2,567 research outputs found
Induced star formation in interacting galaxies
Measurements of H alpha emission line fluxes and FIR fluxes in approx. 100 interacting spirals were used to investigate the effects of close tidal interactions on the disk and nuclear star formation rates in galaxies. Two samples of interacting spirals were studied, a complete sample of close pairs, and a set of strongly perturbed systems from the Arp atlas. Both the integrated H alpha luminosities and FIR luminosities are enhanced in the interacting galaxies, indicating that the encounters indeed trigger massive star formation in many cases. The response of individual galaxies is highly variable, however. A majority of the interacting spirals exhibit normal star formation rates, while a small fraction are undergoing bursts with luminosities which are rarely, if ever, observed in noninteracting systems. Virtually all of the latter are in the Arp sample, indicating that the Arp atlas is heavily biased to the most active star forming systems
A Search for Active Galactic Nuclei in Sc Galaxies with H II Spectra
(Abridged) We have searched for nuclear radio emission from a statistically
complete sample of 40 Sc galaxies within 30 Mpc that are optically classified
as star-forming objects, in order to determine whether weak AGNs might be
present. Only three nuclear radio sources were detected, in NGC 864, NGC 4123,
and NGC 4535. These galaxies have peak 6-cm radio powers of 10^{20} W/Hz at
arcsecond resolution, while upper limits of the non-detected galaxies typically
range from 10^{18.4} to 10^{20} W/Hz. The three nuclear radio sources all are
resolved and appear to have diffuse morphologies, with linear sizes of ~300 pc.
This strongly indicates that circumnuclear star formation has been detected in
these three H II galaxies. Comparison with previous 20-cm VLA results for the
detected galaxies shows that the extended nuclear radio emission has a flat
spectrum in two objects, and almost certainly is generated by thermal emission
from gas ionized by young stars in the centers of those galaxies. The 6-cm
radio powers are comparable to predictions for thermal emission that are based
on the nuclear H-alpha luminosities, and imply nuclear star formation rates of
0.08-0.8 solar masses/yr, while the low-resolution NRAO VLA Sky Survey implies
galaxy-wide star formation rates of 0.3-1.0 solar masses/yr in stars above 5
solar masses. Although the presence of active nuclei powered by massive black
holes cannot be definitively ruled out, the present results suggest that they
are likely to be rare in these late-type galaxies with H II spectra.Comment: To appear in ApJ. 7 page
Cicrumnuclear Supernova Remnants and HII Regions in NGC 253
Archival VLA data has been used to produce arcsecond-resolution 6- and 20-cm
images of the region surrounding the nuclear 200-pc (~15") starburst in NGC
253. Twenty-two discrete sources stronger than 0.4 mJy have been detected
within ~2 kpc (~3') of the galaxy nucleus; almost all these sources must be
associated with the galaxy. None of the radio sources coincides with a detected
X-ray binary, so they appear to be due to supernova remnants and H II regions.
The region outside the central starburst has a derived radio supernova rate of
<~0.1/yr, and may account for at least 20% of the recent star formation in NGC
253. Most of the newly identified sources have steep, nonthermal radio spectra,
but several relatively strong thermal sources also exist, containing the
equivalent of tens of O5 stars. These stars are spread over tens of parsecs,
and are embedded in regions having average ionized gas densities of
20-200/cm^3, much lower than in the most active nuclear star-forming regions in
NGC 253 or in the super star clusters seen in other galaxies. The strongest
region of thermal emission coincides with a highly reddened area seen at
near-infrared wavelengths, possibly containing optically obscured H II regions.Comment: 17 pages, 3 postscript figures, AASTeX format, in press for
Astronomical Journal, July 200
Radio Sources in Low-Luminosity Active Galactic Nuclei. I. VLA Detections of Compact, Flat-Spectrum Cores
We report a 0.2" resolution, 15 GHz survey of a sample of 48 low-luminosity
active galactic nuclei with the Very Large Array. Compact radio emission has
been detected in 57% (17 of 30) of LINERs and low-luminosity Seyferts, at least
15 of which have a flat to inverted radio spectrum (alpha > -0.3). The compact
radio cores are found in both type 1 (i.e. with broad Halpha) and type 2
(without broad Halpha) nuclei. The 2 cm radio power is significantly correlated
with the emission-line ([OI] lambda6300) luminosity. While the present
observations are consistent with the radio emission originating in star-forming
regions, higher resolution radio observations of 10 of the detected sources,
reported in a companion paper (Falcke et al. 2000), show that the cores are
very compact (= 10^8K) and
probably synchrotron self-absorbed, ruling out a starburst origin. Thus, our
results suggest that at least 50% of low-luminosity Seyferts and LINERs in the
sample are accretion powered, with the radio emission presumably coming from
jets or advection-dominated accretion flows. We have detected only 1 of 18
`transition' (i.e. LINER + HII) nuclei observed, indicating their radio cores
are significantly weaker than those of `pure' LINERs.Comment: To appear in the Astrophysical Journal, October 20, 200
Observational Evidence of Accretion Disk-Caused Jet Precession in Galactic Nuclei
We show that the observational data of extragalactic radio sources tend to
support the theoretical relationship between the jet precession period and the
optical luminosity of the sources, as predicted by the model in which an
accretion disk causes the central black hole to precess.Comment: 13 pages, 1 figure, accepted for publication in ApJ Letter
The Pearson-Readhead Survey of Compact Extragalactic Radio Sources From Space. II. Analysis of Source Properties
We have performed a multi-dimensional correlation analysis on the observed
properties of a statistically complete core-selected sample of compact
radio-loud active galactic nuclei, based on data from the VLBI Space Observing
Programme (Paper I) and previously published studies. Our sample is drawn from
the well-studied Pearson-Readhead (PR) survey, and is ideally suited for
investigating the general effects of relativistic beaming in compact radio
sources. In addition to confirming many previously known correlations, we have
discovered several new trends that lend additional support to the beaming
model. These trends suggest that the most highly beamed sources in
core-selected samples tend to have a) high optical polarizations; b) large
pc/kpc-scale jet misalignments; c) prominent VLBI core components; d)
one-sided, core, or halo radio morphology on kiloparsec scales; e) narrow
emission line equivalent widths; and f) a strong tendency for intraday
variability at radio wavelengths. We have used higher resolution space and
ground-based VLBI maps to confirm the bi-modality of the jet misalignment
distribution for the PR survey, and find that the sources with aligned parsec-
and kiloparsec-scale jets generally have arcsecond-scale radio emission on both
sides of the core. The aligned sources also have broader emission line widths.
We find evidence that the BL Lacertae objects in the PR survey are all highly
beamed, and have very similar properties to the high-optically polarized
quasars, with the exception of smaller redshifts. A cluster analysis on our
data shows that after partialing out the effects of redshift, the luminosities
of our sample objects in various wave bands are generally well-correlated with
each other, but not with other source properties.Comment: 21 pages, 10 figures. Accepted for publication in the Astrophysical
Journal. Part I can be found at astro-ph/010227
Line Forces in Keplerian Circumstellar Disks and Precession of Nearly Circular Orbits
We examine the effects of optically thick line forces on orbiting
circumstellar disks, such as occur around Be stars. For radially streaming
radiation, line forces are only effective if there is a strong radial velocity
gradient, as occurs, for example, in a line-driven stellar wind. However,
within an orbiting disk, the radial shear of the azimuthal velocity leads to
strong line-of-sight velocity velocity gradients along nonradial directions. As
such, in the proximity of a stellar surface extending over a substantial cone
angle, the nonradial stellar radiation can impart a significant line force,
even in the case of purely circular orbits. Given the highly supersonic nature
of orbital velocity variations, we use the Sobolev approximation, thereby
extending to the disk case the standard CAK formalism developed for line-driven
winds. We delineate the parameter regimes for which radiative forces might
alter disk properties; but even when radiative forces are small, we
analytically quantify higher-order effects in the linear limit, including the
precession of weakly elliptical orbits. We find that optically thick line
forces can have observable implications for the dynamics of disks around Be
stars, including the generation of either prograde or retrograde precession in
slightly eccentric orbits. However, our analysis suggests a net retrograde
effect, in apparent contradiction with observed long-term variations of
violet/red line profile asymmetries from Be stars, which are generally thought
to result from prograde propagation of a so-called ``one arm mode''. We also
conclude that radiative forces may alter the dynamical properties at the
surface of the disk where disk winds originate, and may even make low-density
disks vulnerable to being blown away.Comment: 31 pages, Latex, aaspp4 macro, 4 figure
Magnetic Fields in Quasar Cores II
Multi-frequency polarimetry with the Very Long Baseline Array (VLBA)
telescope has revealed absolute Faraday Rotation Measures (RMs) in excess of
1000 rad/m/m in the central regions of 7 out of 8 strong quasars studied (e.g.,
3C 273, 3C 279, 3C 395). Beyond a projected distance of ~20 pc, however, the
jets are found to have |RM| < 100 rad/m/m. Such sharp RM gradients cannot be
produced by cluster or galactic-scale magnetic fields, but rather must be the
result of magnetic fields organized over the central 1-100 pc. The RMs of the
sources studied to date and the polarization properties of BL Lacs, quasars and
galaxies are shown to be consistent so far with the predictions of unified
schemes. The direct detection of high RMs in these quasar cores can explain the
low fractional core polarizations usually observed in quasars at centimeter
wavelengths as the result of irregularities in the Faraday screen on scales
smaller than the telescope beam. Variability in the RM of the core is reported
for 3C 279 between observations taken 1.5 years apart, indicating that the
Faraday screen changes on that timescale, or that the projected superluminal
motion of the inner jet components samples a new location in the screen with
time. Either way, these changes in the Faraday screen may explain the dramatic
variability in core polarization properties displayed by quasars.Comment: Accepted to the ApJ. 27 pages, 9 figures including figure 6 in colo
The Nature of Composite LINER/HII Galaxies, As Revealed from High-Resolution VLA Observations
A sample of 37 nearby galaxies displaying composite LINER/HII and pure HII
spectra was observed with the VLA in an investigation of the nature of their
weak radio emission. The resulting radio contour maps overlaid on optical
galaxy images are presented here, together with an extensive literature list
and discussion of the individual galaxies. Radio morphological data permit
assessment of the ``classical AGN'' contribution to the global activity
observed in these ``transition'' LINER galaxies. One in five of the latter
objects display clear AGN characteristics: these occur exclusively in
bulge-dominated hosts.Comment: 31 pages, 27 figures, accepted by ApJ
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