332 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
Optical Variability in Active Galactic Nuclei: Starbursts or Disk Instabilities?
Aperiodic optical variability is a common property of Active Galactic Nuclei
(AGNs), though its physical origin is still open to question. To study the
origin of the optical -- ultraviolet variability in AGN, we compare light
curves of two models to observations of quasar 0957+561 in terms of a structure
function analysis. In the starburst (SB) model, random superposition of
supernovae in the nuclear starburst region produce aperiodic luminosity
variations, while in the disk-instability (DI) model, variability is caused by
instabilities in the accretion disk around a supermassive black hole. We
calculate fluctuating light curves and structure functions, , by
simple Monte-Carlo simulations on the basis of the two models. Each resultant
possesses a power-law portion, , at short time lags (). The two models can be distinguished
by the logarithmic slope, ; 0.74--0.90 in the SB model and
0.41--0.49 in the DI model, while the observed light curves
exhibit 0.35. Therefore, we conclude that the DI model is favored
over the SB model to explain the slopes of the observational structure
function, in the case of 0957+561, though this object is a radio-loud object
and thus not really a fair test for the SB model. In addition, we examine the
time-asymmetry of the light curves by calculating separately for
brightening and decaying phases. The two models exhibit opposite trends of
time-asymmetry to some extent, although the present observation is not long
enough to test this prediction.Comment: 28 pages LaTeX; 9 postscript figures; accepted for publication in
ApJ; e-mail to [email protected]
The SCUBA-2 Cosmology Legacy Survey: The EGS deep field -- III. The evolution of faint submillimeter galaxies at
We present a demographic analysis of the physical and morphological
properties of -selected galaxies from the deep observations
of the SCUBA-2 Cosmology Legacy Survey in the Extended Groth Strip that are
detected below the classical submillimeter-galaxy regime (/beam) and compare them with a sample of
optically-selected star-forming galaxies detected in the Cosmic Assembly
Near-infrared Deep Extragalactic Legacy Survey in the same field. We derive the
evolution of the main sequence of star-forming galaxies, finding a steeper
specific star formation rate versus stellar mass at than previous
studies. Most faint submillimeter-galaxies fall within of the main
sequence, but 40~per cent are classified as starbursts. Faint submillimeter
galaxies have 50~per cent larger sizes at than optically-selected
star-forming galaxies of the same mass range. This is also the redshift bin
where we find the largest fraction of starbursts, and hence we could be
witnessing merging processes, as confirmed by the preference for
visual-morphology classifications of these systems as irregular disk galaxies
and mergers. Both populations show an increment towards lower redshifts ()
of their concentration in -band morphology, but faint submillimeter galaxies
on average show larger concentration values at later times. These findings
support the claim that faint submillimeter galaxies are mostly a population of
massive dust-obscured disk-like galaxies that develop larger bulge components
at later epochs. While the similarities are great, the median sizes, starburst
numbers and -band concentration of faint submillimeter galaxies differ from
those of optically-selected star-forming galaxies of the same stellar mass.Comment: Accepted for publication in MNRA
Subaru Spectroscopy and Spectral Modeling of Cygnus A
We present high angular resolution (0.5) MIR spectra
of the powerful radio galaxy, Cygnus A, obtained with the Subaru telescope. The
overall shape of the spectra agree with previous high angular resolution MIR
observations, as well as previous Spitzer spectra. Our spectra, both on and off
nucleus, show a deep silicate absorption feature. The absorption feature can be
modeled with a blackbody obscured by cold dust or a clumpy torus. The deep
silicate feature is best fit by a simple model of a screened blackbody,
suggesting foreground absorption plays a significant, if not dominant role, in
shaping the spectrum of Cygnus A. This foreground absorption prevents a clear
view of the central engine and surrounding torus, making it difficult to
quantify the extent the torus attributes to the obscuration of the central
engine, but does not eliminate the need for a torus in Cygnus A
AzTEC Millimetre Survey of the COSMOS Field - II. Source Count Overdensity and Correlations with Large-Scale Structure
We report an over-density of bright sub-millimetre galaxies (SMGs) in the
0.15 sq. deg. AzTEC/COSMOS survey and a spatial correlation between the SMGs
and the optical-IR galaxy density at z <~ 1.1. This portion of the COSMOS field
shows a ~ 3-sigma over-density of robust SMG detections when compared to a
background, or "blankfield", population model that is consistent with SMG
surveys of fields with no extragalactic bias. The SMG over-density is most
significant in the number of very bright detections (14 sources with measured
fluxes S(1.1mm) > 6 mJy), which is entirely incompatible with sample variance
within our adopted blank-field number densities and infers an over-density
significance of >> 4. We find that the over-density and spatial correlation to
optical-IR galaxy density are most consistent with lensing of a background SMG
population by foreground mass structures along the line of sight, rather than
physical association of the SMGs with the z <~ 1.1 galaxies/clusters. The SMG
positions are only weakly correlated with weak-lensing maps, suggesting that
the dominant sources of correlation are individual galaxies and the more
tenuous structures in the region and not the massive and compact clusters.
These results highlight the important roles cosmic variance and large-scale
structure can play in the study of SMGs.Comment: 12 pages, 11 figures, 2 tables, accepted for publication in MNRA
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