573 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
The Serendipitous Discovery of a Group or Cluster of young Galaxies at z=2.40 in Deep Hubble Space Telescope WFPC2 Images
We report the serendipitous discovery of a group or cluster of young galaxies
at 2.40 in a 24-orbit HST/WFPC2 exposure of the field around the weak
radio galaxy 53W002. Potential cluster members were identified on ground-based
narrow-band redshifted Ly images and confirmed via spectroscopy.
In addition to the known weak radio galaxy 53W002 at z=2.390, two other
objects were found to have excess narrow-band Ly emission at
2.40. Both have been spectroscopically confirmed, and one clearly
contains a weak AGN. They are located within one arcminute of 53W002, or
Mpc (=0.5) at 2.40, which is the physical
scale of a group or small cluster of galaxies. Profile fitting of the WFPC2
images shows that the objects are very compact, with scale lengths
0\farcs 1 (kpc), and are rather faint
(luminosities < L*), implying that they may be sub-galactic sized objects. We
discuss these results in the context of galaxy and cluster evolution and the
role that weak AGN may play in the formation of young galaxies.Comment: Accepted for publication in The Astrophysical Journal (Letters). 13
pages of gzip compressed and uuencoded PS. Figures are available at
http://www.phys.unsw.edu.au/~spd/bib.htm
A Deep Radio Survey of Abell 2125 III: The Cluster Core - Merging and Stripping
We use radio, near-IR, optical, and X-ray observations to examine dynamic
processes in the central region of Abell 2125. In addition to the central
triple, including members of both major dynamical subsystems identified from a
redshift survey, this region features a galaxy showing strong evidence for
ongoing gas stripping during a high-velocity passage through the gas in the
cluster core. The disk galaxy C153 exhibits a plume stretching toward the
cluster center seen in soft X-rays by Chandra, parts of which are also seen in
[O II] emission and near-UV continuum light. HST imaging shows a distorted
disk, with star-forming knots asymmetrically distributed and remnant spiral
structure possibly defined by dust lanes. The stars and ionized gas in its disk
are kinematically decoupled, demonstrating that pressure stripping must be
important, and that tidal disruption is not the only mechanism at work.
Comparison of the gas properties seen in the X-ray and optical data on the
plume highlight significant features of the history of stripped gas in the
intracluster medium. The nucleus of C153 also hosts an AGN, shown by the weak
and distorted extended radio emission and a radio compact core. The unusual
strength of the stripping signatures in this instance is likely related to the
high relative velocity of the galaxy with respect to the intracluster medium,
during a cluster/cluster merger, and its passage very near the core of the
cluster. Another sign of recent dynamical events is diffuse starlight
asymmetrically placed about the central triple in a cD envelope. Transient and
extreme dynamical events as seen in Abell 2125 may be important drivers of
galaxy evolution in the cores of rich clusters.Comment: 36 pages, 16 figures, accepted AJ, paper with full resolution figures
is available at http:www.aoc.nrao.edu/~fowen/papers/a2125/a2125paper3.ps.g
Optical and X-ray Spectroscopy of 1E 0449.4-1823: Demise of the original type 2 QSO
New optical spectra of the original narrow-line quasar 1E 0449.4-1823 show
that it now has broad emission lines of considerable strength, eliminating it
as a "type 2 QSO" candidate. We suggest that the behavior of 1E 0449.4-1823 is
the same as that of some Seyfert 1.8 and 1.9 galaxies, in which Goodrich
attributed long-term variations of their broad Balmer lines to dynamical
motions of obscuring material located in or around the broad-line region. The
optical continuum and broad emission-line regions of 1E 0449.4-1823 may still
be partly covered in our line of sight, which would explain its large
forbidden-line equivalent widths and flat alpha_ox relative to other
low-redshift QSOs. Also present are apparent absorption features in the broad
Balmer lines and in Mg II, which may be related to the past obscuration and
current emergence of the broad-line region. However, it is difficult to
distinguish absorption from broad emission-line peaks that are displaced in
velocity; we consider the latter a plausible competing interpretation of these
peculiar line profiles. An ASCA X-ray spectrum of 1E 0449.4-1823 can be fitted
with a power-law of Gamma = 1.63, intrinsic N_H < 9 x 10^20 cm-2, and no Fe
Kalpha line emission. Its 2-10 keV luminosity is 6.7 x 10^44 ergs/s. With
regard to the still hypothetical type 2 QSOs, we argue that there is little
evidence for the existence of any among X-ray selected samples.Comment: to appear in ApJ July 1, 1998 (vol. 501). 23 pages including figures.
Uses LaTeX macro aasms4.st
GMOS Spectroscopy of SCUBA Galaxies Behind A851
We have identified counterparts to two submillimeter (submm) sources, SMM
J09429+4659 and SMM J09431+4700, seen through the core of the z=0.41 cluster
Abell 851. We employ deep 1.4-GHz observations and the far-infrared/radio
correlation to refine the submm positions and then optical and near-infrared
imaging to locate their counterparts. We identify an extremely red counterpart
to SMM J09429+4659, while GMOS spectroscopy with Gemini-North shows that the
R=23.8 radio source identified with SMM J09431+4700 is a hyperluminous infrared
galaxy (L_FIR~1.5x10^13 L_sun) at z=3.35, the highest spectroscopic redshift so
far for a galaxy discovered in the submm. The emission line properties of this
galaxy are characteristic of a narrow-line Seyfert-1, although the lack of
detected X-ray emission in a deep XMM-Newton observation suggests that the bulk
of the luminosity of this galaxy is derived from massive star formation. We
suggest that active nuclei, and the outflows they engender, may be an important
part of the evolution of the brightest submm galaxies at high redshifts.Comment: to appear in the Oct 1 issue of ApJ Letter
[OII] Emission, Eigenvector 1 and Orientation in Radio-quiet Quasars
We present supportive evidence that the Boroson and Green eigenvector 1 is
not driven by source orientation. Until recently it was generally accepted that
eigenvector 1 does not depend on orientation as it strongly correlates with
[OIII]5007 emission, thought to be an isotropic property. However, recent
studies of radio-loud AGN have questioned the isotropy of [OIII] emission and
concluded that [OII]3727 emission is isotropic. In this paper we investigate
the relation between eigenvector 1 and [OII] emission in radio-quiet BQS
(Bright Quasar Survey) quasars, and readdress the issue of orientation as the
driver of eigenvector 1. We find significant correlations between eigenvector 1
and orientation independent [OII] emission, which implies that orientation does
not drive eigenvector 1. The luminosities and equivalent widths of [OIII] and
[OII] correlate with one another, and the range in luminosities and equivalent
widths is similar. This suggests that the radio-quiet BQS quasars are largely
free of orientation dependent dust effects and ionization dependent effects in
the narrow-line region. We also conclude that neither the [OIII] emission nor
the [OII]/[OIII] ratio are dependent on orientation in our radio-quiet BQS
quasar sample, contrary to recent results found for radio-loud quasars.Comment: 24 pages, 12 figures, accepted for publication in Ap
Properties of Quasar-Galaxy Associations and Gravitational Mesolensing by Halo Objects
A new catalog of 8382 close quasar-galaxy pairs is presented. The catalog was
composed using published catalogs of quasars and active galactic nuclei
containing 11358 objects, as well as the LEDA catalog of galaxies, which
contains on the order of 100 thousand objects. The search for pairs was carried
out in such a way that the linear distance between the galaxy and projected
quasar does not exceed 150kpc. Based on these new data, the dependence of the
number of pairs on a=z_G/z_Q is analysed, where z_G and z_Q are the redshifts
of the galaxy and quasar, respectively, revealing an excess of pairs with a<0.1
and a>0.9. This means that the galaxies in pairs are preferably located close
to either the observer or the quasar and avoid intermediate distances along the
line of sight to the quasar. Computer simulations demonstrate that it is not
possible to explain this number of pairs with the observed distribution in a as
the result of chance positional coincidences with a uniform spatial
distribution of galaxies. Data on globular clusters show that the excess of
pairs with a0.9 is consistent with the hypothesis that we are
observing distant compact objects that are strongly gravitationally lensed by
transparent lenses with a King mass distribution located in the halos of nearby
galaxies. The Hubble diagram for galaxies and quasars is presented.
Observational tests of the mesolensing hypothesis are formulated.Comment: 11 pages, 7 figure
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