104 research outputs found
Tightly Correlated X-ray/H Emitting Filaments in the Superbubble and Large-Scale Superwind of NGC 3079
Using Chandra and HST we show that X-ray and H filaments that form
the 1.3-kpc diameter superbubble of NGC 3079 have strikingly similar patterns
at 0."8 resolution. This tight match seems to arise from cool disk gas that has
been driven by the wind, with X-rays being emitted from upstream, stand-off
bowshocks or by conductive cooling at the cloud/wind interfaces. We find that
the soft X-ray plasma has thermal and kinetic energies of
and erg
respectively, where is the filling factor of the X-ray gas and may be
small; these are comparable to the energies of the optical line-emitting gas.
X-rays are also seen from the base of the radio counterbubble that is obscured
optically by the galaxy disk, and from the nucleus (whose spectrum shows the Fe
K line). Hydrodynamical simulations reproduce the obbservations well
using large filling factors within both filament systems; assuming otherwise
seriously underestimates the mass loss in the superwind. The superbubble is
surrounded by a fainter conical halo of X-rays that fill the area delineated by
high angle, H-emitting filaments, supporting our previous assertion
that these filaments form the contact discontinuity/shock between galaxy gas
and shocked wind. About 40\arcsec (3 kpc) above the disk, an X-ray arc may
partially close beyond the bubble, but the north-east quadrant remains open,
consistent with the superwind having broken out into at least the galaxy halo.Comment: 8 pages, 3 figures, to appear in Sept. 10 Ap
Enhanced star formation: The importance of bars in spiral galaxies
It was found that among an IR luminous subset of nearby spiral galaxies, nearly all of the systems with IRAS colors and luminosities indicative of enhanced star formation are barred. Radio continuum and IR spectroscopic results support the hypothesis that this emission originates within the central 2 kpc; possibly in a circumnuclear ring. It was also found that outer rings are over represented among these barred systems and suggest possible reasons for this phenomena
Seeing statistics at the upgraded 3.8m UK infrared telescope (UKIRT)
From 1991 until 1997, the 3.8m UK Infrared Telescope (UKIRT) underwent a
programme of upgrades aimed at improving its intrinsic optical performance.
This resulted in images with a FWHM of 0."17 at 2.2 um in September 1998. To
understand and maintain the improvements to the delivered image quality since
the completion of the upgrades programme, we have regularly monitored the
overall atmospheric seeing, as measured by radial displacements of subaperture
images (i.e. seeing-generated focus fluctuations), and the delivered image
diameters. The latter have been measured and recorded automatically since the
beginning of 2001 whenever the facility imager UFTI (UKIRT Fast Track Imager)
has been in use.
In this paper we report the results of these measurements. We investigate the
relation between the delivered image diameter and the RMS atmospheric seeing
(as measured by focus fluctuations, mentioned above). We find that the best
seeing occurs in the second half of the night, generally after 2am HST and that
the best seeing occurs in the summer between the months of July and September.
We also find that the relationship between Zrms and delivered image diameter is
uncertain. As a result Zrms frequently predicts a larger FWHM than that
measured in the images.
Finally, we show that there is no correlation between near-infrared seeing
measured at UKIRT and sub-mm seeing measured at the Caltech Submillimetre
Observatory (CSO).Comment: 10 pages to appear in the SPIE proceeding vol. 4484 on Observatory
Operations to Maximize Scientific Retur
NGC 5291: Implications for the Formation of Dwarf Irregular Galaxies
The possible formation and evolution of dwarf irregular galaxies from material derived from perturbed evolved galaxies is addressed via an HI study of a likely example, the peculiar system NGC 5291. This system, located in the western outskirts of the cluster Abell 3574, contains the lenticular galaxy NGC 5291 which is in close proximity to a disturbed companion and is flanked by an extensive complex of numerous knots extending roughly 4\u27 north and 4\u27 south of the galaxy. In an initial optical and radio study, Longmore et al. (1979, MNRAS, 188, 285) showed that these knots have the spectra of vigorous star-forming regions, and suggested that some may in fact be young dwarf irregular galaxies. High resolution 21-cm line observations taken with the VLA are presented here and reveal that the H I distribution associated with this system encompasses not only the entire N-S complex of optical knots, but also forms an incomplete ring or tail that extends approximately 3\u27 to the west. The HI associated with NGC 5291 itself shows a high velocity range; the Seashell is not detected. The formation mechanism for this unusual system is unclear and two models-a large, low-luminosity ram-swept disk, and a ram-swept interaction-are discussed. The HI in the system contains numerous concentrations, mostly along the N-S arc of the star-forming complexes, which generally coincide with one or more optical knots; the larger HI features contain several X 109 M0 of gas. Each of the knots is compared to a set of criteria designed to determine if these objects are bound against their own internal kinetic energy and are tidally stable relative to the host galaxy. An analysis of the properties of the H I concentrations surrounding the optical star-forming complexes indicates that at least the largest of these is a bound system; it also possesses a stellar component. It is suggested that this object is a genuinely young dwarf irregular galaxy that has evolved from the material associated with the system and that this entire complex contains several proto- or young dwarf irregular galaxies in various stages of development. We are therefore witnessing the early evolution of a number of genuinely young galaxies. Given the evident importance of the NGC 5291 system as a \u27\u27nursery\u27\u27 for young galaxies, careful modeling is required if we are to understand this remarkable galaxy
JHK Standard Stars for Large Telescopes: the UKIRT Fundamental and Extended Lists
We present high-precision JHK photometry with the 3.8m UK Infrared Telescope
(UKIRT) of 83 standard stars, 28 from the widely used preliminary list known as
the "UKIRT Faint Standards" (Casali & Hawarden, 1992), referred to here as the
Fundamental List, and 55 additional stars referred to as the Extended List. The
stars have 9.4<K<15.0 and most should be readily observable with imaging array
detectors in normal operating modes on telescopes of up to 10m aperture. Many
are accessible from the southern hemisphere. Arcsec-accuracy positions (J2000,
Epoch ~1998) are given, together with optical photometry and spectral types
from the literature, where available, or inferred from the J-K colour. Finding
charts are provided for stars with proper motions exceeding 0.3"/yr. On 30
nights between late 1994 and early 1998 the stars from the Fundamental List,
which were used as standards for the whole programme, were observed on an
average of 10 nights each, and those from the Extended List 6 nights. The
average internal standard error of the mean results for K is 0.005 mag; for J-H
it is 0.003 mag for the Fundamental List stars and 0.006 mag for the Extended
List; for H-K the average is 0.004 mag. The results are on the natural system
of the IRCAM3 imager, which used a 256x256 InSb detector array with "standard"
JHK filters, behind gold-coated fore-optics and a gold- or silver-dielectric
coated dichroic. We give colour transformations onto the CIT, Arcetri and
LCO/Palomar NICMOS systems, and preliminary transformations onto the system
defined by the new Mauna Kea Observatory filter set.Comment: 13 pages includes one figure, accepted by MNRAS Feb 2001 Revised
Version with a transformation typo correcte
Water maser emission and the parsec-scale jet in NGC 3079
We have conducted VLBI observations of water maser and radio continuum
emission in the nucleus of NGC 3079. The 22 GHz maser emission arises in
compact clumps, distributed along an axis that is aligned with the major axis
of the galactic disk. The velocities of the masers are consistent with their
lying in the inner parsec of a molecular disk rotating in the same sense as the
rest of the galaxy. However, the velocity field has a significant
non-rotational component, which may indicate supersonic turbulence. The bright
maser emission is not coincident with any detected compact 22 GHz continuum
source, suggesting the high apparent luminosity of the maser may not due to
beamed amplification of continuum emission. We observed two compact continuum
sources that have inverted spectra between 5 and 8 GHz, and steep spectra
between 8 and 22 GHz. NGC 3079 may be a nearby, low-luminosity example of the
class of compact symmetric gigahertz-peaked spectrum (GPS) radio sources. We
detected a third continuum component that lies along the same axis as the other
two, strongly suggesting that this galaxy possesses a nuclear jet. Faint maser
emission was detected near this axis, which may indicate a second population of
masers associated with the jet.Comment: 31 pages, includes 8 figures. To appear in ApJ, Vol 495, 10 March
1998. Full-resolution figures and color plate available at
http://cfa-www.harvard.edu/~trotter/ngc3079.htm
- âŠ