2,232 research outputs found
The ACS LCID Project: Variable Stars in Tucana and LGS3
We present preliminary results concerning the search for short-period
variable stars in Tucana and LGS3 based on very deep HST/ACS imaging. In the
one chip per galaxy we studied so far, a total of 230 and 80 candidates
variables were found, respectively. For Tucana, we identified 134 of them as RR
Lyrae stars (RRL) pulsating in the fundamental mode (RRab), 51 in the
first-overtone mode (RRc), and 37 in both modes simultaneoulsy (RRd), as well
as four candidate anomalous Cepheids (AC). In the case of LGS3, we found 45
RRab and 5 RRc, plus three candidates RRd and five candidate AC. The
metallicities obtained from the mean period of the RRab are [Fe/H]_{Tuc}=-1.7
and [Fe/H]_{LGS3}=-1.8.Comment: 2 pages, 1 figure. To appear in the Proceedings of IAU Symposium 241:
"Stellar Populations as Building Blocks of Galaxies", 10-16 December, 2006 at
La Palma, Canary Islands, Spai
Homogeneous Photometry VI: Variable Stars in the Leo I Dwarf Spheroidal Galaxy
We have characterized the pulsation properties of 164 candidate RR Lyrae
variables (RRLs) and 55 candidate Anomalous and/or short-period Cepheids in Leo
I dwarf spheroidal galaxy. On the basis of its RRLs Leo I is confirmed to be an
Oosterhoff-intermediate type galaxy, like several other dwarfs. We show that in
their pulsation properties, the RRLs representing the oldest stellar population
in the galaxy are not significantly different from those of five other nearby,
isolated dwarf spheroidal galaxies. A similar result is obtained when comparing
them to RR Lyrae stars in recently discovered ultra-faint dwarf galaxies. We
are able to compare the period distributions and period-amplitude relations for
a statistically significant sample of ab type RR Lyrae stars in dwarf galaxies
(~1300stars) with those in the Galactic halo field (~14,000stars) and globular
clusters (~1000stars). Field RRLs show a significant change in their period
distribution when moving from the inner (dG14kpc)
halo regions. This suggests that the halo formed from (at least) two dissimilar
progenitors or types of progenitor. Considered together, the RRLs in classical
dwarf spheroidal and ultra-faint dwarf galaxies-as observed today-do not appear
to follow the well defined pulsation properties shown by those in either the
inner or the outer Galactic halo, nor do they have the same properties as RRLs
in globular clusters. In particular, the samples of fundamental-mode RRLs in
dwarfs seem to lack High Amplitudes and Short Periods ("HASP":AV>1.0mag and P
<0.48d) when compared with those observed in the Galactic halo field and
globular clusters. The observed properties of RRLs do not support the idea that
currently existing classical dwarf spheroidal and ultra-faint dwarf galaxies
are surviving representative examples of the original building blocks of the
Galactic halo.Comment: 49 pages in referee format, 12 figure
The ACS LCID project. IX. Imprints of the early Universe in the radial variation of the star formation history of dwarf galaxies
Based on Hubble Space Telescope observations from the Local Cosmology from
Isolated Dwarfs project, we present the star formation histories, as a function
of galactocentric radius, of four isolated Local Group dwarf galaxies: two dSph
galaxies, Cetus and Tucana, and two transition galaxies (dTrs), LGS-3 and
Phoenix. The oldest stellar populations of the dSphs and dTrs are, within the
uncertainties, coeval () at all galactocentric radii. We find that
there are no significative differences between the four galaxies in the
fundamental properties (such as the normalized star formation rate or
age-metallicity relation) of their outer regions (radii greater than four
exponential scale lengths); at large radii, these galaxies consist exclusively
of old () metal-poor stars. The duration of star formation in
the inner regions vary from galaxy to galaxy, and the extended central star
formation in the dTrs produces the dichotomy between dSph and dTr galaxy types.
The dTr galaxies show prominent radial stellar population gradients: the
centers of these galaxies host young () populations while the age
of the last formation event increases smoothly with increasing radius. This
contrasts with the two dSph galaxies. Tucana shows a similar, but milder,
gradient, but no gradient in age is detected Cetus. For the three galaxies with
significant stellar population gradients, the exponential scale length
decreases with time. These results are in agreement with outside-in scenarios
of dwarf galaxy evolution, in which a quenching of the star formation toward
the center occurs as the galaxy runs out of gas in the outskirts.Comment: Accepted to be published in Ap
Exploring Halo Substructure with Giant Stars. XV. Discovery of a Connection between the Monoceros Ring and the Triangulum-Andromeda Overdensity?
Thanks to modern sky surveys, over twenty stellar streams and overdensity
structures have been discovered in the halo of the Milky Way. In this paper, we
present an analysis of spectroscopic observations of individual stars from one
such structure, "A13", first identified as an overdensity using the M giant
catalog from the Two Micron All-Sky Survey. Our spectroscopic observations show
that stars identified with A13 have a velocity dispersion of 40
, implying that it is a genuine coherent structure rather
than a chance super-position of random halo stars. From its position on the
sky, distance (15~kpc heliocentric), and kinematical properties, A13 is
likely to be an extension of another low Galactic latitude substructure -- the
Galactic Anticenter Stellar Structure (also known as the Monoceros Ring) --
towards smaller Galactic longitude and farther distance. Furthermore, the
kinematics of A13 also connect it with another structure in the southern
Galactic hemisphere -- the Triangulum-Andromeda overdensity. We discuss these
three connected structures within the context of a previously proposed scenario
that one or all of these features originate from the disk of the Milky Way.Comment: 12 pages, 9 figures. Accepted for publication in Ap
Weak Galactic halo--dwarf spheroidal connection from RR Lyrae stars
We discuss the role that dwarf galaxies may have played in the formation of
the Galactic halo (Halo) using RR Lyrae stars (RRL) as tracers of their ancient
stellar component. The comparison is performed using two observables (periods,
luminosity amplitudes) that are reddening and distance independent.
Fundamental mode RRL in six dwarf spheroidals and eleven ultra faint dwarf
galaxies (1,300) show a Gaussian period distribution well peaked around a mean
period of =0.610+-0.001 days (sigma=0.03). The Halo RRL (15,000) are
characterized by a broader period distribution.
The fundamental mode RRL in all the dwarf spheroidals apart from Sagittarius
are completely lacking in High Amplitude Short Period (HASP) variables, defined
as those having P 0.75mag. Such variables are not uncommon
in the Halo and among the globular clusters and massive dwarf irregulars. To
further interpret this evidence, we considered eighteen globulars covering a
broad range in metallicity (-2.3< [Fe/H]< -1.1) and hosting more than 35 RRL
each.
The metallicity turns out to be the main parameter, since only globulars more
metal--rich than [Fe/H] -1.5 host RRL in the HASP region. This finding suggests
that dSphs similar to the surviving ones do not appear to be the major
building-blocks of the Halo. Leading physical arguments suggest an extreme
upper limit of 50% to their contribution. On the other hand, massive dwarfs
hosting an old population with a broad metallicity distribution (Large
Magellanic Cloud, Sagittarius) may have played a primary role in the formation
of the Halo.Comment: accepted for publication in ApJ Lette
The ACS LCID project. X. The Star Formation History of IC 1613: Revisiting the Over-Cooling Problem
We present an analysis of the star formation history (SFH) of a field near
the half light radius in the Local Group dwarf irregular galaxy IC 1613 based
on deep Hubble Space Telescope Advanced Camera for Surveys imaging. Our
observations reach the oldest main sequence turn-off, allowing a time
resolution at the oldest ages of ~1 Gyr. Our analysis shows that the SFH of the
observed field in IC 1613 is consistent with being constant over the entire
lifetime of the galaxy. These observations rule out an early dominant episode
of star formation in IC 1613. We compare the SFH of IC 1613 with expectations
from cosmological models. Since most of the mass is in place at early times for
low mass halos, a naive expectation is that most of the star formation should
have taken place at early times. Models in which star formation follows mass
accretion result in too many stars formed early and gas mass fractions which
are too low today (the "over-cooling problem"). The depth of the present
photometry of IC 1613 shows that, at a resolution of ~1 Gyr, the star formation
rate is consistent with being constant, at even the earliest times, which is
difficult to achieve in models where star formation follows mass assembly.Comment: 13 pages, 12 figures, accepted for publication in the Ap
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