219 research outputs found
Chandra Snapshot Observations of Low-Luminosity AGNs with a Compact Radio Source
The results of Chandra snapshot observations of 11 LINERs (Low-Ionization
Nuclear Emission-line Regions), three low-luminosity Seyfert galaxies, and one
HII-LINER transition object are presented. Our sample consists of all the
objects with a flat or inverted spectrum compact radio core in the VLA survey
of 48 low-luminosity AGNs (LLAGNs) by Nagar et al. (2000). An X-ray nucleus is
detected in all galaxies except one and their X-ray luminosities are in the
range 5x10^38 to 8x10^41 erg/s. The X-ray spectra are generally steeper than
expected from thermal bremsstrahlung emission from an advection-dominated
accretion flow (ADAF). The X-ray to Halpha luminosity ratios for 11 out of 14
objects are in good agreement with the value characteristic of LLAGNs and more
luminous AGNs, and indicate that their optical emission lines are predominantly
powered by a LLAGN. For three objects, this ratio is less than expected.
Comparing with properties in other wavelengths, we find that these three
galaxies are most likely to be heavily obscured AGN. We use the ratio RX = \nu
L\nu (5 GHz)/LX, where LX is the luminosity in the 2-10 keV band, as a measure
of radio loudness. In contrast to the usual definition of radio loudness (RO =
L\nu(5 GHz)/L\nu(B)), RX can be used for heavily obscured (NH >~ 10^23 cm^-2,
AV>50 mag) nuclei. Further, with the high spatial resolution of Chandra, the
nuclear X-ray emission of LLAGNs is often easier to measure than the nuclear
optical emission. We investigate the values of RX for LLAGNs, luminous Seyfert
galaxies, quasars and radio galaxies and confirm the suggestion that a large
fraction of LLAGNs are radio loud.Comment: 15 pages, accepted for publication in Ap
Decomposition of AGN host galaxy images
We describe an algorithm to decompose deep images of Active Galactic Nuclei
into host galaxy and nuclear components. Currently supported are three galaxy
models: A de-Vaucouleurs spheroidal, an exponential disc, and a two-component
disc+bulge model. Key features of the method are: (semi-)analytic
representation of a possibly spatially variable point-spread function; full
two-dimensional convolution of the model galaxy using gradient-controlled
adaptive subpixelling; multiple iteration scheme. The code is computationally
efficient and versatile for a wide range of applications. The quantitative
performance is measured by analysing simulated imaging data. We also present
examples of the application of the method to small test samples of nearby
Seyfert 1 galaxies and quasars at redshifts z < 0.35.Comment: 12 pages, 15 figures, accepted for publication in MNRA
The Chandra High Energy Transmission Grating Observation of an X-ray Ionization Cone in Markarian 3
We present a preliminary analysis of the first high-resolution X-ray spectrum
of a Seyfert 2 galaxy, Mkn 3, obtained with the High Energy Transmission
Grating Spectrometer onboard the Chandra X-ray Observatory. The high-energy
spectrum (lambda < 4 Ang) is dominated by reflection of the AGN continuum
radiation in a cold optically thick medium and contains bright K-alpha
fluorescent lines from iron and silicon, as well as weak, blended lines from
sulfur and magnesium. The soft X-ray emission (4 < lambda < 23 Ang) is
spatially extended along the [O III] ionization cone and shows discrete
signatures of emission following recombination and photoexcitation produced in
a warm photoionized region. The measured iron L line fluxes indicate that
emission from collisionally ionized plasma is almost completely negligible, and
does not contribute significantly to the total energy budget of the X-ray
emission. We find that significant fractions of the H- and He-like resonance
lines, as well as the observed iron L lines are produced through re-emission
from the warm absorbing medium observed in Seyfert 1 galaxies. Its X-ray
spectral properties are qualitatively consistent with those of a typical
Seyfert 1 galaxy viewed at a different orientation, and provide further
convincing evidence for the existence of an obscured Seyfert 1 nucleus in Mkn
3.Comment: 16 pages, including 1 table and 2 figure
The origin of the Narrow Line Region of Mrk 3: an overpressured jet cocoon
We have obtained HST FOC long-slit optical spectroscopy of the Narrow Line
Region of the Seyfert 2 galaxy Mrk 3. In the region cospatial with the
radio-jet the velocity field is highly perturbed and shows two velocity systems
separated by as much as 1700 km/s. We interpret this to be the consequence of
the rapid expansion of a cocoon of hot gas, shocked and heated by the
radio-emitting outflow, which compresses and accelerates the ambient gas. The
NLR itself is essentially a cylindrical shell expanding supersonically. From
the size and velocity of the expanding region, we derive an upper limit to the
radio-source age, ~ 2
E42 erg/s required to inflate the cocoon and estimate that the jet minimum
advance speed is 3 E-3 pc per year. The total kinetic energy of the high
velocity NLR gas can be estimated as ~6 E54 erg, comparable to the total energy
carried by the jet over its lifetime and this quantitatively supports the idea
that the NLR gas is accelerated by the jet. If the advance speed of Mrk 3 is
representative of the Seyfert population then these sources must also be short
lived and probably recurrent. The jet kinetic luminosity of Mrk 3 is between 2
and 3 orders of magnitude smaller than that derived for radio-loud AGNs with
similar emission-line luminosity. On the other hand, the fraction of jet power
dissipated in radio-emission is similar. We speculate that the main distinction
between radio-quiet and radio-loud AGN is ascribed to a difference in jet power
rather than to a different efficiency in synchrotron emission production.Comment: 13 pages, 8 figures, Astrophysical Journal in pres
The Radio Quiescence of Active Galaxies with High Accretion Rates
We present 6 cm Very Large Array observations of the Greene & Ho (2004)
sample of 19 low-mass active galaxies with high accretion rates. This is one of
the only studies of a uniform sample of narrow-line Seyfert 1 (NLS1) galaxies
with such high sensitivity and resolution. Although we detect only one source,
the entire sample is very radio-quiet down to strong limits. GH10 was found to
have a radio power of 8.5 x 10^21 W/Hz, and a ratio R = f(6 cm)/f(4400 A) of
2.8. The 3 sigma upper limits for the remaining nondetections correspond to
radio powers from 3 x 10^20 to 8 x 10^21 W/Hz and 0.47 < R <9.9. Stacking all
nondetections yields an even stronger upper limit of R < 0.27. An assessment of
existing observations in the literature confirms our finding that NLS1s are
consistently radio-quiet, with a radio-loud fraction of 0%-6%, which is
significantly lower than the 10%-20% observed in the general quasar population.
By analogy with stellar-mass black holes, we argue that AGNs undergo a state
transition at L_bol/L_Edd~0.01. Below this value a radiatively inefficient
accretion flow effectively drives an outflow, which disappears when the flow
turns into an optically thick, geometrically thin disk, or a radiation
pressure-dominated slim disk at still higher L_bol/L_Edd.Comment: To appear in ApJ; 8 pages, 3 figures; uses emulateapj5.st
Platelet aggregation and risk of stent thrombosis or bleeding in interventionally treated diabetic patients with acute coronary syndrome
Direct measurement of the jet geometry in Seyfert galaxies
We demonstrate that, by combining optical, radio and X-ray observations of a
Seyfert, it is possible to provide a direct measurement of the angle
between the direction of the radio jet and the normal to the plane of the
spiral host galaxy. To do so, we make the assumptions that the inner radio jet
is perpendicular to the X-ray observed inner accretion disk, and that the
observed jet (or the stronger component, if the jet is two-sided) is physically
closer to Earth than the plane of the galaxy. We draw attention to the
possibility of measurement producing a result which is not self-consistent, in
which case for that galaxy, one of the assumptions must fail.Comment: 11 pages, 1 figure, accepted for publication in The Astrophysical
Journal Letter
Comparison of Nuclear Starburst Luminosities between Seyfert 1 and 2 Galaxies Based on Near-infrared Spectroscopy
We report on infrared K- (2-2.5 micron) and L-band (2.8-4.1 micron) slit
spectroscopy of 23 Seyfert 1 galaxies in the CfA and 12 micron samples. A
polycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 micron in the L
band is primarily used to investigate nuclear star-forming activity in these
galaxies. The 3.3 micron PAH emission is detected in 10 sources (=43%),
demonstrating that detection of nuclear star-formation in a significant
fraction of Seyfert 1 galaxies is now feasible. For the PAH-detected nuclei,
the surface brightness values of the PAH emission are as high as those of
typical starbursts, suggesting that the PAH emission probes the putative
nuclear starbursts in the dusty tori around the central active galactic nuclei
(AGNs). The magnitudes of the nuclear starbursts are quantitatively estimated
from the observed 3.3 micron PAH emission luminosities. The estimated starburst
luminosities relative to some indicators of AGN powers in these Seyfert 1s are
compared with 32 Seyfert 2s in the same samples that we have previously
observed. We find that there is no significant difference in nuclear starburst
to AGN luminosity ratios between Seyfert 1 and 2 galaxies, and that nuclear
starburst luminosity positively correlates with AGN power in both types of
Seyferts. Our results favor a slightly-modified AGN unification model, which
predicts that nuclear starbursts occurring in the dusty tori of Seyferts are
physically connected to the central AGNs, rather than the classical unification
paradigm, in which the dusty tori simply hide the central AGNs of Seyfert 2s
and reprocess AGN radiation as infrared dust emission in Seyferts. No
significant differences in nuclear star formation properties are recognizable
between Seyfert 1s in the CfA and 12 micron samples.Comment: 39 pages, 6 figures, Accepted for publication in ApJ (10 December
2004, vol 617 issue
The Seyfert-Starburst Connection in X-rays. II. Results and Implications
We present the results of X-ray imaging and spectroscopic analysis of a
sample of Seyfert 2 galaxies that contain starbursts, based on their optical
and UV characteristics. These composite galaxies exhibit extended, soft,
thermal X-ray emission, which we attribute to their starburst components.
Comparing their X-ray and far-infrared properties with ordinary Seyfert and
starburst galaxies, we identify the spectral characteristics of their various
intrinsic emission sources. The observed far-infrared emission of the composite
galaxies may be associated almost exclusively with star formation, rather than
the active nucleus. The ratio of the hard X-ray luminosity to the far-infrared
and [O III] 5007 luminosity distinguishes most of these composite galaxies from
``pure'' Seyfert 2 galaxies, while their total observed hard X-ray luminosity
distinguishes them from ``pure'' starbursts. The hard nuclear X-ray source is
generally heavily absorbed (N_H > 10^{23} cm^{-2}) in the composite galaxies.
Based on these results, we suggest that the interstellar medium of the nuclear
starburst is a significant source of absorption. The majority of the sample are
located in groups or are interacting with other galaxies, which may trigger the
starburst or allow rapid mass infall to the central black hole, or both. We
conclude that starbursts are energetically important in a significant fraction
of active galaxies, and starbursts and active galactic nuclei may be part of a
common evolutionary sequence.Comment: 16 pages including 8 figures and 5 tables; to appear in the ApJ, Mar.
10, 200
Multicolour Optical Imaging of IR-Warm Seyfert Galaxies. V. Morphologies and Interactions. Challenging the Orientation Model
This paper is the last in a series, investigating the optical properties of a
sample of mid-IR Warm Seyfert galaxies and of a control sample of mid-IR cold
galaxies. In the present paper we parametrize the morphologies and interaction
properties of the host galaxies and combine these with the major conclusions in
our previous papers. Our results confirm that nuclear activity is linked to
galactic interactions. We suggest an alternative view for the simple
orientation-obscuration model postulated for Seyfert types 1 and 2, that takes
into account the time evolution of their environmental and morphological
properties. Within this view, an evolutionary link between starburst-dominated
and AGN-dominated IR emission is also suggested, to account for the
observational discriminator (mid-IR excess) between our Warm and Cold samples.Comment: 24 pages, including 6 figures and 3 tables (figure 5 included as
independent file), Submitted to Ap
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