over even strongly humanizing frames points towards a utilitarian rather than a social-psychological model of casualty aversion. Either way, civilian casualties deserve a more prominent place in the literature on public support for war
Using VO tools to investigate distant radio starbursts hosting obscured AGN in the HDF(N) region
A 10-arcmin field around the HDF(N) contains 92 radio sources >40 uJy,
resolved by MERLIN+VLA at 0".2-2".0 resolution. 55 have Chandra X-ray
counterparts including 18 with a hard X-ray photon index and high luminosity
characteristic of a type-II (obscured) AGN. >70% of the radio sources have been
classified as starbursts or AGN using radio morphologies, spectral indices and
comparisons with optical appearance and MIR emission. Starbursts outnumber
radio AGN 3:1. This study extends the VO methods previously used to identify
X-ray-selected obscured type-II AGN to investigate whether very luminous radio
and X-ray emission originates from different phenomena in the same galaxy. The
high-redshift starbursts have typical sizes of 5--10 kpc and star formation
rates of ~1000 Msun/yr. There is no correlation between radio and X-ray
luminosities nor spectral indices at z>~1.3. ~70% of both the radio-selected
AGN and the starburst samples were detected by Chandra. The X-ray luminosity
indicates the presence of an AGN in at least half of the 45 cross-matched radio
starbursts, of which 11 are type-II AGN including 7 at z>1.5. This distribution
overlaps closely with the X-ray detected radio sources which were also detected
by SCUBA. Stacked 1.4-GHz emission at the positions of radio-faint X-ray
sources is correlated with X-ray hardness. Most extended radio starbursts at
z>1.3 host X-ray selected obscured AGN. Radio emission from most of these
ultra-luminous objects is dominated by star formation but it contributes less
than 1/3 of their X-ray luminosity. Our results support the inferences from
SCUBA and IR data, that at z>1.5, star formation is an order of magnitude more
extended and more copious, it is closely linked to AGN activity and it is
triggered differently, compared with star formation at lower redshifts.Comment: 24 pages, 12 figures, uses graphicx, rotating, natbib, supertabular
packages and aa.cls. Accepted for publication in A&
The Unusual Infrared Object HDF-N J123656.3+621322
We describe an object in the Hubble Deep Field North with very unusual
near-infrared properties. It is readily visible in Hubble Space Telescope
NICMOS images at 1.6um and from the ground at 2.2um, but is undetected (with
signal-to-noise <~ 2) in very deep WFPC2 and NICMOS data from 0.3 to 1.1um. The
f_nu flux density drops by a factor >~ 8.3 (97.7% confidence) from 1.6 to
1.1um. The object is compact but may be slightly resolved in the NICMOS 1.6um
image. In a low-resolution, near-infrared spectrogram, we find a possible
emission line at 1.643um, but a reobservation at higher spectral resolution
failed to confirm the line, leaving its reality in doubt. We consider various
hypotheses for the nature of this object. Its colors are unlike those of known
galactic stars, except perhaps the most extreme carbon stars or Mira variables
with thick circumstellar dust shells. It does not appear to be possible to
explain its spectral energy distribution as that of a normal galaxy at any
redshift without additional opacity from either dust or intergalactic neutral
hydrogen. The colors can be matched by those of a dusty galaxy at z >~ 2, by a
maximally old elliptical galaxy at z >~ 3 (perhaps with some additional
reddening), or by an object at z >~ 10 whose optical and 1.1um light have been
suppressed by the intergalactic medium. Under the latter hypothesis, if the
luminosity results from stars and not an AGN, the object would resemble a
classical, unobscured protogalaxy, with a star formation rate >~ 100 M_sun/yr.
Such UV-bright objects are evidently rare at 2 < z < 12.5, however, with a
space density several hundred times lower than that of present-day L* galaxies.Comment: Accepted for publication in the Astrophysical Journal. 27 pages,
LaTeX, with 7 figures (8 files); citations & references updated + minor
format change
Millimetre observations of a sample of high-redshift obscured quasars
We present observations at 1.2 mm with MAMBO-II of a sample of z>~2
radio-intermediate obscured quasars, as well as CO observations of two sources
with the Plateau de Bure Interferometer. Five out of 21 sources (24%) are
detected at a significance of >=3sigma. Stacking all sources leads to a
statistical detection of = 0.96+-0.11 mJy and stacking only the
non-detections also yields a statistical detection, with = 0.51+-0.13
mJy. This corresponds to a typical far-infrared luminosity L_FIR~4x10^12 Lsol.
If the far-infrared luminosity is powered entirely by star-formation, and not
by AGN-heated dust, then the characteristic inferred star-formation rate is
~700 Msol yr-1. This far-infrared luminosity implies a dust mass of
M_dust~3x10^8 Msol. We estimate that such large dust masses on kpc scales can
plausibly cause the obscuration of the quasars. We present dust SEDs for our
sample and derive a mean SED for our sample. This mean SED is not well fitted
by clumpy torus models, unless additional extinction and far-infrared
re-emission due to cool dust are included. There is a hint that the host
galaxies of obscured quasars must have higher far-infrared luminosities and
cool-dust masses and are therefore often found at an earlier evolutionary phase
than those of unobscured quasars. For one source at z=2.767, we detect the
CO(3-2) transition, with S_CO Delta nu=630+-50 mJy km s-1, corresponding to
L_CO(3-2)= 3.2x10^7 Lsol, or L'_CO(3-2)=2.4x10^10 K km s-1 pc2. For another
source at z=4.17, the lack of detection of the CO(4-3) line yields a limit of
L'_CO(4-3)<1x10^10 K km s-1 pc2. Molecular gas masses, gas depletion timescales
and gas-to-dust ratios are estimated (Abridged).Comment: Accepted by ApJ, 25 pages, 11 figures, 4 table
BLAST: the Redshift Survey
The Balloon-borne Large Aperture Submillimeter Telescope (BLAST) has recently
surveyed ~8.7 deg^2 centered on GOODS-South at 250, 350, and 500 microns. In
Dye et al. (2009) we presented the catalogue of sources detected at 5-sigma in
at least one band in this field and the probable counterparts to these sources
in other wavebands. In this paper, we present the results of a redshift survey
in which we succeeded in measuring redshifts for 82 of these counterparts. The
spectra show that the BLAST counterparts are mostly star-forming galaxies but
not extreme ones when compared to those found in the Sloan Digital Sky Survey.
Roughly one quarter of the BLAST counterparts contain an active nucleus. We
have used the spectroscopic redshifts to carry out a test of the ability of
photometric redshift methods to estimate the redshifts of dusty galaxies,
showing that the standard methods work well even when a galaxy contains a large
amount of dust. We have also investigated the cases where there are two
possible counterparts to the BLAST source, finding that in at least half of
these there is evidence that the two galaxies are physically associated, either
because they are interacting or because they are in the same large-scale
structure. Finally, we have made the first direct measurements of the
luminosity function in the three BLAST bands. We find strong evolution out to
z=1, in the sense that there is a large increase in the space-density of the
most luminous galaxies. We have also investigated the evolution of the
dust-mass function, finding similar strong evolution in the space-density of
the galaxies with the largest dust masses, showing that the luminosity
evolution seen in many wavebands is associated with an increase in the
reservoir of interstellar matter in galaxies.Comment: Accepted for publication in the Astrophysical Journal. Maps and
associated results are available at http://blastexperiment.info
Unveiling the Nature of Submillimeter Galaxy SXDF850.6
We present an 880 micron Submillimeter Array (SMA) detection of the
submillimeter galaxy SXDF850.6. SXDF850.6 is a bright source (S(850 micron) = 8
mJy) detected in the SCUBA Half Degree Extragalactic Survey (SHADES), and has
multiple possible radio counterparts in its deep radio image obtained at the
VLA. Our new SMA detection finds that the submm emission coincides with the
brightest radio emission that is found ~8" north of the coordinates determined
from SCUBA. Despite the lack of detectable counterparts in deep UV/optical
images, we find a source at the SMA position in near-infrared and longer
wavelength images. We perform SED model fits to UV-optical-IR photometry (u, B,
V, R, i', z', J, H, K, 3.6 micron, 4.5 micron, 5.8 micron, and 8.0 micron) and
to submm-radio photometry (850 micron, 880 micron, 1100 micron, and 21 cm)
independently, and we find both are well described by starburst templates at a
redshift of z ~= 2.2 (+/- 0.3). The best-fit parameters from the UV-optical-IR
SED fit are a redshift of z = 1.87 (+0.15/-0.07), a stellar mass of M_star =
2.5 +2.2/-0.3 x 10^11 M_sun, an extinction of A_V = 3.0 (+0.3/-1.0) mag, and an
age of 720 (+1880/-210) Myr. The submm-radio SED fit provides a consistent
redshift of z ~ 1.8-2.5, an IR luminosity of L_IR = (7-26) x 10^12 L_sun, and a
star formation rate of 1300-4500 M_sun/yr. These results suggest that SXDF850.6
is a mature system already having a massive amount of old stellar population
constructed before its submm bright phase and is experiencing a dusty
starburst, possibly induced by major mergers.Comment: 7 pages, 5 figures, Accepted for publication in Astrophysical Journa
The physical scale of the far-infrared emission in the most luminous submillimetre galaxies II: evidence for merger-driven star formation
We present high-resolution 345 GHz interferometric observations of two
extreme luminous (L_{IR}>10^{13} L_sun), submillimetre-selected galaxies (SMGs)
in the COSMOS field with the Submillimeter Array (SMA). Both targets were
previously detected as unresolved point-sources by the SMA in its compact
configuration, also at 345 GHz. These new data, which provide a factor of ~3
improvement in resolution, allow us to measure the physical scale of the
far-infrared in the submillimetre directly. The visibility functions of both
targets show significant evidence for structure on 0.5-1 arcsec scales, which
at z=1.5 translates into a physical scale of 5-8 kpc. Our results are
consistent with the angular and physical scales of two comparably luminous
objects with high-resolution SMA followup, as well as radio continuum and CO
sizes. These relatively compact sizes (<5-10 kpc) argue strongly for
merger-driven starbursts, rather than extended gas-rich disks, as the preferred
channel for forming SMGs. For the most luminous objects, the derived sizes may
also have important physical consequences; under a series of simplifying
assumptions, we find that these two objects in particular are forming stars
close to or at the Eddington limit for a starburst.Comment: 9 pages, 3 Figures, submitted to MNRA
A Study of 3CR Radio Galaxies from z = 0.15 to 0.65. II. Evidence for an Evolving Radio Structure
Radio structure parameters were measured from the highest quality radio maps
available for a sample of 3CR radio galaxies in the redshift range 0.15 < z <
0.65. Combined with similar data for quasars in the same redshift range, these
morphology data are used in conjunction with a quantification of the richness
of the cluster environment around these objects (the amplitude of the
galaxy-galaxy spatial covariance function, Bgg) to search for indirect evidence
of a dense intracluster medium (ICM). This is done by searching for confinement
and distortions of the radio structure that are correlated with Bgg.
Correlations between physical size and hot spot placement with Bgg show
evidence for an ICM only at z 0.4,
suggesting an epoch of z ~ 0.4 for the formation of an ICM in these Abell
richness class 0-1, FR2-selected clusters. X-ray selected clusters at
comparable redshifts, which contain FR1 type sources exclusively, are
demonstrably richer than the FR2-selected clusters found in this study. The
majority of the radio sources with high Bgg values at z < 0.4 can be described
as ``fat doubles'' or intermediate FR2/FR1s. The lack of correlation between
Bgg and bending angle or Bgg and lobe length asymmetry suggests that these
types of radio source distortion are caused by something other than interaction
with a dense ICM. Thus, a large bending angle cannot be used as an unambiguous
indicator of a rich cluster around powerful radio sources. These results
support the hypothesis made in Paper 1 that cluster quasars fade to become
FR2s, then FR1s, on a timescale of 0.9 Gyrs (for H0 = 50 km s^-1 Mpc^-1).Comment: 44 pages, 8 figures, 2 tables; to be published in the September 2002
issue of The Astronomical Journa
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