arXiv subjects

J. S. Gallagher III

Publications and source records attributed to J. S. Gallagher III.

75 records · Page 5Linked to original sources

arXiv2000

The Metallicity Distribution Function of Red Giants in the LMC

We report new metallicity determinations for 39 red giants in a 220 square arcminute region, 1.8 degrees southwest of the bar of the Large Magellanic Cloud. These abundance measurements are based on spectroscopy of the Ca II infrared triplet. The metallicity distribution function (MDF) shows a strong peak at [Fe/H] = -0.57 +/- 0.04. Half the red giants in our field fall within the range -0.83 < [Fe/H] < -0.41. We find a striking contrast in the shape of the MDF below [Fe/H] < -1 between our inner disk field and the distant outer field studied by Olszewski (1993). Our field-star MDF seems similar to that of the intermediate-age (1-3 Gyr) star clusters. We have also obtained abundance estimates using Stromgren photometry for ~1000 red giants in the same field. The Stromgren measurements, which are sensitive to a combination of cyanogen and iron lines, correlate well with the Ca II measurements, but a metallicity-dependent offset is found (abridged).

astro-ph↗

arXiv1999

Observations and Implications of the Star Formation History of the LMC

We present derivations of star formation histories based on color-magnitude diagrams of three fields in the LMC from HST/WFPC2 observations. A significant component of stars older than 4 Gyr is required to match the observed color-magnitude diagrams. Models with a dispersion-free age-metallicity relation are unable to reproduce the width of the observed main sequence; models with a range of metallicity at a given age provide a much better fit. Such models allow us to construct complete ``population boxes'' for the LMC based entirely on color-magnitude diagrams; remarkably, these qualitatively reproduce the age-metallicity relation observed in LMC clusters. We discuss some of the uncertainties in deriving star formation histories. We find, independently of the models, that the LMC bar field has a larger relative component of older stars than the outer fields. The main implications suggested by this study are: 1) the star formation history of field stars appears to differ from the age distribution of clusters, 2) there is no obvious evidence for bursty star formation, but our ability to measure bursts shorter in duration than $\sim$ 25% of any given age is limited by the statistics of the observed number of stars, 3) there may be some correlation of the star formation rate with the last close passage of the LMC/SMC/Milky Way, but there is no dramatic effect, and 4) the derived star formation history is probably consistent with observed abundances, based on recent chemical evolution models.

astro-ph↗

arXiv1999

Kinematics and Structure of the Starburst Galaxy NGC 7673

The morphology and kinematics of the luminous blue starburst galaxy NGC 7673 are explored using the WIYN 3.5m telescope. Signs of a past kinematic disturbance are detected in the outer galaxy; the most notable feature is a luminous ripple located 1.55 arc minutes from the center of NGC 7673. Sub-arc second imaging in B and R filters also reveals red dust lanes and blue star clusters that delineate spiral arms in the bright inner disk and narrow band Halpha imaging shows that the luminous star clusters are associated with giant H II regions. The Halpha kinematics measured with echelle imaging spectroscopy using the WIYN DensePak fiber array imply that these HII regions are confined to a smoothly rotating disk. The velocity dispersion in ionized gas in the disk is approximately 24 km/s, which sets an upper bound on the dispersion of young stellar populations. Broad emission components with velocity dispersion approx. 63 km/s are found in some regions are likely produced by mechanical power supplied by massive, young stars; a violent starburst is occurring in a kinematically calm disk. Although the asymmetric outer features point to a merger or interaction as the starburst trigger, the inner disk structure constrains the strength of the event to the scale of a minor merger or weak interaction that occurred at least an outer disk dynamical time scale in the past.

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