910 research outputs found
Plausible fluorescent Ly-alpha emitters around the z=3.1 QSO0420-388
We report the results of a survey for fluorescent Ly-alpha emission carried
out in the field surrounding the z=3.1 quasar QSO0420-388 using the FORS2
instrument on the VLT. We first review the properties expected for fluorescent
Ly-alpha emitters, compared with those of other non-fluorescent Ly-alpha
emitters. Our observational search detected 13 Ly-alpha sources sparsely
sampling a volume of ~14000 comoving Mpc^3 around the quasar. The properties of
these in terms of i) the line equivalent width, ii) the line profile and iii)
the value of the surface brightness related to the distance from the quasar,
all suggest that several of these may be plausibly fluorescent. Moreover, their
number is in good agreement with the expectation from theoretical models. One
of the best candidates for fluorescence is sufficiently far behind QSO0420-388
that it would imply that the quasar has been active for (at least) ~60 Myrs.
Further studies on such objects will give information about proto-galactic
clouds and on the radiative history (and beaming) of the high-redshift quasars.Comment: 10 pages, 4 figures.Update to match the version published on ApJ 657,
135, 2007 March
An extreme, blueshifted iron line profile in the Narrow Line Seyfert 1 PG 1402+261; an edge-on accretion disk or highly ionized absorption?
We report on a short XMM-Newton observation of the radio-quiet Narrow Line
Seyfert 1 PG 1402+261. The EPIC X-ray spectrum of PG 1402+261 shows a strong
excess of counts between 6-9 keV in the rest frame. This feature can be modeled
by an unusually strong (equivalent width 2 keV) and very broad (FWHM velocity
of 110000 km/s) iron K-shell emission line. The line centroid energy at 7.3 keV
appears blue-shifted with respect to the iron Kalpha emission band between
6.4-6.97 keV, while the blue-wing of the line extends to 9 keV in the quasar
rest frame. The line profile can be fitted by reflection from the inner
accretion disk, but an inclination angle of >60 deg is required to model the
extreme blue-wing of the line. Furthermore the extreme strength of the line
requires a geometry whereby the hard X-ray emission from PG 1402+261 above 2
keV is dominated by the pure-reflection component from the disk, while little
or none of the direct hard power-law is observed. Alternatively the spectrum
above 2 keV may instead be explained by an ionized absorber, if the column
density is sufficiently high (N_H > 3 x 10^23 cm^-2) and if the matter is
ionized enough to produce a deep (tau~1) iron K-shell absorption edge at 9 keV.
This absorber could originate in a large column density, high velocity outflow,
perhaps similar to those which appear to be observed in several other high
accretion rate AGN. Further observations, especially at higher spectral
resolution, are required to distinguish between the accretion disk reflection
or outflow scenarios.Comment: Accepted for publication in ApJ (18 pages, 5 figures, 1 table
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
A Survey for H2O Megamasers III. Monitoring Water Vapor Masers in Active Galaxies
We present single-dish monitoring of the spectra of 13 extragalactic water
megamasers taken over a period of 9 years and a single epoch of sensitive
spectra for 7 others. Our data include the first K-band science observations
taken with the new 100 m Green Bank Telescope (GBT). In the context of a
circumnuclear, molecular disk model, our results suggest that either (a) the
maser lines seen are systemic features subject to a much smaller acceleration
than present in NGC 4258, presumably because the gas is farther from the
nuclear black hole, or (b) we are detecting ``satellite'' lines for which the
acceleration is in the plane of the sky.
We also report a search for water vapor masers towards the nuclei of 58
highly inclined, nearby galaxies.Comment: accepted by ApJ
The Circum-Galactic Environment of Bright IRAS Galaxies
This paper studies systematically, for the first time, the circumgalactic
environment of bright IRAS galaxies as defined by Soifer et al. (1989). While
the role of gravitational interaction for luminous and ultraluminous IRAS
galaxies has been well established by various studies, the situation is by far
more obscure in the IR luminosity range of the bright IRAS sample, 10^{10}Lsol
< Lfir < 10^{11} Lsol. To easily identify nearby companion galaxies, the bright
IRAS sample was restricted to 87 objects with redshift range 0.008 < z < 0.018
and galactic latitude > 30^{o}. A control sample, selected from the Center for
Astrophysics redshift survey catalogue, includes 90 objects matching the Bright
IRAS sample for distribution of isophotal diameter, redshift, and morphological
type. From a search of nearby companion galaxies within 250 Kpc on the
second-generation Digitized Sky Survey (DSS-II), we found that the
circumgalactic environment of the Bright IRAS galaxies contains more large
companions than the galaxies in the optically selected control sample, and is
similar to that of Seyfert 2 galaxies. We found a weak correlation over a wide
range of far IR luminosity (10^9 Lsol < Lfir < 10^{12.5}Lsol) between projected
separation and Lfir, which confirms a very close relationship between star
formation rate of a galaxy and the strength of gravitational perturbations. We
also find that the far IR colors depend on whether a source is isolated or
interacting. Finally, we discuss the intrinsic difference and evolution
expectations for the bright IRAS galaxies and the control sample, as well as
the relationship between starbursting and active galaxies.Comment: 10 pages, 5 figs, 2 tables. Accepted for publication in Ap
On the Effects of Dissipative Turbulence on the Narrow Emission-Line Ratios in Seyfert Galaxies
We present a photoionization model study of the effects of micro-turbulence
and dissipative heating on emission lines for number and column densities,
elemental abundances, and ionizations typical for the narrow emission line
regions (NLRs) of Seyfert galaxies. Earlier studies of NLR spectra generally
found good agreement between the observations and the model predictions for
most strong emission lines, such as [O III] 5007, [O II]
3727, [N II] 6583, [Ne III] 3869, and the H and He
recombination lines. Nevertheless, the strengths of lines from species with
ionization potentials greater than that of He(54.4 eV), e.g. N and
Ne, were often under-predicted. Among the explanations suggested for
these discrepancies were (selectively) enhanced elemental abundances and
contributions from shock heated gas. Interestingly, the NLR lines have widths
of several 100 km s, well in excess of the thermal broadening. If this
is due to micro-turbulence, and the turbulence dissipates within the
emission-line gas, the gas can be heated in excess of that due to
photoionization. We show that the combined effects of turbulence and
dissipative heating can strongly enhance N V 1240 (relative to He II
1640), while the heating alone can boost the strength of [Ne V]
3426. We suggest that this effect is present in the NLR, particularly
within 100 pc of the central engine. Finally, since micro-turbulence
would make clouds robust against instabilities generated during acceleration,
it is not likely to be a coincidence that the radially outflowing emission-line
gas is turbulent.Comment: 29 oages, including 10 figures. Accepted for publication in the
Astrophysical Journa
Coevolution of Supermassive Black Holes and Circumnuclear Disks
We propose a new evolutionary model of a supermassive black hole (SMBH) and a
circumnuclear disk (CND), taking into account the mass-supply from a host
galaxy and the physical states of CND. In the model, two distinct accretion
modes depending on gravitational stability of the CND play a key role on
accreting gas to a SMBH. (i) If the CMD is gravitationally unstable, energy
feedback from supernovae (SNe) supports a geometrically thick, turbulent gas
disk. The accretion in this mode is dominated by turbulent viscosity, and it is
significantly larger than that in the mode (ii), i.e., the CMD is supported by
gas pressure. Once the gas supply from the host is stopped, the high accretion
phase () changes to the low one (mode
(ii), ), but there is a delay with yr. Through this evolution, the gas-rich CND turns into the gas poor
stellar disk. We found that not all the gas supplied from the host galaxy
accrete onto the SMBH even in the high accretion phase (mode (i)), because the
part of gas is used to form stars. As a result, the final SMBH mass () is not proportional to the total gas mass supplied from the host
galaxy (); decreases with .This would indicate that it is difficult to form a SMBH with observed at high- QSOs. The evolution of the SMBH and CND would
be related to the evolutionary tracks of different type of AGNs.Comment: 11 pages, 11 figures, accepted for publication in Ap
The composite starburst/AGN nature of the superwind galaxy NGC 4666
We report the discovery of a Compton-thick AGN and of intense star-formation
activity in the nucleus and disk, respectively, of the nearly edge-on superwind
galaxy NGC 4666. Spatially unresolved emission is detected by BeppoSAX only at
energies <10 keV, whereas spatially resolved emission from the whole disk is
detected by XMM-Newton. A prominent (EW ~ 1-2 keV) emission line at ~6.4 keV is
detected by both instruments. From the XMM-Newton data alone the line is
spectrally localized at E ~ 6.42 +/- 0.03 keV, and seems to be spatially
concentrated in the nuclear region of NGC 4666. This, together with the
presence of a flat (Gamma ~ 1.3) continuum in the nuclear region, suggests the
existence of a strongly absorbed (i.e., Compton-thick) AGN, whose intrinsic
2-10 keV luminosity is estimated to be L_{2-10} > 2 x 10^{41} erg/s. At
energies <1 keV the integrated (BeppoSAX) spectrum is dominated by a ~0.25 keV
thermal gas component distributed throughout the disk (resolved by XMM-Newton).
At energies ~2-10 keV, the integrated spectrum is dominated by a steep (G > 2)
power-law (PL) component. The latter emission is likely due to unresolved
sources with luminosity L ~ 10^{38} - 10^{39} erg/s that are most likely
accreting binaries (with BH masses <8 M_sun). Such binaries, which are known to
dominate the X-ray point-source luminosity in nearby star-forming galaxies,
have Gamma ~ 2 PL spectra in the relevant energy range. A Gamma ~ 1.8 PL
contribution from Compton scattering of (the radio-emitting) relativistic
electrons by the ambient FIR photons may add a truly diffuse component to the
2-10 keV emission.Comment: A&A, in press (10 pages, 14 figures.) Full gzipped psfile obtainable
from http://www.bo.iasf.cnr.it/~malaguti/r_stuff.htm
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