311 research outputs found
Faint Blue Galaxies as a Probe of the X-ray Background at High Redshift
We present a formalism describing the physical content of cross-correlation
functions between a diffuse background and a population of discrete sources.
The formalism is used to interpret cross-correlation signals between the
unresolved X-ray background and a galaxy population resolved to high redshift
in another spectral band. Specifically, we apply it to the so-called faint blue
galaxy population and constrain their X-ray emissivity and clustering
properties. A model is presented which satisfies the recently measured
constraints on all 3 correlation functions (galaxy/galaxy,
background/background and galaxy/background). This model predicts that faint
galaxies in the magnitude range B=18-23 (cvering redshifts z \lsim 0.5) make
up of the X-ray background in the 0.5-2 keV band. At the mean
redshift of the galaxy sample, , the comoving volume emissivity is
ergs sMpc . When extrapolated
to fainter magnitudes, the faint blue galaxy population can account for most of
the residual background at soft energy. We show how the measurement of the
angular and zero-lag cross-correlation functions between increasingly faint
galaxies and the X-ray background can allow us to map the X-ray emissivity as a
function of redshift.Comment: uuencoded compressed postscript, without figures. The preprint is
available with figures at http://www.ast.cam.ac.uk/preprint/PrePrint.htm
An Ultraviolet-Selected Galaxy Redshift Survey - II: The Physical Nature of Star Formation in an Enlarged Sample
We present further spectroscopic observations for a sample of galaxies
selected in the vacuum ultraviolet (UV) at 2000 \AA from the FOCA balloon-borne
imaging camera of Milliard et al. (1992). This work represents an extension of
the initial study of Treyer et al. (1998). Our enlarged catalogue contains 433
sources; 273 of these are galaxies, nearly all with redshifts z=0-0.4. Nebular
emission line measurements are available for 216 galaxies, allowing us to
address issues of reddening and metallicity. The UV and Halpha luminosity
functions strengthen our earlier assertions that the local volume-averaged star
formation rate is higher than indicated from earlier surveys. Moreover,
internally within our sample, we do not find a steep rise in the UV luminosity
density with redshift over 0<z<0.4. Our data is more consistent with a modest
evolutionary trend as suggested by recent redshift survey results. We find no
evidence for a significant number of AGN in our sample. We find the UV flux
indicates a consistently higher mean star formation rate than that implied by
the Halpha luminosity for typical constant or declining star formation
histories. Following Glazebrook et al. (1999), we interpret this discrepancy in
terms of a starburst model for our UV-luminous sources. Whilst we can explain
most of our observations in this way, there remains a small population with
extreme UV-optical colours which cannot be understood.Comment: 27 pages, 19 figures, accepted for publication in MNRA
On the Classification of UGC1382 as a Giant Low Surface Brightness Galaxy
We provide evidence that UGC1382, long believed to be a passive elliptical
galaxy, is actually a giant low surface brightness (GLSB) galaxy which rivals
the archetypical GLSB Malin 1 in size. Like other GLSB galaxies, it has two
components: a high surface brightness disk galaxy surrounded by an extended low
surface brightness (LSB) disk. For UGC1382, the central component is a
lenticular system with an effective radius of 6 kpc. Beyond this, the LSB disk
has an effective radius of ~38 kpc and an extrapolated central surface
brightness of ~26 mag/arcsec^2. Both components have a combined stellar mass of
~8x10^10 M_sun, and are embedded in a massive (10^10 M_sun) low-density (<3
M_sun/pc^2) HI disk with a radius of 110 kpc, making this one of the largest
isolated disk galaxies known. The system resides in a massive dark matter halo
of at least 2x10^12 M_sun. Although possibly part of a small group, its low
density environment likely plays a role in the formation and retention of the
giant LSB and HI disks. We model the spectral energy distributions and find
that the LSB disk is likely older than the lenticular component. UGC1382 has
UV-optical colors typical of galaxies transitioning through the green valley.
Within the LSB disk are spiral arms forming stars at extremely low
efficiencies. The gas depletion time scale of ~10^11 yr suggests that UGC1382
may be a very long term resident of the green valley. We find that the
formation and evolution of the LSB disk is best explained by the accretion of
gas-rich LSB dwarf galaxies.Comment: 17 pages, 16 figures, 4 tables; accepted to the Astrophysical Journa
Les enjeux de la conférence de Paris : penser autrement la question climatique
International audienc
Rice markets in Madagascar in disarray : Policy options for increased efficiency and price stabilization. Washington, World Bank.
Faced with a production shortfall in early 2004 and a sharp rise in the price of imported rice due to a depreciation of the Malagasy franc and a spike in international rice prices, Madagascar attempted to stabilize domestic rice prices through public tenders for rice imports and subsidized sales at official prices. This paper discusses the 2004 rice crisis, chronicling the events that triggered the crisis and the subsequent interventions by the government, and analyzes the impacts of the policies adopted and steps taken to spur development of the domestic rice market. Using a partial equilibrium model, the paper also quantifies the overall costs and benefits of a change in import duties for various household groups, and compares this intervention to a policy of targeted food transfers or security stocks. as well as other options for price stabilization, including a reduction in import duties and a security stock policy
CNN photometric redshifts in the SDSS at
We release photometric redshifts, reaching 0.7, for 14M galaxies
at in the 11,500 deg of the SDSS north and south galactic caps.
These estimates were inferred from a convolution neural network (CNN) trained
on stamp images of galaxies labelled with a spectroscopic redshift from
the SDSS, GAMA and BOSS surveys. Representative training sets of 370k
galaxies were constructed from the much larger combined spectroscopic data to
limit biases, particularly those arising from the over-representation of
Luminous Red Galaxies. The CNN outputs a redshift classification that offers
all the benefits of a well-behaved PDF, with a width efficiently signaling
unreliable estimates due to poor photometry or stellar sources. The dispersion,
mean bias and rate of catastrophic failures of the median point estimate are of
order , , on a representative test sample at ,
out-performing currently published estimates. The distributions in narrow
intervals of magnitudes of the redshifts inferred for the photometric sample
are in good agreement with the results of tomographic analyses. The inferred
redshifts also match the photometric redshifts of the redMaPPer galaxy clusters
for the probable cluster members. The CNN input and output are available at:
https://deepdip.iap.fr/treyer+2023.Comment: Submitted to MNRA
The Space Density of Extended Ultraviolet (XUV) Disks in the Local Universe and Implications for Gas Accretion on to Galaxies
We present results of the first unbiased search for extended UV (XUV)-disk
galaxies undertaken to determine the space density of such galaxies. Our sample
contains 561 local (0.001 < z < 0.05) galaxies that lie in the intersection of
available GALEX deep imaging (exposure time > 1.5 x 10^4 s) and SDSS DR7
footprints. We explore modifications to the standard classification scheme for
our sample that includes both disk- and bulge-dominated galaxies. Visual
classification of each galaxy in the sample reveals an XUV-disk frequency of up
to 20% for the most nearby portion of our sample. On average over the entire
sample (out to z=0.05) the frequency ranges from a hard limit of 4% to 14%. The
GALEX imaging allows us to detect XUV-disks beyond 100 Mpc. The XUV regions
around XUV-disk galaxies are consistently bluer than the main bodies. We find a
surprisingly high frequency of XUV emission around luminous red (NUV-r > 5) and
green valley (3 < NUV-r < 5) galaxies. The XUV-disk space density in the local
universe is > 1.5-4.2 x 10^-3 Mpc^-3. Using the XUV emission as an indicator of
recent gas accretion, we estimate that the cold gas accretion rate onto these
galaxies is > 1.7-4.6 x 10^-3 Msun Mpc^-3 yr^-1. The number of XUV-disks in the
green valley and the estimated accretion rate onto such galaxies points to the
intriguing possibility that 7%-18% of galaxies in this population are
transitioning away from the red sequence.Comment: 19 pages, 24 figures, ApJ in Pres
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