228 research outputs found
Optimal slit orientation for long multi-object spectroscopic exposures
Historically, long-slit spectroscopic observations were carried out using the
parallactic angle for the slit orientation if slit loss was an important
consideration (either to maximize the signal-to-noise or to do
spectrophotometry). This requires periodic realignment of the slit position
angle as the parallactic angle changes. This is not possible for multi-slit
observations where one slit position angle must be chosen for the entire
exposure. Common wisdom suggests using the parallactic angle at the meridian
(HA=0). In this paper, I examine what the best strategy is for long, multi-slit
exposures. I find that in extreme cases (very long exposure time) the best
choice is to orient the slit \emph{perpendicular} to the parallactic angle at
the meridian. There are two effects to consider: the increasing dispersion with
increasing airmass and the changing angle between the parallactic angle and the
slit. In the case of \emph{traditional} slit orientation, the two effects
amplify each other, thus rendering a significant fraction of the observation
useless. Using the perpendicular orientation, the two processes work against
each other, thus most of the observation remains useful. I will use, as an
example, our 8 hour Lockman Hole observations using the Keck telescope, but
generic methods are given to evaluate a particular observation. I also make the
tools available to the community.Comment: Accepted by A&A (20/06/2005
A deep VLA survey at 6 cm in the Lockman Hole
We have obtained a deep radio image with the Very Large Array at 6 cm in the
Lockman Hole. The noise level in the central part of the field is about 11
microJy. From these data we have extracted a catalogue of 63 radio sources. The
analysis of the radio spectral index suggests a flattening of the average radio
spectra and an increase of the population of flat spectrum radio sources in the
faintest flux bin. Cross correlation with the ROSAT/XMM X-ray sources list
yields 13 reliable radio/X-ray associations, corresponding to about 21 per cent
of the radio sample. Most of these associations (8 out of 13) are classified as
Type II AGN. Using optical CCD (V and I) and K' band data we found an optical
identification for 58 of the 63 radio sources. This corresponds to an
identification rate of about 92 per cent, one of the highest percentages so far
available. From the analysis of the colour-colour diagram and of the radio flux
- optical magnitude diagram we have been able to select a subsample of radio
sources whose optical counterparts are likely to be high redshift (z>0.5)
early-type galaxies, hosting an Active Galactic Nucleus responsible of the
radio activity. We also find evidence that at these faint radio limits a large
fraction (about 60 per cent) of the faintest optical counterparts (i.e. sources
in the magnitude range 22.5<I<24.5 mag) of the radio sources are Extremely Red
Objects (EROs) with I-K'>4.Comment: 20 pages, Astronomy and Astrophysics, accepte
A Deep VLA survey at 6 cm in the Lockman Hole
This is the published version, also available here: http://dx.doi.org/10.1051/0004-6361:20021721.We have obtained a deep radio image with the Very Large Array at 6 cm in the Lockman Hole. The noise level in the central part of the field is ~ Jy. From these data we have extracted a catalogue of 63 radio sources with a maximum distance of 10 arcmin from the field center and with peak flux density greater than 4.5 times the local rms noise. The differential source counts are in good agreement with those obtained by other surveys. The analysis of the radio spectral index suggests a flattening of the average radio spectra and an increase of the population of flat spectrum radio sources in the faintest flux bin. Cross correlation with the ROSAT/XMM X-ray sources list yields 13 reliable radio/X-ray associations, corresponding to ~21% of the radio sample. Most of these associations (8 out of 13) are classified as type II AGN.
Using optical CCD ( V and I) and band data with approximate limits of mag, mag and mag, we found an optical identification for 58 of the 63 radio sources. This corresponds to an identification rate of ~92%, one of the highest percentages so far available. From the analysis of the colour-colour diagram and of the radio flux - optical magnitude diagram we have been able to select a subsample of radio sources whose optical counterparts are likely to be high redshift ( z>0.5) early-type galaxies, hosting an Active Galactic Nucleus responsible of the radio activity. This class of objects, rather than a population of star-forming galaxies, appears to be the dominant population ( 50%) in a 5 GHz selected sample with a flux limit as low as 50 Jy.
We also find evidence that at these faint radio limits a large fraction (~60%) of the faintest optical counterparts (i.e. sources in the magnitude range 22.54$ and combining our radio data with existing ISO data we conclude that these EROs sources are probably associated with high redshift, passively evolving elliptical galaxies. The six radio selected EROs represent only ~2% of the optically selected EROs present in the field. If their luminosity is indeed a sign of AGN activity, the small number of radio detections suggests that a small fraction of the EROS population contains an active nucleus
Photometric Redshift of X-Ray Sources in the Chandra Deep Field South
Based on the photometry of 10 near-UV, optical, and near-infrared bands of
the Chandra Deep Field South, we estimate the photometric redshifts for 342
X-ray sources, which constitute ~99% of all the detected X-ray sources in the
field. The models of spectral energy distribution are based on galaxies and a
combination of power-law continuum and emission lines. Color information is
useful for source classifications: Type-I AGN show non-thermal spectral
features that are distinctive from galaxies and Type-II AGN. The hardness ratio
in X-ray and the X-ray-to-optical flux ratio are also useful discriminators.
Using rudimentary color separation techniques, we are able to further refine
our photometric redshift estimations. Among these sources, 137 have reliable
spectroscopic redshifts, which we use to verify the accuracy of photometric
redshifts and to modify the model inputs. The average relative dispersion in
redshift distribution is ~8%, among the most accurate for photometric surveys.
The high reliability of our results is attributable to the high quality and
broad coverage of data as well as the applications of several independent
methods and a careful evaluation of every source. We apply our redshift
estimations to study the effect of redshift on broadband colors and to study
the redshift distribution of AGN. Our results show that both the hardness ratio
and U-K color decline with redshift, which may be the result of a K-correction.
The number of Type-II AGN declines significantly at z>2 and that of galaxies
declines at z>1. However, the distribution of Type-I AGN exhibits less redshift
dependence. As well, we observe a significant peak in the redshift distribution
at z=0.6. We demonstrate that our photometric redshift estimation produces a
reliable database for the study of X-ray luminosity of galaxies and AGN.Comment: 40 pages, 11 figures. Accepted for publication in the Astrophysical
Journa
A Puzzling X-Ray Source Found in the chandra Deep Field South
In this letter we report the detection of an extremely strong X-ray emission
line in the 940ks chandra ACIS-I spectrum of CXO CDFS J033225.3-274219. The
source was identified as a Type1 AGN at redshift of z = 1.617, with 2.0 -- 10.0
keV rest frame X-ray luminosity of ~ 10^44 ergs s^-1. The emission line was
detected at 6.2^{+0.2}_{-0.1} keV, with an equivalent width (EW) of
4.4^{+3.2}_{-1.4} keV, both quantities referring to the observed frame. In the
rest frame, the line is at 16.2^{+0.4}_{-0.3} keV with an EW of
11.5^{+8.3}_{-3.7} keV. An X-ray emission line at similar energy (~ 17 keV,
rest frame) in QSO PKS 2149-306 was discovered before using ASCA data. We
reject the possibility that the line is due to a statistical or instrumental
artifact. The line is most likely due to blueshifted Fe-K emission from an
relativistic outflow, probably an inner X-ray jet, with velocities of the order
of ~ 0.6-0.7c. Other possible explanations are also discussed
XMM-Newton observation of the Lockman Hole II. Spectral analysis
We present the results of the X-ray spectral analysis of the first deep X-ray
survey with the XMM-Newton observatory during Performance Verification. We
restrict the analysis to the sample of 98 sources with more than 70 net counts
(flux limit in the [0.5-7] keV band of 1.6 10^{-15} erg cm^{-2} s^{-1}) of
which 61 have redshift identification. We find no correlation between the
spectral index Gamma and the intrinsic absorption column density N_H and, for
both the Type-1 and Type-2 AGN populations, we obtain ~2. The
progressive hardening of the mean X-ray source spectrum with decreasing flux is
essentially due to an increase in intrinsic absorption. The marked separation
between the two AGN populations in several diagnostics diagrams, involving
X-ray colour, X-ray flux, optical/near IR colour and optical brightness, is
also a consequence of different absorption column densities and enables the
classification of optically faint obscured AGN. About 27% of the subsample with
R-K' colour are EROs (R-K>5) and most of these 18 X-ray selected EROs contain
anobscured AGN as revealed by their high X-ray-to-optical/near IR flux ratios.
There are six sources in our sample with L_X[0.5-10]>10^44 erg s^{-1} and
log(N_H)>10^22 cm^{-2}: which are likely Type-2 QSOs and we thus derive a
density of ~69 objects of this class per square degree.Comment: Accepted by Astronomy & Astrophysics, 15 pages, 10 figure
A serendipitous survey for galaxy clusters by the XMM-Newton Survey Science Center
We describe the initial results of a programme to detect and identify
extended X-ray sources found serendipitously in XMM-Newton observations. We
have analysed 186 EPIC-PN images at high galactic latitude with a limiting flux
of \ergcms and found 62 cluster candidates. Thanks to the
enhanced sensitivity of the XMM-Newton telescopes, the new clusters found in
this pilot study are on the average fainter, more compact, and more distant
than those found in previous X-ray surveys. At our survey limit the surface
density of clusters is about 5 deg. We also present the first results of
an optical follow-up programme aiming at the redshift measurement of a large
sample of clusters. The results of this pilot study give a first glimpse on the
potential of serendipitous cluster science with XMM-Newton based on real data.
The largest, yet to be fulfilled promise is the identification of a large
number of high-redshift clusters for cosmological studies up to or 1.5.Comment: Proc World Space Conf. Houston, October 2002, Adv. Space Res., in
pres
A new wide pair of cool white dwarfs in the Solar neighbourhood
We report the discovery of a wide pair (93 arcsec angular separation) of
extremely cool ( K) white dwarfs with a very large common proper
motion ( arcsec/yr). The objects were discovered in a high proper
motion survey in the poorly investigated southern sky region with
using SuperCOSMOS Sky Survey (SSS) data. Both objects,
SSSPM J2231-7514 and SSSPM J2231-7515, show featureless optical spectra. Fits
of black-body models to the spectra yield effective temperatures of 3810 K and
3600 K, respectively for the bright (V=16.60) and faint (V=16.87) component.
Both degenerates are much brighter than other recent discoveries of cool white
dwarfs with comparable effective temperatures and/or colours.
Therefore, they should be relatively nearby objects. The comparison with other
cool white dwarfs and a photometric distance determination yield distance
estimates between 9 pc and 14 pc. The latter seems to be more realistic, since
the good agreement of the proper motion of both components within the errors of
about 8 mas/yr and the angular separation between the two stars support a
distance of about 15 pc with relatively small masses of the components. With
smaller distance we should be able to measure a differential proper motion due
to orbital motion if the orbital plane is not strongly inclined and the present
orbital velocity vector is not close to the line of sight. The space velocity
based on that distance and assumptions on radial velocity makes the new pair of
extremely cool white dwarfs some of the probably oldest members of the Galactic
disk population, although the possibility that these objects are part of a
Galactic halo dark matter component can also not yet be ruled out.Comment: 9 pages (with emulateapj5.sty), 3 figures, accepted for publication
in The Astrophysical Journa
The bright optical/NIR afterglow of the faint GRB 080710 - Evidence for a jet viewed off axis
We investigate the optical/near-infrared light curve of the afterglow of GRB
080710 in the context of rising afterglows. Optical and near-infrared
photometry was performed using the seven channel imager GROND and the
Tautenburg Schmidt telescope. X-ray data were provided by the X-ray Telescope
onboard the Swift satellite. The optical/NIR light curve of the afterglow of
GRB 080710 is dominated by an initial increase in brightness, which smoothly
turns over into a shallow power law decay. The initially rising achromatic
light curve of the afterglow of GRB 080710 can be accounted for with a model of
a burst viewed off-axis or a single jet in its pre deceleration phase and in an
on-axis geometry. An unified picture of the afterglow light curve and prompt
emission properties can be obtained with an off-axis geometry, suggesting that
late and shallow rising optical light curves of GRB afterglows might be
produced by geometric effects.Comment: 9 pages, 4 figures, accepted by A and
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