58 research outputs found
Balmer and Metal Absorption Feature Gradients in M32
Spectra from MDM Observatory are used to assess Lick/IDS feature strength
gradients inside the half-light radius of the compact Local Group elliptical
galaxy M32. All but a few (of 24 measured) indices show a statistically
significant gradient. Comparing with models, the index gradients indicate a
mean age and abundance gradient in the sense that the nucleus is a factor of
2.5 younger and a factor of 0.3 dex more metal-rich than at 1 effective radius.
This conclusion is only weakly dependent on which index combinations are used
and is robust to high accuracy. Stars near the M32 nucleus have a mean age and
heavy element abundance [M/H] of (4.7 Gyr, +0.02), judging from models by
Worthey with variable abundance ratios. This result has very small formal
random errors, although, of course, there is significant age-metallicity
degeneracy along an (age, abundance) line segment from (5.0 Gyr, 0.00) to (4.5
Gyr, +0.05). An abundance pattern of [C/M]=+0.077, [N/M]=-0.13, [Mg/M]=-0.18,
[Fe/M]~0.0, and [Na/M]=+0.12 is required to fit the feature data, with a
fitting precision of about 0.01 dex. Model uncertainties make the accuracies of
these values at least twice the magnitude of the precision. Forcing
scaled-solar abundances does not change the age very much, but it increases the
rms goodness of model-data fit by a factor of 3 and broadens the allowed range
of age to Gyr. The overall abundance pattern contrasts with larger
elliptical galaxies, in which all measurable lighter elements are enhanced
relative to iron and calcium.Comment: 23 pages, 9 figures, Astronomical Journal, in pres
A 2dF spectroscopic study of globular clusters in NGC 5128: Probing the formation history of the nearest giant Elliptical
We have performed a spectroscopic study of globular clusters (GCs) in the
giant elliptical NGC 5128 using the 2dF facility at the Anglo-Australian
telescope. We obtained integrated optical spectra for a total of 254 GCs, 79 of
which are newly confirmed on the basis of their radial velocities and spectra.
In addition, we obtained an integrated spectrum of the galaxy starlight along
the southern major axis. We derive an empirical metallicity distribution
function (MDF) for 207 GCs (~14 of the estimated total GC system) based upon
Milky Way GCs. This MDF is multimodal at high statistical significance with
peaks at [Z/H]~-1.3 and -0.5. A comparison between the GC MDF and that of the
stellar halo at 20 kpc (~4 Reff) reveals close coincidence at the metal-rich
ends of the distributions. However, an inner 8 kpc stellar MDF shows a clear
excess of metal-rich stars when compared to the GCs. We compare a higher S/N
subsample (147 GCs) with two stellar population models which include non-solar
abundance ratio corrections. The vast majority of our sample (~90%) appears
old, with ages similar to the Milky Way GC system. There is evidence for a
population of intermediate-age (~4-8 Gy) GCs (<15% of the sample) which are on
average more metal-rich than the old GCs. We also identify at least one younger
cluster (~1-2 Gy) in the central regions of the galaxy. Our observations are
consistent with a picture where NGC 5128 has undergone at least two mergers
and/or interactions involving star formation and limited GC formation since
z=1, however the effect of non-canonical hot stellar populations on the
integrated spectra of GCs remains an outstanding uncertainty in our GC age
estimates.Comment: 17 figures, some long table
Detection of low Eu abundances in extremely metal-poor stars and the origin of r-process elements
We report abundance analyses of three extremely metal-poor stars with [Fe/H]
, using the Subaru High Dispersion Spectrograph (HDS). All are
found to have sub-solar values of [Eu/Fe]. Comparison with our chemical
evolution model of the Galactic halo implies the dominant source of Eu to be
the low-mass end of the supernova mass range. Future studies of stars with low
Eu abundances will be important to determine the r-process site.Comment: 7 pages, 2 figures, accepted for publication in the Astrophysical
Journal Letter
The Frequency of Barred Spiral Galaxies in the Near-IR
We have determined the fraction of barred galaxies in the H-band for a
statistically well-defined sample of 186 spirals drawn from the Ohio State
University Bright Spiral Galaxy survey. We find 56% of our sample to be
strongly barred at H, while another 16% is weakly barred. Only 27% of our
sample is unbarred in the near-infrared. The RC3 and the Carnegie Atlas of
Galaxies both classify only about 30% of our sample as strongly barred. Thus
strong bars are nearly twice as prevalent in the near-infrared as in the
optical. The frequency of genuine optically hidden bars is significant, but
lower than many claims in the literature: 40% of the galaxies in our sample
that are classified as unbarred in the RC3 show evidence for a bar in the
H-band, while for the Carnegie Atlas this fraction is 66%. Our data reveal no
significant trend in bar fraction as a function of morphology in either the
optical or H-band. Optical surveys of high redshift galaxies may be strongly
biased against finding bars, as bars are increasingly difficult to detect at
bluer rest wavelengths.Comment: LaTeX with AASTeX style file, 23 pages with 6 figures. Accepted for
publication in The Astronomical Journal (Feb. 2000
Population Synthesis in the Blue IV: Accurate Model Predictions for Lick Indices and UBV Colors in Single Stellar Populations
[Abridged] We present new model predictions for 16 Lick absorption line
indices from Hdelta through Fe5335, and UBV colors for single stellar
populations (SPs) with ages ranging between 1 and 15 Gyr, [Fe/H] ranging from
-1.3 to +0.3, and variable abundance ratios. We develop a method to estimate
mean ages and abundances of Fe, C, N, Mg, and Ca that explores the sensitivity
of the various indices to those parameters. When applied to high-S/N Galactic
cluster data, the models match the clusters' elemental abundances and ages with
high precision. Analyzing stacked SDSS spectra of early-type galaxies brighter
than Lstar, we find mean luminosity-weighted ages of the order of ~ 8 Gyr and
iron abundances slightly below solar. Abundance ratios, [X/Fe], are higher than
solar, and correlate positively with galaxy luminosity. Nitrogen is the element
whose abundance correlates the most strongly with luminosity, which seems to
indicate secondary enrichment. This result may impose a lower limit of 50-200
Myr to the time-scale of star formation in early-type galaxies. Unlike in the
case of clusters, in galaxies bluer Balmer lines yield younger ages than Hbeta.
This age discrepancy is stronger for lower luminosity galaxies. We examine four
scenarios to explain this trend. The most likely is the presence of small
amounts of a young/intermediate-age SP component. Two-component models provide
a better match to the data when the mass fraction of the young component is a
few %. This result implies that star formation has been extended in early-type
galaxies, and more so in less massive galaxies, lending support to the
``downsizing'' scenario. It also implies that SP synthesis models are capable
of constraining not only the mean ages of SPs in galaxies, but also their age
spread.Comment: To appear in the Astrophysical Journal Supplement Series. 55 Pages,
using emulateapj5.sty. Full version, containing all (enlarged) figures can be
found at http://www.astro.virginia.edu/~rps7v/Models/ms.pdf . A number of
useful tables in the Appendix can be obtained in advance of publication by
request to the autho
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