303 research outputs found
Parsec-scale HI absorption structure in a low-redshift galaxy seen against a Compact Symmetric Object
We present global VLBI observations of the 21-cm transition of atomic
hydrogen seen in absorption against the radio source J0855+5751. The foreground
absorber (SDSS~J085519.05+575140.7) is a dwarf galaxy at = 0.026. As the
background source is heavily resolved by VLBI, the data allow us to map the
properties of the foreground HI gas with a spatial resolution of 2pc. The
absorbing gas corresponds to a single coherent structure with an extent
35pc, but we also detect significant and coherent variations, including a
change in the HI optical depth by a factor of five across a distance of
6pc. The large size of the structure provides support for the Heiles &
Troland model of the ISM, as well as its applicability to external galaxies.
The large variations in HI optical depth also suggest that caution should be
applied when interpreting measurements from radio-detected DLAs. In
addition, the distorted appearance of the background radio source is indicative
of a strong jet-cloud interaction in its host galaxy. We have measured its
redshift ( = 0.54186) using optical spectroscopy on the William Herschel
Telescope and this confirms that J0855+5751 is a FRII radio source with a
physical extent of 1kpc and supports the previous identification of this
source as a Compact Symmetric Object. These sources often show absorption
associated with the host galaxy and we suggest that both HI and OH should be
searched for in J0855+5751.Comment: 14 pages and 10 figures. Accepted for publication in MNRA
On the use of photometric redshifts for X-ray selected AGNs
(Abridged) In this paper we present photometric redshift estimates for a
sample of X-ray selected sources detected in the wide field (~2 deg^2), bright
[f_{X} (0.5-8 keV)~10^{-14} cgs] XMM-Newton/2dF survey. Unlike deeper X-ray
samples comprising a large fraction of sources with colours dominated by the
host galaxy, our bright survey primarily probes the QSO X-ray population.
Therefore photometric redshift methods employing both galaxy and QSO templates
need to be used. We employ the photometric redshift technique of
Hatziminaoglou, Mathez & Pello (2000) using 5-band photometry from the SDSS. We
separate our X-ray sources according to their optical profile to point-like and
extended. We apply QSO and galaxy templates to the point-like and extended
sources respectively. X-ray sources associated with Galactic stars are
identified and discarded from our point-like sample on the basis of their low
X-ray--to--optical flux ratio and their broad band colours that are best fit by
stellar templates. Comparison of our results with spectroscopic redshifts
available, allows calibration of our method and estimation of the photometric
redshift accuracy. For ~70 per cent of the point-like sources photometric
redshifts are correct within dz <= 0.3 (or ~75 per cent have dz/(1+z) <= 0.2),
and the rms scatter is estimated to be sigma_z = 0.30. For the optically
extended objects the photometric redshifts work only in the case of red (g - r
> 0.5 mag) sources yielding dz <= 0.15 and dz/(1+z) <= 0.2 for 73 and 93 per
cent respectively. However, we find that the above photometric redshift
technique does not work in the case of extended sources with blue colours (g -
r < 0.5): such sources cannot be fit successfully by QSO or galaxy templates,
or any linear combination of the two.Comment: Replaced due to extended revision; 11 pages, 4 figures; Accepted in
A&
On the multiplicity of ALMA Compact Array counterparts of far-infrared bright quasars
We present ALMA Atacama Compact Array (ACA) 870 micron continuum maps of 28
infrared-bright SDSS quasars with Herschel/SPIRE detections at redshifts 2-4,
the largest such sample ever observed with ALMA. The ACA detections are centred
on the SDSS coordinates to within 1 arcsec for about 80 per cent of the sample.
Larger offsets indicate that the far-infrared (FIR) emission detected by
Herschel might come from a companion source. The majority of the objects (about
70 per cent) have unique ACA counterparts within the SPIRE beam down to 3-4
arcsec resolution. Only 30 per cent of the sample shows clear evidence for
multiple sources with secondary counterparts contributing to the total 870
micron flux within the SPIRE beam to at least 25 per cent. We discuss the
limitations of the data based on simulated pairs of point-like sources at the
resolution of the ACA and present an extensive comparison of our findings with
recent works on the multiplicities of sub-millimetre galaxies. We conclude
that, despite the coarse resolution of the ACA, our data support the idea that,
for a large fraction of FIR-bright quasars, the sub-mm emission comes from
single sources. Our results suggest that, on average, optically bright quasars
with strong FIR emission are not triggered by early-stage mergers but are,
instead, together with their associated star formation rates, the outcome of
either late-stage mergers or secular processes.Comment: 16 pages, 10 figure
Smooth and Clumpy Dust Distribution in AGN: a Direct Comparison of two Commonly Explored Infrared Emission Models
The geometry of the dust distribution within the inner regions of Active
Galactic Nuclei (AGN) is still a debated issue and relates directly with the
AGN unified scheme. Traditionally, models discussed in the literature assume
one of two distinct dust distributions in what is believed to be a toroidal
region around the Supermassive Black Holes: a continuous distribution,
customarily referred to as smooth, and a concentration of dust in clumps or
clouds, referred to as clumpy.
In this paper we perform a thorough comparison between two of the most
popular models in the literature, namely the smooth models by Fritz. et al.
2006 and the clumpy models by Nenkova et al. 2008a, in their common parameters
space. Particular attention is paid to the silicate features at ~9.7 and ~18
micron, the width of the infrared bump, the near-infrared index and the
luminosity at 12.3 micron, all previously reported as possible diagnostic tools
to distinguish between the two dust distributions. We find that, due to the
different dust chemical compositions used in the two models, the behaviour of
the silicate features at 9.7 and 18 micron is quite distinct between the two
models. The width of the infrared bump and the peak of the infrared emission
can take comparable values, their distributions do, however, vary. The
near-infrared index is also quite different, due partly to the primary sources
adopted by the two models. Models with matched parameters do not produce
similar SEDs and virtually no random parameter combinations can result in
seemingly identical SEDs.Comment: 9 Pages, 6 Figures, 1 Table. Accepted for publication in MNRA
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