57 research outputs found
WIYN Survey for Carbon Stars in the M31 and Cetus Dwarf Spheroidal Galaxies: Evolutionary Implications
We report results of a photometric survey with the WIYN telescope for carbon
stars in the M 31 dwarf spheroidal (dSph) companions And III, And V, And VI,
and And VII, as well as in the relatively isolated Local Group dSph Cetus. We
find three carbon-star candidates in And VII and one carbon star in each And VI
and Cetus. Comparing the carbon star content with other Local Group dwarf
galaxies, we argue against the presence of substantial intermediate-age stellar
populations in the all of the galaxies surveyed with the exception of And VII.
We discuss these results in the context of the origin of the Andromeda dSphs
and conclude that these are ancient galaxies, most of which ceased star
formation long before the main merger events in M31. The M31 dSphs therefore
show less diversity in star formation histories than the Galactic dSph
companions, or the M31 dE companions, as illustrated by NGC147 which was
surveyed as a calibration object. All of our dSph targets except And V have
candidate carbon stars below the tip of the RGB, which resemble CH stars found
in globular clusters. We estimate that 0.3% of stars in the dSphs are CH stars,
presumably as a result of C pollution from a binary companion. Comparisons with
CH star frequencies in globular clusters could constrain the impact of dense
environments on the frequency of this form of binary star evolution.Comment: accepted for publication in A
A spectroscopy-based Age-Metallicity Relation of the SMC
The Small Magellanic Cloud (SMC) is the only dwarf galaxy in the Local Group that is known to have formed and preserved populous star clusters continuously over the past 12 Gyr. Due to its proximity (â 60 kpc), stars can be resolved well below the oldest main sequence turnoff points. This facilitates accurate age and metallicity determinations without suffering from the age-metallicity degeneracy. Therefore, the SMC star clusters provide a unique closely spaced set of single-age, single-metallicity tracers to derive a well-sampled age-metallicity relation required for the understanding of the star formation history of this satellite galaxy. Up to date spectroscopically based metallicity estimates exist only for the small number of 7 clusters (Da Costa & Hatzidimitriou 1998). Our project now more than doubles the available data set by the observation of 10 additional cluster
Dark Matter and Stellar Mass in the Luminous Regions of Disk Galaxies
We investigate the correlations among stellar mass (M_*), disk scale length
(R_d), and rotation velocity at 2.2 disk scale lengths (V_2.2) for a sample of
81 disk-dominated galaxies (disk/total >= 0.9) selected from the SDSS. We
measure V_2.2 from long-slit H-alpha rotation curves and infer M_* from galaxy
i-band luminosities (L_i) and g-r colors. We find logarithmic slopes of
2.60+/-0.13 and 3.05+/-0.12 for the L_i-V_2.2 and M_*-V_2.2 relations, somewhat
shallower than most previous studies, with intrinsic scatter of 0.13 dex and
0.16 dex. Our direct estimates of the total-to-stellar mass ratio within
2.2R_d, assuming a Kroupa IMF, yield a median ratio of 2.4 for M_*>10^10 Msun
and 4.4 for M_*=10^9-10^10 Msun, with large scatter at a given M_* and R_d. The
typical ratio of the rotation speed predicted for the stellar disk alone to the
observed rotation speed at 2.2R_d is ~0.65. The distribution of R_d at fixed
M_* is broad, but we find no correlation between disk size and the residual
from the M_*-V_2.2 relation, implying that this relation is an approximately
edge-on view of the disk galaxy fundamental plane. Independent of the assumed
IMF, this result implies that stellar disks do not, on average, dominate the
mass within 2.2R_d. We discuss our results in the context of infall models of
disk formation in cold dark matter halos. A model with a disk-to-halo mass
ratio m_d=0.05 provides a reasonable match to the R_d-M_* distribution for spin
parameters \lambda ranging from ~0.04-0.08, and it yields a reasonable match to
the mean M_*-V_2.2 relation. A model with m_d=0.1 predicts overly strong
correlations between disk size and M_*-V_2.2 residual. Explaining the wide
range of halo-to-disk mass ratios within 2.2R_d requires significant scatter in
m_d values, with systematically lower m_d for galaxies with lower .Comment: 18 pages, 2 tables, 7 figures, Accepted to ApJ, Table 1 updated,
otherwise minor change
Low-Surface-Brightness Galaxies in the Sloan Digital Sky Survey. I. Search Method and Test Sample
In this paper we present results of a pilot study to use imaging data from
the Sloan Digital Sky Survey (SDSS) to search for low-surface-brightness (LSB)
galaxies. For our pilot study we use a test sample of 92 galaxies from the
catalog of Impey et al. (1996) distributed over 93 SDSS fields of the Early
Data Release (EDR). Many galaxies from the test sample are either LSB or dwarf
galaxies. To deal with the SDSS data most effectively a new photometry software
was created, which is described in this paper. We present the results of the
selection algorithms applied to these 93 EDR fields. Two galaxies from the
Impey et al. test sample are very likely artifacts, as confirmed by follow-up
imaging. With our algorithms, we were able to recover 87 of the 90 remaining
test sample galaxies, implying a detection rate of 96.5%. The three
missed galaxies fall too close to very bright stars or galaxies. In addition,
42 new galaxies with parameters similar to the test sample objects were found
in these EDR fields (i.e., 47% additional galaxies). We present the main
photometric parameters of all identified galaxies and carry out first
statistical comparisons. We tested the quality of our photometry by comparing
the magnitudes for our test sample galaxies and other bright galaxies with
values from the literature. All these tests yielded consistent results. We
briefly discuss a few unusual galaxies found in our pilot study, including an
LSB galaxy with a two-component disk and ten new giant LSB galaxies.Comment: 36 pages, 16 figures, accepted for publication by AJ, some figures
were bitmapped to reduce the siz
Complexity on Small Scales: The Metallicity Distribution of the Carina Dwarf Spheroidal Galaxy
The Carina dwarf spheroidal (dSph) galaxy is the only galaxy of this type
that shows clearly episodic star formation separated by long pauses. Here we
present metallicities for 437 radial velocity members of this Galactic
satellite. We obtained medium-resolution spectroscopy with the multi-object
spectrograph FLAMES at the ESO VLT. Our target red giants cover the entire
projected surface area of Carina. Our spectra are centered at the near-infrared
Ca triplet, which is a well-established metallicity indicator for old and
intermediate-age red giants. The resulting data sample provides the largest
collection of spectroscopically derived metallicities for a Local Group dSph to
date. Four of our likely radial velocity members of Carina lie outside of this
galaxy's nominal tidal radius, supporting earlier claims of the possible
existence of such stars beyond the main body of Carina. We find a mean
metallicity of [Fe/H]=-1.7 dex. The formal full width at half maximum of the
metallicity distribution is 0.92 dex, while the full range of metallicities
spans ~-3.0<[Fe/H]<0.0 dex. The metallicity distribution might be indicative of
several subpopulations. There appears to be a mild radial gradient such that
more metal-rich populations are more centrally concentrated, matching a similar
trend for an increasing fraction of intermediate-age stars. This as well as the
photometric colors of the more metal-rich red giants suggest that Carina
exhibits an age-metallicity relation. Indeed the age-metallicity degeneracy
seems to conspire to form a narrow red giant branch despite the considerable
spread in metallicity and wide range of ages. The metallicity distribution is
not well-matched by a simple closed-box model of chemical evolution, but
requires models that take into account also infall and outflows. (Abridged)Comment: 29 pages, 13 figures, 6 tables, accepted for publication in the A
A Survey of z>5.7 Quasars in the Sloan Digital Sky Survey II: Discovery of Three Additional Quasars at z>6
We present the discovery of three new quasars at z>6 in 1300 deg^2 of SDSS
imaging data, J114816.64+525150.3 (z=6.43), J104845.05+463718.3 (z=6.23) and
J163033.90+401209.6 (z=6.05). The first two objects have weak Ly alpha emission
lines; their redshifts are determined from the positions of the Lyman break.
They are only accurate to 0.05 and could be affected by the presence of broad
absorption line systems. The last object has a Ly alpha strength more typical
of lower redshift quasars. Based on a sample of six quasars at z>5.7 that cover
2870 deg^2 presented in this paper and in Paper I, we estimate the comoving
density of luminous quasars at z 6 and M_{1450} < -26.8 to be (8 +/-
3)x10^{-10} Mpc^{-3} (for H_0 = 50 km/s/Mpc, Omega = 1). HST imaging of two
z>5.7 quasars and high-resolution ground-based images (seeing 0.4'') of three
additional z>5.7 quasars show that none of them is gravitationally lensed. The
luminosity distribution of the high-redshfit quasar sample suggests the bright
end slope of the quasar luminosity function at z 6 is shallower than Psi
L^{-3.5} (2-sigma), consistent with the absence of strongly lensed objects.Comment: AJ in press (Apr 2003), 26 pages, 9 figure
Population genomic analysis of elongated skulls reveals extensive female-biased immigration in Early Medieval Bavaria
Modern European genetic structure demonstrates strong correlations with geography, while genetic analysis of prehistoric humans has indicated at least two major waves of immigration from outside the continent during periods of cultural change. However, population-level genome data that could shed light on the demographic processes occurring during the intervening periods have been absent. Therefore, we generated genomic data from 41 individuals dating mostly to the late 5th/early 6th century AD from present-day Bavaria in southern Germany, including 11 whole genomes (mean depth 5.56Ă). In addition we developed a capture array to sequence neutral regions spanning a total of 5 Mb and 486 functional polymorphic sites to high depth (mean 72Ă) in all individuals. Our data indicate that while men generally had ancestry that closely resembles modern northern and central Europeans, women exhibit a very high genetic heterogeneity; this includes signals of genetic ancestry ranging from western Europe to East Asia. Particularly striking are women with artificial skull deformations; the analysis of their collective genetic ancestry suggests an origin in southeastern Europe. In addition, functional variants indicate that they also differed in visible characteristics. This example of female-biased migration indicates that complex demographic processes during the Early Medieval period may have contributed in an unexpected way to shape the modern European genetic landscape. Examination of the panel of functional loci also revealed that many alleles associated with recent positive selection were already at modern-like frequencies in European populations âŒ1,500 years ago
Complexity on Small Scales II: Metallicities and Ages in the Leo II Dwarf Spheroidal Galaxy
We present metallicities and ages for 52 red giants in the remote Galactic
dwarf spheroidal (dSph) galaxy Leo II. These stars cover the entire surface
area of Leo II and are radial velocity members. We obtained medium-resolution
multi-fiber spectroscopy with ESO/VLT's FLAMES spectrograph. The metallicities
were determined based on the near-infrared Ca II triplet. The resulting
metallicity distribution (MD) is asymmetric and peaks at [Fe/H]=-1.74 dex on
the Carretta & Gratton scale. The full range in metallicities extends from -2.4
to -1.1 dex. As in other dSphs, no extremely metal-poor red giants were found.
We compare Leo II's observed MD with model predictions for several other
Galactic dSphs from the literature. Leo II clearly exhibits a lack of more
metal poor stars, in analogy to the classical G-dwarf problem, which may
indicate a comparable `K-giant problem'. Moreover, its evolution appears to
have been affected by galactic winds. We use our inferred metallicities as an
input parameter for isochrone fits to SDSS photometry and derive approximate
ages. The resulting age-metallicity distribution covers the full age range from
2-15 Gyr on our adopted isochrone scale. During the first 7 Gyr relative to the
oldest stars [Fe/H] appears to have remained almost constant. The almost
constant metallicity at higher ages and a slight drop by about 0.3 dex
thereafter may be indicative of rejuvenation by low metallicity gas. Overall,
the age-metallicity relation appears to support the formation of Leo II from
pre-enriched gas. Evidence for enrichment is seen during the recent 2-4 Gyr.
Our findings support earlier photometric findings of Leo II as a galaxy with a
prominent old and a dominant intermediate-age population. We do not find a
significant radial metallicity gradient nor age gradient in our data.(Abridged)Comment: 23 pages, 12 Figures, accepted for publication in the A
Selection of Metal-poor Giant Stars Using the Sloan Digital Sky Survey Photometric System
We present a method for photometric selection of metal-poor halo giants from
the imaging data of the Sloan Digital Sky Survey (SDSS). These stars are offset
from the stellar locus in the (g-r) vs. (u-g) color-color diagram. Based on a
sample of 29 candidates for which spectra were taken, we derive a selection
efficiency of the order of 50%, for stars brighter than . The
candidates selected in 400 deg of sky from the SDSS Early Data Release
trace the known halo structures (tidal streams from the Sagittarius dwarf
galaxy, the Draco dwarf spheroidal galaxy), indicating that such a
color-selected sample can be used to study the halo structure even without
spectroscopic information. This method, and supplemental techniques for
selecting halo stars, such as RR Lyrae stars and other blue horizontal branch
stars, can produce an unprecedented three-dimensional map of the Galactic halo
based on the SDSS imaging survey.Comment: 8 pages, 3 figures, 1 table. Accepted by Ap
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