334 research outputs found
The HELLAS2XMM survey: IV. Optical identifications and the evolution of the accretion luminosity in the Universe
We present results from the photometric and spectroscopic identification of
122 X-ray sources recently discovered by XMM-Newton in the 2-10 keV band (the
HELLAS2XMM 1dF sample). Their flux cover the range 8E-15-4E-13 cgs and the
total area surveyed is 0.9 deg2. About 20% of the hard X-ray selected sources
have an X-ray to optical flux ratio (X/O) ten times or more higher than that of
optically selected AGN. Unlike the faint sources found in the ultra-deep
Chandra and XMM-Newton surveys, which reach X-ray (and optical) fluxes more
than one order of magnitude lower than the HELLAS2XMM survey sources, many of
the extreme X/O sources in our sample have R<=25 and are therefore accessible
to optical spectroscopy. We report the identification of 13 sources with
X/O>10: 8 are narrow line QSO (i.e. QSO2), four are broad line QSO. We use a
combined sample of 317 hard X-ray selected sources (HELLAS2XMM 1dF, CDFN 1Msec,
SSA13 and Lockman Hole flux limited samples), 221 with measured z, to evaluate
the cosmological evolution of the hard X-ray source's number and luminosity
densities. Looking backward in time, the low luminosity sources (logL(2-10keV)
= 43-44 erg/s) increase in number at a rate different than the high luminosity
sources (logL(2-10keV)>44.5 erg/s), reaching a maximum around z=1 and then
levelling off beyond z=2. This translates into an accretion driven luminosity
density which is dominated by sources with logL(2-10keV) < 44.5 erg/s up to at
least z=1, while the contribution of the same sources and of those with
logL(2-10keV)>44.5 erg/s appear to be comparable between z=2 and 4.Comment: v2, minor changes, A&A in pres
Very Red and Extremely Red Galaxies in the Fields of z ~ 1.5 Radio-Loud Quasars
We previously identified an excess of mostly red galaxies around 31 RLQs at
z=1-2. These fields have an ERO (extremely red object, R-K>6) density 2.7 times
higher than the field. Assuming the EROs are passively evolved galaxies at the
quasar redshifts, they have characteristic luminosities of only ~L^*. We also
present new observations of four z~1.54 RLQ fields: (1) Wide-field J & Ks data
confirm an Abell richness ~2 excess within 140" of Q0835+580 but an excess only
within 50" of Q1126+101. (2) In 3 fields we present deep narrow-band redshifted
H-alpha observations. We detect five candidate galaxies at the quasar
redshifts, a surface density 2.5x higher than the field. (3) SCUBA sub-mm
observations of 3 fields detect 2 quasars and 2 galaxies with SEDs best fit as
highly reddened galaxies at the quasar z. (4) H-band adaptive optics (AO)
imaging is used to estimate redshifts for 2 red, bulge-dominated galaxies using
the Kormendy relation. Both have structural redshifts foreground to the quasar,
but these are not confirmed by photometric redshifts, possibly because their
optical photometry is corrupted by scattered light from the AO guidestar. (5)
We use quantitative SED fits to constrain the photometric redshifts z_ph for
some galaxies. Most galaxies near Q0835+580 are consistent with being at its
redshift, including a candidate very old passively evolving galaxy. Many very &
extremely red objects have z_ph z_q, and dust reddening is required to fit most
of them, including many objects whose fits also require relatively old stellar
populations. Large reddenings of E(B-V)~0.6 are required to fit four J-K
selected EROs, though all but one of them have best-fit z_ph>z_q. These objects
may represent a population of dusty high-z galaxies underrepresented in
optically selected samples. (Abridged)Comment: Missing object 1126.424 added to Table 4; title changed to save
people the apparent trouble of reading the abstract. 38 pages, 16 figures, 2
in color; all-PostScript figure version available from
http://astro.princeton.edu/~pathall/tp3.ps.g
The usage of Stromgren photometry in studies of Local Group Dwarf Spheroidal Galaxies - Application to Draco: a new catalogue of Draco members and a study of the metallicity distribution function and radial gradients
In this paper we demonstrate how Stromgren uvby photometry can be efficiently
used to: 1. Identify red giant branch stars that are members in a dwarf
spheroidal galaxy. 2. Derive age-independent metallicities for the same stars
and quantify the associated errors. Stromgren uvby photometry in a 11 x 22
arcmin field centered on the Draco dwarf spheroidal galaxy was obtained using
the Isaac Newton Telescope on La Palma. Members of the Draco dSph galaxy were
identified using the surface gravity sensitive c_1 index which discriminates
between red giant and dwarf stars. Thus enabling us to distinguish the (red
giant branch) members of the dwarf spheroidal galaxy from the foreground dwarf
stars in our galaxy. The method is evaluated through a comparison of our
membership list with membership classifications in the literature based on
radial velocities and proper motions. The metallicity sensitive m_1 index was
used to derive individual and age-independent metallicities for the members of
the Draco dSph galaxy. The derived metallicities are compared to studies based
on high resolution spectroscopy and the agreement is found to be very good. We
present metallicities for 169 members of the red giant branch in the Draco
dwarf spheroidal galaxy (the largest sample to date). The metallicity
distribution function for the Draco dSph galaxy shows a mean [Fe/H] = -1.74 dex
with a spread of 0.24 dex. The correlation between metallicity and colour for
the stars on the red giant branch is consistent with a dominant old, and coeval
population. There is a possible spatial population gradient over the field with
the most metal-rich stars being more centrally concentrated than the metal-poor
stars.Comment: Accepted for publication in A&A. 26 pages, 27 figures (some at
reduced resolution). High resolution version available at
http://www.astro.lu.se/~daniel/draco/faria.p
Galactic Kinematics Towards the South Galactic Pole. First Results from the Yale-San Juan Southern Proper-Motion Program
The predictions from a Galactic Structure and Kinematic model code are
compared to the color counts and absolute proper-motions derived from the
Southern Proper-Motion survey covering more than 700 toward the South
Galactic Pole in the range . The theoretical assumptions
and associated computational procedures, the geometry for the kinematic model,
and the adopted parameters are presented in detail and compared to other
Galactic Kinematic models of its kind. The data to which the model is compared
consists of more than 30,000 randomly selected stars, and it is best fit by
models with a solar peculiar motion of +5 km s in the V-component
(pointing in the direction of Galactic rotation), a large LSR speed of 270 km
s, and a (disk) velocity ellipsoid that always points towards the
Galactic center. The absolute proper-motions in the U-component indicate a
solar peculiar motion of km s, with no need for a local
expansion or contraction term. The fainter absolute motions show an indication
that the thick-disk must exhibit a rather steep velocity gradient of about -36
km s kpc with respect to the LSR. We are not able to set
constraints on the overall rotation for the halo, nor on the thick-disk or halo
velocity dispersions. Some substructure in the U & V proper-motions could be
present in the brighter bins , and it might be indicative
of (disk) moving groups.Comment: 24 double-column pages, 12 tables, AAS Latex macros v4.0, 19 B&W
figures, 1 color figure. Accepted for publication on The Astronomical Journa
A single low-energy, iron-poor supernova as the source of metals in the star SMSS J 031300.36-670839.3
The element abundance ratios of four low-mass stars with extremely low
metallicities indicate that the gas out of which the stars formed was enriched
in each case by at most a few, and potentially only one low-energy, supernova.
Such supernovae yield large quantities of light elements such as carbon but
very little iron. The dominance of low-energy supernovae is surprising, because
it has been expected that the first stars were extremely massive, and that they
disintegrated in pair-instability explosions that would rapidly enrich galaxies
in iron. What has remained unclear is the yield of iron from the first
supernovae, because hitherto no star is unambiguously interpreted as
encapsulating the yield of a single supernova. Here we report the optical
spectrum of SMSS J031300.36- 670839.3, which shows no evidence of iron (with an
upper limit of 10^-7.1 times solar abundance). Based on a comparison of its
abundance pattern with those of models, we conclude that the star was seeded
with material from a single supernova with an original mass of ~60 Mo (and that
the supernova left behind a black hole). Taken together with the previously
mentioned low-metallicity stars, we conclude that low-energy supernovae were
common in the early Universe, and that such supernovae yield light element
enrichment with insignificant iron. Reduced stellar feedback both chemically
and mechanically from low-energy supernovae would have enabled first-generation
stars to form over an extended period. We speculate that such stars may perhaps
have had an important role in the epoch of cosmic reionization and the chemical
evolution of early galaxies.Comment: 28 pages, 6 figures, Natur
The Dynamical Equilibrium of Galaxy Clusters
If a galaxy cluster is effectively in dynamical equilibrium then all galaxy
populations within the cluster must have distributions in velocity and position
that individually reflect the same underlying mass distribution, although the
derived virial masses can be quite different. Specifically, within the CNOC
cluster sample the virial radius of the red galaxy population is, on the
average, a factor of smaller than that of the blue population.
The red galaxies also have a smaller RMS velocity dispersion, a factor of within our sample. Consequently, the virial mass calculated from the
blue galaxies is times larger than from the red galaxies.
However, applying the Jeans equation of stellar-hydrodynamical equilibrium to
the red and blue subsamples separately give statistically identical cluster
mass profiles. This is strong evidence that these clusters are effectively
equilibrium systems, and therefore empirically demonstrates that the masses in
the virialized region are reliably estimated using dynamical techniques.Comment: Submitted for publication in ApJLetts. 12 pages as a uufile, also
available at http://manaslu.astro.utoronto.ca/~carlberg/cnoc/br/br.ps.g
The VVDS type-1 AGN sample: The faint end of the luminosity function
In a previous paper (Gavignaud et al. 2006), we presented the type-1 Active
Galactic Nuclei (AGN) sample obtained from the first epoch data of the
VIMOS-VLT Deep Survey (VVDS). The sample consists of 130 faint, broad-line AGN
with redshift up to z=5 and 17.5< I <24.0, selected on the basis of their
spectra. In this paper we present the measurement of the Optical Luminosity
Function up to z=3.6 derived from this sample, we compare our results with
previous results from brighter samples both at low and at high redshift. Our
data, more than one magnitude fainter than previous optical surveys, allow us
to constrain the faint part of the luminosity function up to high redshift. By
combining our faint VVDS sample with the large sample of bright AGN extracted
from the SDSS DR3 (Richards et al., 2006b) and testing a number of different
evolutionary models, we find that the model which better represents the
combined luminosity functions, over a wide range of redshift and luminosity, is
a luminosity dependent density evolution (LDDE) model, similar to those derived
from the major X-surveys. Such a parameterization allows the redshift of the
AGN space density peak to change as a function of luminosity and explains the
excess of faint AGN that we find at 1.0< z <1.5. On the basis of this model we
find, for the first time from the analysis of optically selected samples, that
the peak of the AGN space density shifts significantly towards lower redshift
going to lower luminosity objects. This result, already found in a number of
X-ray selected samples of AGN, is consistent with a scenario of "AGN cosmic
downsizing", in which the density of more luminous AGN, possibly associated to
more massive black holes, peaks earlier in the history of the Universe, than
that of low luminosity ones.Comment: 13 pages, 10 figures, submitted to A&
Strategies for prompt searches for GRB afterglows: the discovery of the GRB 001011 optical/near-infrared counterpart using colour-colour selection
We report the discovery of the optical and near-infrared counterparts to GRB
001011. The GRB 001011 error box determined by Beppo-SAX was simultaneously
imaged in the near-infrared by the 3.58-m New Technology Telescope and in the
optical by the 1.54-m Danish Telescope ~8 hr after the gamma-ray event. Here we
implement the colour-colour discrimination technique proposed by Rhoads (2001)
and extend it using near-IR data as well. We present the results provided by an
automatic colour-colour discrimination pipe-line developed to discern the
different populations of objects present in the GRB 001011 error box. Our
software revealed three candidates based on single-epoch images. Second-epoch
observations carried out ~3.2 days after the burst revealed that the most
likely candidate had faded, thus identifying it with the counterpart to the
GRB. In deep R-band images obtained 7 months after the burst a faint
(R=25.38+/-0.25) elongated object, presumably the host galaxy of GRB 001011,
was detected at the position of the afterglow. The GRB 001011 afterglow is the
first discovered with the assistance of colour-colour diagram techniques. We
discuss the advantages of using this method and its application to error boxes
determined by future missions.Comment: Accepted for publication in Astronomy and Astrophysics, 13 pages, 16
figure
Sustainability and Substitution of Exhaustible Natural Resources: How Resource Prices Affect Long-term R&D Investments
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