1,635 research outputs found
Tidally Triggered Star Formation in Close Pairs of Galaxies: Major and Minor Interactions
We study star formation in a sample of 345 galaxies in 167 pairs and compact
groups drawn from the original CfA2 Redshift Survey and from a follow-up search
for companions. We construct our sample with attention to including pairs with
luminosity contrast |\Delta m_R| >= 2. These 57 galaxies with |\Delta m_R| >= 2
provide a set of nearby representative cases of minor interactions, a central
feature of the hierarchical galaxy formation model. Here we report the
redshifts and positions of the 345 galaxies in our sample, and of 136 galaxies
in apparent pairs that are superpositions. In the pairs sample as a whole,
there are strong correlations between the equivalent width of the H\alpha
emission line and the projected spatial and the line-of-sight velocity
separation of the pair. For pairs of small luminosity contrast, |\Delta m_R| <
2, the member galaxies show a correlation between the equivalent width of
H\alpha and the projected spatial separation of the pair. However, for pairs
with large luminosity contrast, |\Delta m_R| >= 2, we detect no correlation
between the equivalent width of H\alpha and the projected spatial separation.
The relative luminosity of the companion galaxy is more important in a
gravitational tidal interaction than the intrinsic luminosity of the galaxy.
Central star formation across the entire pairs sample depends strongly on the
luminosity ratio, |\Delta m_R|, a reasonable proxy for the mass ratio of the
pair; pairs composed of similarly luminous galaxies produce the strongest
bursts of star formation. Pairs with |\Delta m_R| >= 2 rarely have EW(H\alpha)
>~ 70 Ang.Comment: Minor revisions following journal proof
Induced Gamma-band Activity Elicited by Visual Representation of Unattended Objects
Peer reviewedPostprin
The Radio Properties of Composite LINER/HII Galaxies
Arcsec-resolution VLA observations -- newly obtained as well as published --
of 40 nearby galaxies are discussed, completing a study of the radio properties
of a magnitude-limited sample of nearby galaxies of the composite LINER/HII
type. Our results reveal an overall detection rate of at least 25% AGN
candidates among these composite sources. The general properties of these AGN
candidates, as compared to non-AGN composite sources and HII galaxies, are
discussed.Comment: Accepted for publication in ApJ
A Close Look at Star Formation around Active Galactic Nuclei
We analyse star formation in the nuclei of 9 Seyfert galaxies at spatial
resolutions down to 0.085arcsec, corresponding to length scales of less than
10pc in some objects. Our data were taken mostly with the near infrared
adaptive optics integral field spectrograph SINFONI. The stellar light profiles
typically have size scales of a few tens of parsecs. In two cases there is
unambiguous kinematic evidence for stellar disks on these scales. In the
nuclear regions there appear to have been recent - but no longer active -
starbursts in the last 10-300Myr. The stellar luminosity is less than a few
percent of the AGN in the central 10pc, whereas on kiloparsec scales the
luminosities are comparable. The surface stellar luminosity density follows a
similar trend in all the objects, increasing steadily at smaller radii up to
10^{13}L_sun/kpc^2 in the central few parsecs, where the mass surface density
exceeds 10^4M_sun/pc^2. The intense starbursts were probably Eddington limited
and hence inevitably short-lived, implying that the starbursts occur in
multiple short bursts. The data hint at a delay of 50--100Myr between the onset
of star formation and subsequent fuelling of the black hole. We discuss whether
this may be a consequence of the role that stellar ejecta could play in
fuelling the black hole. While a significant mass is ejected by OB winds and
supernovae, their high velocity means that very little of it can be accreted.
On the other hand winds from AGB stars ultimately dominate the total mass loss,
and they can also be accreted very efficiently because of their slow speeds.Comment: 51 pages, including 27 figures; accepted by ApJ (paper reorganised,
but results & conclusions the same
Infrared Spectroscopy of a Massive Obscured Star Cluster in the Antennae Galaxies (NGC 4038/4039) with NIRSPEC
We present infrared spectroscopy of the Antennae Galaxies (NGC 4038/4039)
with NIRSPEC at the W. M. Keck Observatory. We imaged the star clusters in the
vicinity of the southern nucleus (NGC 4039) in 0.39" seeing in K-band using
NIRSPEC's slit-viewing camera. The brightest star cluster revealed in the
near-IR (M_K(0) = -17.9) is insignificant optically, but coincident with the
highest surface brightness peak in the mid-IR (12-18 micron) ISO image
presented by Mirabel et al. (1998). We obtained high signal-to-noise 2.03 -
2.45 micron spectra of the nucleus and the obscured star cluster at R ~ 1900.
The cluster is very young (4 Myr old), massive (16e6 M_sun), and compact
(density ~ 115 M_sun pc^(-3) within a 32 pc half-light radius), assuming a
Salpeter IMF (0.1 - 100 M_sun). Its hot stars have a radiation field
characterized by T_eff ~ 39,000 K, and they ionize a compact H II region with
n_e ~ 1e4 cm^(-3). The stars are deeply embedded in gas and dust (A_V ~ 9-10
mag), and their strong FUV field powers a clumpy photodissociation region with
densities n_H >= 1e5 cm^(-3) on scales of up to 200 pc, radiating L[H_2 1-0
S(1)] = 9600 L_sun.Comment: 4 pages, 5 embedded figures. To appear in proceedings of 33d ESLAB
Symposium: Star Formation from the Small to the Large Scale, held in
Noordwijk, The Netherlands, Nov. 1999. Also available at
http://astro.berkeley.edu/~agilber
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
K-band Spectroscopy of Clusters in NGC 4038/4039
Integral field spectroscopy in the K-band (1.9-2.4um) was performed on four
IR-bright star clusters and the two nuclei in NGC 4038/4039 (``The Antennae'').
Two of the clusters are located in the overlap region of the two galaxies, and
together comprise ~25% of the total 15um and ~10% of the total 4.8 GHz emission
from this pair of merging galaxies. The other two clusters, each of them
spatially resolved into two components, are located in the northern galaxy, one
in the western and one in the eastern loop of blue clusters. Comparing our
analysis of Brgamma, CO band-heads, He I (2.058um), Halpha (from archival HST
data), and V-K colors with stellar population synthesis models indicates that
the clusters are extincted (A_V ~ 0.7 - 4.3 mags) and young, displaying a
significant age spread (4-13 Myrs). The starbursts in the nuclei are much older
(65 Myrs), with the nucleus of NGC 4038 displaying a region of recent star
formation northward of its K-band peak. Using our derived age estimates and
assuming the parameters of the IMF (Salpeter slope, upper mass cut-off of 100
M_sun, Miller-Scalo between 1 M_sun and 0.1 M_sun), we find that the clusters
have masses between 0.5 and 5 * 10^6M_sun.Comment: 10 pages, 3 figures, ApJ accepte
Deuteron Magnetic and Quadrupole Moments with a Poincar\'e Covariant Current Operator in the Front-Form Dynamics
The deuteron magnetic and quadrupole moments are unambiguosly determined
within the front-form Hamiltonian dynamics, by using a new current operator
which fulfills Poincar\'e, parity and time reversal covariance, together with
hermiticity and the continuity equation. For both quantities the usual
disagreement between theoretical and experimental results is largely removed.Comment: To appear in Phys. Rev. Let
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 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
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