128 research outputs found
Photometric Variability and Astrometric Stability of the Radio Continuum Nucleus in the Seyfert Galaxy NGC 5548
The NRAO VLA and VLBA were used from 1988 November to 1998 June to monitor
the radio continuum counterpart to the optical broad line region (BLR) in the
Seyfert galaxy NGC 5548. Photometric and astrometric observations were obtained
at 21 epochs. The radio nucleus appeared resolved, so comparisons were limited
to observations spanning 10-60 days and 3-4 yr, and acquired at matched
resolutions of 1210 mas = 640 pc (9 VLA observations), 500 mas = 260 pc (9 VLA
observations), or 2.3 mas = 1.2 pc (3 VLBA observations). The nucleus is
photometrically variable at 8.4 GHz by % and % between VLA
observations separated by 41 days and 4.1 yr, respectively. The 41-day changes
are milder (%) at 4.9 GHz and exhibit an inverted spectrum (, ) from 4.9 to 8.4 GHz. The nucleus
is astrometrically stable at 8.4 GHz, to an accuracy of 28 mas = 15 pc between
VLA observations separated by 4.1 yr and to an accuracy of 1.8 mas = 0.95 pc
between VLBA observations separated by 3.1 yr. Such photometric variability and
astrometric stability is consistent with a black hole being the ultimate energy
source for the BLR, but is problematic for star cluster models that treat the
BLR as a compact supernova remnant and, for NGC 5548, require a new supernova
event every 1.7 yr within an effective radius 42 mas = 22 pc. A deep
image at 8.4 GHz with resolution 660 mas = 350 pc was obtained by adding data
from quiescent VLA observations. This image shows faint bipolar lobes
straddling the radio nucleus and spanning 12 arcsec = 6.4 kpc. These
synchrotron-emitting lobes could be driven by twin jets or a bipolar wind from
the Seyfert 1 nucleus.Comment: with 9 figures, to appear in the Astrophysical Journal, 2000 March
10, volume 53
A Radio Study of the Seyfert galaxy Markarian 6: Implications for Seyfert life-cycles
We have carried out an extensive radio study with the Very Large Array on the
Seyfert 1.5 galaxy Mrk 6 and imaged a spectacular radio structure in the
source. The radio emission occurs on three different spatial scales, from ~7.5
kpc bubbles to ~1.5 kpc bubbles lying nearly orthogonal to them and a ~1 kpc
radio jet lying orthogonal to the kpc-scale bubble. To explain the complex
morphology, we first consider a scenario in which the radio structures are the
result of superwinds ejected by a nuclear starburst. However, recent Spitzer
observations of Mrk 6 provide an upper limit to the star formation rate (SFR)
of ~5.5 M_sun/yr, an estimate much lower than the SFR of ~33 M_sun/yr derived
assuming that the bubbles are a result of starburst winds energized by
supernovae explosions. Thus, a starburst alone cannot meet the energy
requirements for the creation of the bubbles in Mrk 6. We show that a single
plasmon model is energetically infeasible, and we argue that a jet-driven
bubble model while energetically feasible does not produce the complex radio
morphologies. Finally, we consider a model in which the complex radio structure
is a result of an episodically-powered precessing jet that changes its
orientation. This model is the most attractive as it can naturally explain the
complex radio morphology, and is consistent with the energetics, the spectral
index and the polarization structure. Radio emission in this scenario is a
short-lived phenomenon in the lifetime of a Seyfert galaxy which results due to
an accretion event.Comment: Accepted for publication in Ap
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
Probing the Complex and Variable X-ray Absorption of Markarian 6 with XMM-Newton
We report on an X-ray observation of the Seyfert 1.5 galaxy Mrk 6 obtained
with the EPIC instruments onboard XMM-Newton. Archival BeppoSAX PDS data from
18-120 keV were also used to constrain the underlying hard power-law continuum.
The results from our spectral analyses generally favor a double
partial-covering model, although other spectral models such as absorption by a
mixture of partially ionized and neutral gas cannot be firmly ruled out. Our
best-fitting model consists of a power law with a photon index of 1.81+/-0.20
and partial covering with large column densities up to 10^{23} cm**-2. We also
detect a narrow emission line consistent with Fe Kalpha fluorescence at
6.45+/-0.04 keV with an equivalent width of ~93+/-25 eV. Joint analyses of
XMM-Newton, ASCA, and BeppoSAX data further provide evidence for both spectral
variability (a factor of ~2 change in absorbing column) and
absorption-corrected flux variations (by ~60%) during the ~4 year period probed
by the observations.Comment: 7 pages, 2 figures. accepted for publication in the Astronomical
Journa
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
Are Seyfert Narrow Line Regions Powered by Radio Jets?
We argue that the narrow line regions of Seyfert galaxies are powered by the
transport of energy and momentum by the radio-emitting jets and consequently
that the ratio of the radio power to jet energy flux is much smaller than is
usually assumed for radio galaxies. This can be partially attributed to the
smaller ages () of Seyferts compared to radio galaxies but one
also requires that either the magnetic energy density is more than an order of
magnitude below the equipartition value, or more likely, that the internal
energy densities of Seyfert jets are dominated by thermal plasma. If Seyfert
jets are initially dominated by relativistic plasma, then an analysis of the
data on jets in five Seyfert galaxies shows that all but one of these would
have mildly relativistic jet velocities near 100 pc in order to power the
respective narrow-line regions. However, observations of jet-cloud interactions
in the NLR provide additional information on jet velocities and composition via
the momentum budget. Our analysis of a jet-cloud interaction in NGC 1068,
implies a shocked jet pressure much larger than the minimum pressure of the
radio knot, a velocity and a jet temperature
implying mildly relativistic electrons but thermal protons. The jet mass flux
at this point , is an order of magnitude higher than
the mass accretion rate into the black hole, strongly indicating entrainment.
The initial jet mass flux , comparable to the mass
accretion rate and is consistent with the densities inferred for accretion disc
coronae from high energy observations, together with an initially mildly
relativistic velocity and an initial jet radius of order 10 gravitational
radii.Comment: LaTeX2e, uses astrobib.sty, 4 postscript figures; submitted to Ap
Statistical Properties of Radio Emission from the Palomar Seyfert Galaxies
We have carried out an analysis of the radio and optical properties of a
statistical sample of 45 Seyfert galaxies from the Palomar spectroscopic survey
of nearby galaxies. We find that the space density of bright galaxies (-22 mag
<= M_{B_T} <= -18 mag) showing Seyfert activity is (1.25 +/- 0.38) X 10^{-3}
Mpc^{-3}, considerably higher than found in other Seyfert samples. Host galaxy
types, radio spectra, and radio source sizes are uncorrelated with Seyfert
type, as predicted by the unified schemes for active galaxies. Approximately
half of the detected galaxies have flat or inverted radio spectra, more than
expected based on previous samples. Surprisingly, Seyfert 1 galaxies are found
to have somewhat stronger radio sources than Seyfert 2 galaxies at 6 and 20 cm,
particularly among the galaxies with the weakest nuclear activity. We suggest
that this difference can be accommodated in the unified schemes if a minimum
level of Seyfert activity is required for a radio source to emerge from the
vicinity of the active nucleus. Below this level, Seyfert radio sources might
be suppressed by free-free absorption associated with the nuclear torus or a
compact narrow-line region, thus accounting for both the weakness of the radio
emission and the preponderance of flat spectra. Alternatively, the flat spectra
and weak radio sources might indicate that the weak active nuclei are fed by
advection-dominated accretion disks.Comment: 18 pages using emulateapj5, 13 embedded figures, accepted by Ap
BLR kinematics and Black Hole Mass in Markarian 6
We present results of the optical spectral and photometric observations of
the nucleus of Markarian 6 made with the 2.6-m Shajn telescope at the Crimean
Astrophysical Observatory. The continuum and emission Balmer line intensities
varied more than by a factor of two during 1992-2008. The lag between the
continuum and Hbeta emission line flux variations is 21.1+-1.9 days. For the
Halpha line the lag is about 27 days but its uncertainty is much larger. We use
Monte-Carlo simulation of the random time series to check the effect of our
data sampling on the lag uncertainties and we compare our simulation results
with those obtained by random subset selection (RSS) method of Peterson et al.
(1998). The lag in the high-velocity wings are shorter than in the line core in
accordance with the virial motions. However, the lag is slightly larger in the
blue wing than in the red wing. This is a signature of the infall gas motion.
Probably the BLR kinematic in the Mrk 6 nucleus is a combination of the
Keplerian and infall motions. The velocity-delay dependence is similar for
individual observational seasons. The measurements of the Hbeta line width in
combination with the reverberation lag permits us to determine the black hole
mass, M_BH=(1.8+-0.2)x10^8 M_sun. This result is consistent with the AGN
scaling relationships between the BLR radius and the optical continuum
luminosity (R_BLR is proportional to L^0.5) as well as with the black-hole
mass-luminosity relationship (M_BH-L) under the Eddington luminosity ratio for
Mrk 6 to be L_bol/L_Edd ~ 0.01.Comment: 17 pages, 10 figures, accepted for publication in MNRA
The Megamaser Cosmology Project. III. Accurate Masses of Seven Supermassive Black Holes in Active Galaxies with Circumnuclear Megamaser Disks
Observations of HO masers from circumnuclear disks in active galaxies for
the Megamaser Cosmology Project allow accurate measurement of the mass of
supermassive black holes (BH) in these galaxies. We present the Very Long
Baseline Interferometry (VLBI) images and kinematics of water maser emission in
six active galaxies: NGC~1194, NGC~2273, NGC~2960 (Mrk~1419), NGC~4388,
NGC~6264 and NGC~6323. We use the Keplerian rotation curves of these six
megamaser galaxies, plus a seventh previously published, to determine accurate
enclosed masses within the central pc of these galaxies, smaller than
the radius of the sphere of influence of the central mass in all cases. We also
set lower limits to the central mass densities of between 0.12 and 60 ~pc. For six of the seven disks, the high central
densities rule out clusters of stars or stellar remnants as the central
objects, and this result further supports our assumption that the enclosed mass
can be attributed predominantly to a supermassive black hole. The seven BHs
have masses ranging between 0.76 and 6.510. The BH mass
errors are \%, dominated by the uncertainty of the Hubble constant.
We compare the megamaser BH mass determination with other BH mass measurement
techniques. The BH mass based on virial estimation in four galaxies is
consistent with the megamaser BH mass given the latest empirical value of
, but the virial mass uncertainty is much greater. MCP
observations continue and we expect to obtain more maser BH masses in the
future.Comment: 18 pages, 4 figures. This paper has been submitted to ApJ. An updated
version of this paper will be posted when it gets accepte
Spectral Energy Distributions of type 2 QSOs: obscured star formation at high redshifts
We present new mid-infrared and submillimetre observations for a sample of
eight high redshift type-2 QSOs located in the Chandra Deep Field South. The
sources are X-ray absorbed with luminosities in excess of 10^44 erg/s. Two of
the targets have robust detections, S/N > 4, while a further three targets are
marginally detected with S/N > =2.5. All sources are detected in multiple
mid-infrared bands with the Spitzer Space Telescope. The multiwavelength
spectral energy distributions (SEDs) of the type-2 QSOs are compared to those
of two local ultraluminous galaxies (Arp220 and IR22491) in order to assess
contributions from a star-forming component in various parts of the SED. We
suggest that their submillimetre emission is possibly due to a starburst while
a large fraction of the mid-infrared energy is likely to originate in the
obscured central quasar. Using the mid-infrared and submm observations we
derive infrared luminosities which are found to be in excess of L>10^12Lsun.
The submillimetre (850micron) to X-ray (2 keV) spectral indices (alpha_SX) span
a wide range. About half of the type-2 QSOs have values typical for a
Compton-thick AGN with only 1 per cent of the nuclear emission seen through
scattering and, the remaining with values typical of submm-bright galaxies.
Combining the available observational evidence we outline a possible scenario
for the early stages of evolution of these sources.Comment: Accepted for publication in MNRA
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