587 research outputs found
A Chandra Observation of M51: Active Nucleus and Nuclear Outflows
We present a Chandra ACIS-S observation of the nuclear region of the nearby
spiral galaxy M51 (NGC 5194), which has a low-luminosity Seyfert 2 nucleus. The
X-ray image shows the nucleus, southern extranuclear cloud, and northern loop,
the morphology of the extended emission being very similar to those seen in
radio continuum and optical emission line images. The X-ray spectrum of the
nucleus is well represented by a model consisting of soft thermal plasma with
kT ~0.5 keV, a very hard continuum, and an Fe Kalpha emission line at 6.45 keV
with an equivalent width of >2 keV. The very strong Fe line and the flat
continuum indicate that the nucleus is obscured by a column density in excess
of 10^24 cm^-2 and the spectrum is dominated by reflected emission from cold
matter near the nucleus. The X-ray spectra of the extranuclear clouds are well
fitted by a thermal plasma model with kT ~0.5 keV. This spectral shape and
morphology strongly suggest that the clouds are shock heated by the bi-polar
outflow from the nucleus. The shock velocities of the extranuclear cloud and
northern loop inferred from the temperatures of the X-ray gas are 690 km/s and
660 km/s, respectively. By assuming a steady-state situation in which the
emission of the extranuclear clouds is powered by the jets, the mechanical
energy in the jets is found to be comparable to the bolometric luminosity of
the nucleus.Comment: 7 pages, 5 figures, Accepted for publication in the Astrophyscal
Jouna
High resolution X-ray spectroscopy and imaging of Mrk573
We present a detailed analysis of the XMM-Newton RGS high resolution X-ray
spectra of the Seyfert 2 galaxy, Mrk573. This analysis is complemented by the
study of the Chandra image, and its comparison to optical (HST) and radio (VLA)
data. The soft X-ray emission is mainly due to gas photoionised by the central
AGN, as indicated by the detection of radiative recombination continua from
OVII and OVIII, as well as by the prominence of the OVII forbidden line. This
result is confirmed by the best fit obtained with a self-consistent CLOUDY
photoionisation model. However, a collisionally excited component is also
required, in order to reproduce the FeXVII lines, accounting for about 1/3 of
the total luminosity in the 15-26 A band. Once adopted the same model in the
Chandra ACIS data, another photoionised component, with higher ionisation
parameter, is needed to take into account emission from higher Z metals. The
broadband ACIS spectrum also confirms the Compton-thick nature of the source.
The imaging analysis shows the close morphological correspondence between the
soft X-ray and the [OIII] emission. The radio emission appears much more
compact, although clearly aligned with the narrow line region. The collisional
phase of the soft X-ray emission may be due to starburst, requiring a star
formation rate of M yr, but there is no clear
evidence of this kind of activity from other wavelengths. On the other hand, it
may be related to the radio ejecta, responsible for the heating of the plasma
interacting with the outflow, but the estimated pressure of the hot gas is much
larger than the pressure of the radio jets, assuming equipartition and under
reasonable physical parameters.Comment: 12 pages, 6 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society Main Journa
Physical Conditions of Accreting Gas in T Tauri Star Systems
We present results from a low resolution (R~300) near-infrared spectroscopic
variability survey of actively accreting T Tauri stars (TTS) in the
Taurus-Auriga star forming region. Paschen and Brackett series H I
recombination lines were detected in 73 spectra of 15 classical T Tauri
systems. The values of the Pan/PaB, Brn/BrG, and BrG/Pan H I line ratios for
all observations exhibit a scatter of < 20% about the weighted mean, not only
from source to source, but also for epoch-to-epoch variations in the same
source. A representative or `global' value was determined for each ratio in
both the Paschen and Brackett series as well as the BrG/Pan line ratios. A
comparison of observed line ratio values was made to those predicted by the
temperature and electron density dependent models of Case B hydrogen
recombination line theory. The measured line ratios are statistically well-fit
by a tightly constrained range of temperatures (T < 2000 K) and electron
densities 1e9 < n_e < 1e10 cm^-3. A comparison of the observed line ratio
values to the values predicted by the optically thick and thin local
thermodynamic equilibrium cases rules out these conditions for the emitting H I
gas. Therefore, the emission is consistent with having an origin in a non-LTE
recombining gas. While the range of electron densities is consistent with the
gas densities predicted by existing magnetospheric accretion models, the
temperature range constrained by the Case B comparison is considerably lower
than that expected for accreting gas. The cooler gas temperatures will require
a non-thermal excitation process (e.g., coronal/accretion-related X-rays and UV
photons) to power the observed line emission.Comment: 12 pages, emulateapj format, Accepted for publication in Ap
A Chandra Observation of the Circinus Galaxy
We report on a recent Chandra ACIS-S observation of the Circinus galaxy.
These observations confirm that the nuclear spectrum results from reflection of
a hard X-ray continuum by ``neutral'' matter. The nuclear X-ray emission is
extended by \sim 60 pc in the general direction of the optical ``ionization
cone''. An image in the Fe K\alpha line has been made and shows that this
emission extends up to 200 pc from the nucleus. There is also large-scale X-ray
emission both along and perpendicular to the galaxy disk. Thermal plasma models
for this extended gas indicate temperatures kT \sim 0.6 keV, though cooler
photoionized gas is also possible. The X-ray emission from gas in the disk is
probably associated with the starburst ring of radius 150--250 pc. The gas
extending \sim 600 pc perpendicular to the disk is closely correlated with the
high-excitation optical-line emission. In addition to its soft X-ray emission,
we tentatively detect a hard component from the gas above the plane; this hard
emission may represent nuclear X-rays scattered into our line of sight by
electrons in the outflowing wind. Ten compact sources are found in the central
kpc of the galaxy. The most luminous has an X-ray luminosity of \simeq 10^{40}
erg s^{-1} and seems to be an X-ray binary in the Circinus galaxy with a
black-hole mass exceeding 80 M_{\odot}.Comment: 25 pages, 13 figures. Accepted for publication in The Astrophysical
Journa
Parsec-Scale Images of Flat-Spectrum Radio Sources in Seyfert Galaxies
We present high angular resolution (~2 mas) radio continuum observations of
five Seyfert galaxies with flat-spectrum radio nuclei, using the VLBA at 8.4
GHz. The goal of the project is to test whether these flat-spectrum cores
represent thermal emission from the accretion disk, as inferred previously by
Gallimore et al. for NGC 1068, or non-thermal, synchrotron self-absorbed
emission, which is believed to be responsible for more powerful, flat-spectrum
nuclear sources in radio galaxies and quasars. In four sources (T0109-383, NGC
2110, NGC 5252, Mrk 926), the nuclear source is detected but unresolved by the
VLBA, indicating brightness temperatures in excess of 10^8 K and sizes, on
average, less than 1 pc. We argue that the radio emission is non-thermal and
synchrotron self-absorbed in these galaxies, but Doppler boosting by
relativistic outflows is not required. Synchrotron self-absorption brightness
temperatures suggest intrinsic source sizes smaller than ~0.05-0.2 pc, for
these four galaxies, the smallest of which corresponds to a light-crossing time
of ~60 light days or 10^4 gravitational radii for a 10^8 M_sun black hole.
We also present MERLIN and VLA observations of NGC 4388, which was undetected
by the VLBA, and argue that the observed, flat-spectrum, nuclear radio emission
in this galaxy represents optically thin, free-free radiation from dense
thermal gas on scales ~0.4 to a few pc. It is notable that the two Seyfert
galaxies with detected thermal nuclear radio emission (NGC 1068 and NGC 4388)
both have large X-ray absorbing columns, suggesting that columns in excess of
\~10^{24} cm^{-2} are needed for such disks to be detectable. (Abridged)Comment: 36 pages including 5 tables and 4 figures; accepted for publication
in Ap
The Spitzer Extragalactic Representative Volume Survey (SERVS): The Environments of High-z SDSS Quasi-Stellar-Objects
This paper presents a study of the environments of SDSS Quasi-Stellar-Objects
(QSOs) in the Spitzer Extragalactic Representative Volume Survey (SERVS). We
concentrate on the high-redshift QSOs as these have not been studied in large
numbers with data of this depth before. We use the IRAC 3.6-4.5{\mu}m colour of
objects and ancillary r-band data to filter out as much foreground
contamination as possible. This technique allows us to find a significant (>
4-{\sigma}) over-density of galaxies around QSOs in a redshift bin centred on z
~ 2.0 and a (> 2-{\sigma}) over-density of galaxies around QSOs in a redshift
bin centred on z ~ 3.3. We compare our findings to the predictions of a
semi-analytic galaxy formation model, based on the {\Lambda}CDM millennium
simulation, and find for both redshift bins that the model predictions match
well the source-density we have measured from the SERVS data.Comment: 13 pages, 12 figures, Accepted by Ap
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