161 research outputs found
The Star Formation History of the Carina Dwarf Galaxy
We have analyzed deep B and V photometry of the Carina dwarf spheroidal
reaching below the old main-sequence turnoff to about V = 25. Using simulated
color-magnitude diagrams to model a range of star formation scenarios, we have
extracted a detailed, global star formation history. Carina experienced three
significant episodes of star formation at about 15 Gyr, 7 Gyr, and 3 Gyr.
Contrary to the generic picture of galaxy evolution, however, the bulk of star
formation, at least 50%, occured during the episode 7 Gyr ago, which may have
lasted as long as 2 Gyr. For unknown reasons, Carina formed only 10-20% of its
stars at an ancient epoch and then remained quiescent for more than 4 Gyr. The
remainder (~30%) formed relatively recently, only 3 Gyr ago. Interest in the
local population of dwarf galaxies has increased lately due to their potential
importance in the understanding of faint galaxy counts. We surmise that objects
like Carina, which exhibits the most extreme episodic behavior of any of the
dwarf spheroidal companions to the Galaxy, are capable of contributing to the
observed excess of blue galaxies at B = 24 only if the star formation occurred
instantaneously.Comment: 23 pages of text, 20 figures, 8 tables. AJ, in pres
The Star Formation History of the Local Group dwarf galaxy Leo I
We present a quantitative analysis of the star formation history (SFH) of the
Local Group dSph galaxy Leo I, from the information in its HST [(V-I),I]
color-magnitude diagram (CMD). The method we use is based in comparing, via
synthetic CMDs, the expected distribution of stars in the CMD for different
evolutionary scenarios, with the observed distribution. We consider the SFH to
be composed by the SFR(t), the Z(t), the IMF, and a function ,
controlling the fraction and mass ratio distribution of binary stars.
The comparison between the observed CMD and the model CMDs is done through
chi-square minimization of the differences in the number of stars in a set of
regions of the CMD.
Our solution for the SFH of Leo I defines a minimum of chi-square in a well
defined position of the parameter space, and the derived SFR(t) is robust, in
the sense that its main characteristics are unchanged for different
combinations of the remaining parameters. However, only a narrow range of
assumptions for Z(t), IMF and result in a good agreement between
the data and the models, namely: Z=0.0004, a Kroupa et al. (1993) IMF or
slightly steeper, and a relatively large fraction of binary stars. Most star
formation activity (70% to 80%) occurred between 7 and 1 Gyr ago. At 1 Gyr ago,
it abruptly dropped to a negligible value, but seems to have been active until
at least ~ 300 Myr ago. Our results don't unambiguously answer the question of
whether Leo I began forming stars around 15 Gyr ago, but it appears that the
amount of this star formation, if existing at all, would be small.Comment: 25 pages + 14 figures. Accepted by The Astronomical Journa
Dwarf Cepheids in the Carina Dwarf Spheroidal Galaxy
We have discovered 20 dwarf Cepheids (DC) in the Carina dSph galaxy from the
analysis of individual CCD images obtained for a deep photometric study of the
system. These short-period pulsating variable stars are by far the most distant
(~100 kpc) and faintest (V ~ 23.0) DCs known. The Carina DCs obey a
well-defined period-luminosity relation, allowing us to readily distinguish
between overtone and fundamental pulsators in nearly every case. Unlike RR Lyr
stars, the pulsation mode turns out to be uncorrelated with light-curve shape,
nor do the overtone pulsators tend towards shorter periods compared to the
fundamental pulsators. Using the period-luminosity (PL) relations from Nemec et
al. (1994 AJ, 108, 222) and McNamara (1995, AJ, 109, 1751), we derive (m-M)_0 =
20.06 +/- 0.12, for E(B-V) = 0.025 and [Fe/H] = -2.0, in good agreement with
recent, independent estimates of the distance/reddening of Carina. The error
reflects the uncertainties in the DC distance scale, and in the metallicity and
reddening of Carina. The frequency of DCs among upper main sequence stars in
Carina is approximately 3%. The ratio of dwarf Cepheids to RR Lyr stars in
Carina is 0.13 +/- 0.10, though this result is highly sensitive to the
star-formation history of Carina and the evolution of the Horizontal Branch. We
discuss how DCs may be useful to search effectively for substructure in the
Galactic halo out to Galactocentric distances of ~100 kpc.Comment: 20 pages of text, 7 figure
The Effect of Star Formation History on the Inferred Initial Stellar Mass Function
Peaks and lulls in the star formation rate (SFR) over the history of the
Galaxy produce plateaux and declines in the present day mass function (PDMF)
where the main-sequence lifetime overlaps the age and duration of the SFR
variation. These PDMF features can be misinterpreted as the form of the
intrinsic stellar initial mass function (IMF) if the star formation rate is
assumed to be constant or slowly varying with time. This effect applies to all
regions that have formed stars for longer than the age of the most massive
stars, including OB associations, star complexes, and especially galactic field
stars. Related problems may apply to embedded clusters. Evidence is summarized
for temporal SFR variations from parsec scales to entire galaxies, all of which
should contribute to inferred IMF distortions. We give examples of various star
formation histories to demonstrate the types of false IMF structures that might
be seen. These include short-duration bursts, stochastic histories with
log-normal amplitude distributions, and oscillating histories with various
periods and phases. The inferred IMF should appear steeper than the intrinsic
IMF over mass ranges where the stellar lifetimes correspond to times of
decreasing SFRs; shallow portions of the inferred IMF correspond to times of
increasing SFRs. If field regions are populated by dispersed clusters and
defined by their low current SFRs, then they should have steeper inferred IMFs
than the clusters. The SFRs required to give the steep field IMFs in the LMC
and SMC are determined. Structure observed in several determinations of the
Milky Way field star IMF can be accounted for by a stochastic and bursty star
formation history.Comment: accepted by ApJ for 1 Jan 2006, Vol 636, 12 pages + 6 figure
The Chemical Enrichment History of the Small Magellanic Cloud and Its Gradients
We present stellar metallicities derived from Ca II triplet spectroscopy in
over 350 red giant branch stars in 13 fields distributed in different positions
in the SMC, ranging from 1\arcdeg\@ to 4\arcdeg\@ from its center.
In the innermost fields the average metallicity is [Fe/H] . This value
decreases when we move away towards outermost regions. This is the first
detection of a metallicity gradient in this galaxy. We show that the
metallicity gradient is related to an age gradient, in the sense that more
metal-rich stars, which are also younger, are concentrated in the central
regions of the galaxy.Comment: 30 pages, 13 figures, accepted for publication in Astronomical
Journa
Deep Hubble Space Telescope Imaging of IC 1613 II. The Star Formation History
We present deep HST WFPC2 imaging of the Local Group dwarf irregular galaxy
IC 1613. The photometry is the deepest to date for an isolated dwarf irregular
galaxy. The resulting color-magnitude diagram (CMD) is analyzed using three
different methods to derive a star formation history (SFH). All three find an
enhanced star formation rate (SFR), from 3 to 6 Gyr ago, and similar
age-metallicity relationships (AMR). A comparison of the newly observed outer
field with an earlier studied central field of IC 1613 shows that the SFR in
the outer field has been significantly depressed during the last Gyr. This
implies that the optical scale length of the galaxy has been decreasing with
time and that comparison of galaxies at intermediate redshift with present day
galaxies should take this effect into account. We find strong similarities
between IC 1613 and the more distant Milky Way dSph companions in that all are
dominated by star formation at intermediate ages. In particular, the SFH and
AMR for IC 1613 and Leo I are indistinguishable. This implies that dIrr
galaxies cannot be distinguished from dSphs by their intermediate age stellar
populations. This type of a SFH may also be evidence for slower or suppressed
early star formation in dwarf galaxies due to photoionization after the
reionization of the universe by background radiation. Assuming that IC 1613 is
typical of a dIrr evolving in isolation, since most of the star formation
occurs at intermediate ages, these dwarf systems cannot be responsible for the
fast chemical enrichment of the IGM which is seen at high redshift. There is no
evidence for any large amplitude bursts of star formation in IC 1613, and we
find it highly unlikely that analogs of IC 1613 have contributed to the excess
of faint blue galaxies in existing galaxy redshift surveys.Comment: 32 pages, including 1 table and 17 figures, accepted for publication
in the Astrophysical Journal October 10, 2003 issu
HST observations of the Local Group dwarf galaxy Leo I
We present deep HST F555W (V) and F814W (I) observations of a central field
in the Local Group dwarf spheroidal (dSph) galaxy Leo I. The resulting
color-magnitude diagram (CMD) reaches I \simeq 26 and reveals the oldest ~10-15
Gyr old turnoffs. Nevertheless, a horizontal branch is not obvious in the CMD.
Given the low metallicity of the galaxy, this likely indicates that the first
substantial star formation in the galaxy may have been somehow delayed in Leo I
in comparison with the other dSph satellites of the Milky Way. The subgiant
region is well and uniformly populated from the oldest turnoffs up to the 1 Gyr
old turnoff, indicating that star formation has proceeded in a continuous way,
with possible variations in intensity but no big gaps between successive
bursts, over the galaxy's lifetime. The structure of the red-clump of core
He-burning stars is consistent with the large amount of intermediate-age
population inferred from the main sequence and the subgiant region. In spite of
the lack of gas in Leo I, the CMD clearly shows star formation continuing until
1 Gyr ago and possibly until a few hundred Myrs ago in the central part of the
galaxy.Comment: 26 pages with 8 figures (fig 2 not available electronically). To be
published in ApJ, April 1 1999 (vol.514, #2
Basic parameters of three star clusters in the Small Magellanic Cloud: Kron 11, Kron 63 and NGC 121
We present observations for three star clusters, Kron 11, Kron 63 and NGC
121, in the Small Magellanic Cloud. We have studied their structure and derived
their fundamental parameters by means of their luminosity functions, their
color magnitude diagrams and the Padova suite of isochrones. NGC 121 is a well
studied object, for which we confirm previous evidence about its old age and
low metal content, and have found that it is undergoing mass segregation. Kron
11 and Kron 63 are poorly populated clusters which had never been studied so
far. Kron 11 is several gigayears younger than NGC 121, while Kron 63 is
basically a very young star aggregate. Both clusters are immersed in dense
stellar fields which share the same population properties, suggesting that in
their cases, cluster ages are consistent with typical ages of field stars.Comment: 9 pages; 7 figures, MNRAS pape
Multi-Element Abundance Measurements from Medium-Resolution Spectra. III. Metallicity Distributions of Milky Way Dwarf Satellite Galaxies
We present metallicity distribution functions (MDFs) for the central regions
of eight dwarf satellite galaxies of the Milky Way: Fornax, Leo I and II,
Sculptor, Sextans, Draco, Canes Venatici I, and Ursa Minor. We use the
published catalog of abundance measurements from the previous paper in this
series. The measurements are based on spectral synthesis of iron absorption
lines. For each MDF, we determine maximum likelihood fits for Leaky Box,
Pre-Enriched, and Extra Gas (wherein the gas supply available for star
formation increases before it decreases to zero) analytic models of chemical
evolution. Although the models are too simplistic to describe any MDF in
detail, a Leaky Box starting from zero metallicity gas fits none of the
galaxies except Canes Venatici I well. The MDFs of some galaxies, particularly
the more luminous ones, strongly prefer the Extra Gas Model to the other
models. Only for Canes Venatici I does the Pre-Enriched Model fit significantly
better than the Extra Gas Model. The best-fit effective yields of the less
luminous half of our galaxy sample do not exceed 0.02 Z_sun, indicating that
gas outflow is important in the chemical evolution of the less luminous
galaxies. We surmise that the ratio of the importance of gas infall to gas
outflow increases with galaxy luminosity. Strong correlations of average [Fe/H]
and metallicity spread with luminosity support this hypothesis.Comment: 17 pages, 5 figures; accepted for publication in ApJ; minor
corrections in v3; corrected typographical errors in Tables 1 and 3 in v
A Prescription for Building the Milky Way's Halo from Disrupted Satellites
We develop a semi-analytic method for determining the phase-space population
of tidal debris along the orbit of a disrupting satellite galaxy and illustrate
its use with a number of applications. We use this method to analyze Zhao's
proposal that the microlensing events towards the Large Magellanic Cloud (LMC)
might be explained by an appropriately placed tidal streamer, and find that his
scenarios lead either to unacceptably high overdensities (10 -- 100%) in faint
star counts (apparent magnitudes 17.5 -- 20.5) away from the Galactic plane or
short timescales for the debris to disperse (10^8 years). We predict that the
tidal streamers from the LMC and the Sagittarius dwarf galaxy currently extend
over more than in azimuth along their orbits. Assuming that each
satellite has lost half of its primordial mass, we find that the streamers will
have overdensities in faint star counts of 10 -- 100% and < 1% respectively,
and conclude that this mass loss rate is unlikely for the LMC, but possible for
Sagittarius. If the Galaxy has accreted one hundred
objects (comparable to its current population of globular clusters) at
distances of 20 -- 100 kpc during its lifetime then 10% of the sky will now be
covered by tidal streamers.Comment: 35 pages, LaTeX, 12 postscript figures included. Submitted to Ap
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