878 research outputs found
Neutral Gas Distributions and Kinematics of Five Blue Compact Dwarf Galaxies
We present the results of high spatial resolution HI observations of five
intrinsically compact dwarf galaxies which are currently experiencing a strong
burst of star formation. The HI maps indicate that these systems have a complex
and clumpy interstellar medium. Unlike typical dwarf irregular galaxies, these
Blue Compact Dwarf (BCD) galaxies exhibit strong central concentrations in
their neutral gas distributions which may provide a clue to the origin of their
strong star-burst activity. Furthermore, while all of the systems do appear to
be rotating, based on observed velocity gradients, the kinematics are complex.
All systems have non-ordered kinematic structure at some level; some of the
extended gas is not necessarily kinematically connected to the main system.
The observed gas distributions and kinematics place constraints on
evolutionary scenarios for BCDs. Evolutionary links between BCDs, dwarf
irregulars, and dwarf ellipticals have been postulated to explain their high
star formation rates and low luminosity, low metallicity nature. The BCDs
appear to have higher central mass concentrations in both gas and stellar
content than the dwarf irregulars, indicating that evolutionary scenarios
connecting these two classes will require mass redistribution. In addition, the
fact that BCDs are rotationally supported systems indicates that BCDs are
unlikely to evolve into dwarf ellipticals without substantial loss of angular
momentum. Thus, while such evolutionary scenarios may still be possible with
the aid of mergers or tidal interactions, the isolated nature of BCDs suggests
that the majority of BCDs will not fade to become objects similar to the
present day dwarf ellipticals.Comment: 19 pages, 15 figures. To appear in A
High chemical abundances in stripped Virgo spiral galaxies
Based on a comparison of the oxygen abundances in H 2 regions in field and Virgo cluster late type spiral galaxies, Shields, Skillman, & Kennicutt (1991) suggested that the highly stripped spiral galaxies in the Virgo cluster have systematically higher abundances than comparable field galaxies. In April 1991 and May 1992 we used the blue channel spectrograph on the MMT to obtain new observations of 30 H 2 regions in Virgo spiral galaxies. These spectra cover the wavelength range from (O II) lambda 3727 to (S II) lambda 6731. We now have observed at least 4 H II regions in 9 spiral galaxies in the Virgo cluster. Combining (O II) and (O III) line strengths, we calculate the H II region oxygen abundances based on the empirical calibration of Edmunds & Pagel (1984). These observations show: (1) The stripped, low luminosity Virgo spirals (N4689, N4571) truly have abundances characteristic of much more luminous field spirals; (2) Virgo spirals which show no evidence of stripping (N4651, N4713) have abundances comparable to field galaxies; and (3) Evidence for transition galaxies (e.g., N4254, N4321), with marginally stripped disks and marginal abundance enhancements. The new observations presented here confirm the validity of the oxygen over-abundances in the stripped Virgo spirals. Shields et al. (1991) discussed two different mechanisms for producing the higher abundances in the disks of stripped galaxies in Virgo. The first is the supression of infall of near-primordial material, the second is the suppression of radial inflow of metal-poor gas. Distinguishing between the two cases will require more observations of the Virgo cluster spirals and a better understanding of which parameters determine the variation of abundance with radius in field spirals (cf., Garnett & Shields 1987)
The Metallicity of the Red Giant Branch in the Disk of NGC 6822
Deep J, H, and K' images obtained with the Canada-France-Hawaii Telescope
adaptive optics system are used to investigate the metallicity of red giant
branch (RGB) stars in three fields in the disk of the Local Group dwarf
irregular galaxy NGC 6822. The slope of the RGB on the (K, J-K) color-magnitude
diagrams indicates that = -1.0 +/- 0.3. The locus of the RGB is bluer
than that of globular clusters with the same RGB slope, by an amount that is
consistent with the majority of RGB stars in these fields having an age near 3
Gyr. It is demonstrated that if RGB stars in NGC 6822 are this young then the
metallicity computed from the RGB slope may be ~ 0.05 dex too low.Comment: 19 pages of text; 10 figures; to appear in the PAS
Using the Tip of the Red Giant Branch as a Distance Indicator in the Near Infrared
The tip of the red giant branch (TRGB) is a well-established standard candle
used to measure distances to nearby galaxies. The TRGB luminosity is typically
measured in the I-band, where the luminosity has little dependency on stellar
age or stellar metallicity. As the TRGB is brighter at wavelengths redder than
the I-band, observational gains can be made if the TRGB luminosity can be
robustly calibrated at longer wavelengths. This is of particular interest given
the infrared capabilities that will be available with the James Webb Space
Telescope and an important calibration consideration for using TRGB distances
as part of an independent measurement of the Hubble constant. Here, we use
simulated photometry to investigate the dependency of the TRGB luminosity on
stellar age and metallicity as a function of wavelength (475 nm - 4.5 micron).
We find intrinsic variations in the TRGB magnitude to increase from a few
hundredths of a magnitude at 800-900 nm to ~0.6 mag by 1.5 micron. We show that
variations at the longer infrared wavelengths can be reduced to 0.02-0.05 mag
(1-2% accuracy in distance) with careful calibrations that account for changes
in age and metal content. These represent the minimum uncertainties;
observational uncertainties will be higher. Such calibration efforts may also
provide independent constraints of the age and metallicity of stellar halos
where TRGB distances are best measured. At 3.6 and 4.5 micron, the TRGB
magnitude is predicted to vary up to ~0.15 mag even after corrections for
stellar age and metallicity, making these wavelengths less suitable for
precision distances.Comment: 11 pages, 7 figures, 1 table, Accepted to the Astrophysical Journa
Rapid Oscillations in Cataclysmic Variables. XV. HT Camelopardalis (= RX J0757.0+6306)
We present photometry and spectroscopy of HT Camelopardalis, a recently
discovered X-ray-bright cataclysmic variable. The spectrum shows bright lines
of H, He I, and He II, all moving with a period of 0.059712(1) d, which we
interpret as the orbital period. The star's brightness varies with a strict
period of 515.0592(2) s, and a mean full amplitude of 0.11 mag. These
properties qualify it as a /bona fide/ DQ Herculis star (intermediate polar) --
in which the magnetism of the rapidly rotating white dwarf channels accretion
flow to the surface. Normally at V=17.8, the star shows rare and very brief
outbursts to V=12-13. We observed one in December 2001, and found that the 515
s pulse amplitude had increased by a factor of ~100 (in flux units). A
transient orbital signal may also have appeared.Comment: PDF, 19 pages, 3 tables, 6 figures; accepted, in press, to appear
June 2002, PASP; more info at http://cba.phys.columbia.edu
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