264 research outputs found
Do sub-mm sources and quasars form an evolutionary sequence?
The high redshift sub-mm sources discovered by SCUBA are widely believed to
represent the dust-enshrouded formation of massive elliptical galaxies. Given
the strong evidence for a link between the formation of the spheroid and the
growth of the central black hole, one might expect to see a luminous quasar at
the nucleus of every SCUBA source. Somewhat surprisingly, however, only a very
small fraction (~5%) are detected by Chandra with quasar luminosities. In this
paper I discuss some of the implications of these results and discuss the
accumulating evidence that sub-mm sources and quasars may represent different
stages in the evolutionary sequence of a massive proto-spheroid.Comment: 4 pages, 2 figures. Astron. Nachrichten, in press. Proceedings of the
"X-ray Surveys in the light of new observatories" workshop, 4-6 September
2002, Santander, Spai
The Search for AGN in Distant Galaxy Clusters
We are undertaking the first systematic study of the prevalence of AGN
activity in a large sample of high redshift galaxy clusters. Local clusters
contain mainly red elliptical galaxies, and have little or no luminous AGN
activity. However, recent studies of some moderate to high redshift clusters
have revealed significant numbers of luminous AGN within the cluster. This
effect may parallel the Butcher-Oemler effect - the increase in the fraction of
blue galaxies in distant clusters compared to local clusters. Our aim is to
verify and quantify recent evidence that AGN activity in dense environments
increases with redshift, and to evaluate the significance of this effect. As
cluster AGN are far less prevalent than field sources, a large sample of over
120 cluster fields at z > 0.1 has been selected from the Chandra archives and
is being analysed for excess point sources. The size of the excess, the radial
distribution and flux of the sources and the dependence of these on cluster
redshift and luminosity will reveal important information about the triggering
and fueling of AGN.Comment: 2 pages, to appear in proceedings of 'Multi-wavelength AGN surveys',
Cozumel, 200
Do nuclear starbursts obscure the X-ray background?
We propose a model for the source of the X-ray background (XRB) in which low
luminosity active nuclei (L<10^43 erg/s) are obscured by nuclear starbursts
within the inner 100pc. The obscuring material covers most of the sky as seen
from the central source, rather than being distributed in a toroidal structure,
and hardens the averaged X-ray spectrum by photoelectric absorption. The gas is
turbulent with velocity dispersion of a few 100 km/s and cloud-cloud collisions
lead to copious star formation. Although supernovae tend to produce outflows,
most of the gas is trapped in the gravity field of the starforming cluster
itself and the central black hole. A hot (T=10^6-10^7 K) virialised phase of
this gas, comprising a few per cent of the total obscuring material, feeds the
central engine of 10^7 solar masses through Bondi accretion, at a sub-Eddington
rate appropriate for the luminosity of these objects. If starburst-obscured
objects give rise to the residual XRB, then only 10 per cent of the accretion
in active galaxies occurs close to the Eddington limit in unabsorbed objects.Comment: 5 pages, 2 PS figures included in the text, MNRAS in the press. Also
at http://www.ifca.unican.es/~barcons/preprints.htm
Short time-scale optical variability of the dwarf Seyfert nucleus in NGC 4395
We present optical spectroscopic observations of the least-luminous known
Seyfert 1 galaxy, NGC 4395, which was monitored every half-hour over the course
of 3 nights. The continuum emission varied by ~35 per cent over the course of 3
nights, and we find marginal evidence for greater variability in the blue
continuum than the red. A number of diagnostic checks were performed on the
data in order to constrain any systematic or aperture effects. No correlations
were found that adequately explained the observed variability, hence we
conclude that we have observed real intrinsic variability of the nuclear
source. No simultaneous variability was measured in the broad H-beta line,
although given the difficulty in deblending the broad and narrow components it
is difficult to comment on the significance of this result. The observed short
time-scale continuum variability is consistent with NGC 4395 having an
intermediate-mass (~10^5 solar masses) central supermassive black hole, rather
than a very low accretion rate. Comparison with the Seyfert 1 galaxy NGC 5548
shows that the observed variability seems to scale with black hole mass in
roughly the manner expected in accretion models. However the absolute
time-scale of variability differs by several orders of magnitude from that
expected in simple accretion disc models in both cases.Comment: 16 pages, 14 figures, 5 tables, accepted for publication in MNRA
Rest-frame ultra-violet spectra of massive galaxies at z=3: evidence of high-velocity outflows
Galaxy formation models invoke the presence of strong feedback mechanisms
that regulate the growth of massive galaxies at high redshifts. In this paper
we aim to: (1) confirm spectroscopically the redshifts of a sample of massive
galaxies selected with photometric redshifts z > 2.5; (2) investigate the
properties of their stellar and interstellar media; (3) detect the presence of
outflows, and measure their velocities. To achieve this, we analysed deep,
high-resolution (R~2000) FORS2 rest-frame UV spectra for 11 targets. We
confirmed that 9 out of 11 have spectroscopic redshifts z > 2.5. We also
serendipitously found two mask fillers at redshift z > 2.5, which originally
were assigned photometric redshifts 2.0 < z < 2.5. In the four highest-quality
spectra we derived outflow velocities by fitting the absorption line profiles
with models including multiple dynamical components. We found strongly
asymmetric, high-ionisation lines, from which we derived outflow velocities
ranging from 480 to 1518 km/s. The two galaxies with highest velocity show
signs of AGN. We revised the spectral energy distribution fitting U-band
through 8 micron photometry, including the analysis of a power-law component
subtraction to identify the possible presence of active galactic nuclei (AGN).
The revised stellar masses of all but one of our targets are >1e10 Msun, with
four having stellar masses > 5e10 Msun. Three galaxies have a significant
power-law component in their spectral energy distributions, which indicates
that they host AGN. We conclude that massive galaxies are characterised by
significantly higher velocity outflows than the typical Lyman break galaxies at
z ~ 3. The incidence of high-velocity outflows (~40% within our sample) is also
much higher than among massive galaxies at z < 1, which is consistent with the
powerful star formation and nuclear activity that most massive galaxies display
at z > 2.Comment: 17 pages, 14 figures, Accepted for publication in A&
AXJ1749+684: a narrow emission-line galaxy with a flat X-ray spectrum
We report the serendipitous detection of an X-ray source, AXJ1749+684, with
the ASCA Gas Imaging Spectrometer. AXJ1749+684 is identified with a
LINER/starburst-type spiral galaxy KUG 1750+683A at a redshift z = 0.05. It has
a hard X-ray spectrum, consistent with that of the X-ray background (XRB) in
the 1-10 keV band. Despite the optical classification, the X-ray luminosity
cannot be explained by starburst activity. Combined with spatial variations in
the optical emission line ratios, this suggests the presence of an obscured
Seyfert nucleus embedded within a starforming galaxy. Similar behaviour could
explain the ambiguous properties of the faint narrow-line X-ray galaxies
(NLXGs) emerging from deep X-ray surveys.Comment: MNRAS Letters in press, 6 pages, 7 figures in MNRAS LaTex styl
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