1,113 research outputs found
Model color-magnitude diagrams for Hubble Space Telescope observations of Local Group dwarf galaxies
In this paper, we discuss a method to conduct a quantitative study of the
star formation history (SFH) of Local Group (LG) galaxies using (HST) data.
This method has proven to be successful in the analysis of the SFH of the same
kind of galaxies using ground-based observations. It is based on the comparison
of observed CMDs with a set of model CMDs. The latter are computed assuming
different evolutionary scenarios, and include a detailed simulation of
observational effects. HST CMDs are ~3 mags deeper than typical ground-based
CMDs, allowing the observation, for all LG galaxies, of a part of the CMD that
up till now had remained accessible only for the very nearest galaxies. A very
important feature that will become accessible is the HB+red-clump. The
distribution of stars along this structure is quite sensitive to age and
metallicity and should provide a very important improvement in the time
resolution of the SFH for stars older than ~2-3 Gyr. We show and discuss four
model CMDs which would be comparable with CMDs from deep HST observations.
These model CMDs represent the following evolutionary scenarios corresponding
to a wide range of dwarf galaxy sub-types from dI to dE: A) a constant SFR from
15Gyr ago to the present time; B) as A), but with the SFR stopped 0.5 Gyr ago;
C) a constant SFR in the age range 10-9Gyr and D) as C) but in the age range
15-12 Gyr. In all four cases a range of metallicity from Z=0.0001 to Z=0.004
has been assumed. The present analysis is just a first qualitative approach to
what one may expect to find in the CMDs of LG galaxies. However a complete set
of model CMDs must be computed to analize the data for each galaxy, using the
crowding effects derived for that particular galaxy.Comment: 2 fi
The Stellar Content and the Star Formation History of the Local Group Dwarf Galaxy LGS 3
The star formation history (SFH) and the properties of the dwarf galaxy LGS3
are analyzed using color-magnitude (CM) diagrams plotted from VRI photometry of
736 stars. The distance to the galaxy is estimated through the position of the
tip or the red giant branch. Two acceptable results have been obtained:
0.77+/-0.07 Mpc and 0.96+/-0.07 Mpc, although the first value is favored by
complementary considerations on the stellar content of the galaxy. Both values
make LGS3 a possible satellite of M31 or of M33. The SFH is investigated for
each of the two adopted distances comparing the observed CM diagrams with model
CM diagrams computed for different star formation rates (psi(t)) and chemical
enrichment laws (Z(t)).
The results are compatible with LGS3 having been forming stars since an early
epoch, 15-12 Gyr ago, at an almost constant rate if distance is 0.77 Mpc or at
an exponentially decreasing rate if distance is 0.96 Mpc. According to our
models, the current metallicity would range from Z~0.0007 to Z~0.002. Other
results are the current psi(t): (0.55+/-0.04)x10^(-10) Mo yr^(-1) pc^(-2) or
(0.47+/-0.07)x10^(-10) Mo yr^(-1) pc^(-2), depending of the distance, and its
average for the entire life of the galaxy, =(1.4+/-0.1)x10^(-10) Mo
yr^(-1) pc^(-2). At the present psi(t), the probability of LGS3 having an HII
region is 0.2, which is compatible with the fact that no HII regions have been
found in the galaxy. Its fraction of gas relative to the mass intervening in
the chemical evolution is about 0.40 and its percentage of dark matter (that
which cannot be explained as stellar remnants or by extrapolation of the used
IMF to low masses) is 95%. The results for psi(t) and Z(t) for d=0.77 Mpc are
compatible with a moderate outflow of well mixed material (lambda=3), but largeComment: 23 pages, 3 tables, 14 figures. AJ in pres
The stellar content of the Local Group dwarf galaxy Phoenix
We present new deep ground-based photometry of the Local Group dwarf
galaxy Phoenix. Our results confirm that this galaxy is mainly dominated by red
stars, with some blue plume stars indicating recent (100 Myr old) star
formation in the central part of the galaxy. We have performed an analysis of
the structural parameters of Phoenix based on an ESO/SRC scanned plate, in
order to search for differentiated component. The results were then used to
obtain the color-magnitude diagrams for three different regions of Phoenix in
order to study the variation of the properties of its stellar population. The
young population located in the central component of Phoenix shows a clear
asymmetry in its distribution, that could indicate a propagation of star
formation across the central component. The HI cloud found at 6 arcmin
Southwest by Young & Lo (1997) could have been involved in this process.
We also find the presence of a substantial intermediate-age population in the
central region of Phoenix that would be less abundant or absent in its outer
regions. This result is also consistent with the gradient found in the number
of horizontal branch stars, whose frequency relative to red giant branch stars
increases towards the outer part of the galaxy. These results, together with
those of our morphological study, suggest the existence of an old, metal-poor
population with a spheroidal distribution surrounding the younger inner
component of Phoenix. This two-component structure may resemble the halo-disk
structure observed in spirals, although more data, in particular on kinematics,
are necessary to confirm this.Comment: 46 pages, 21 figures, 9 Tables, to be published in AJ, August 9
Surface Brightness and Stellar Populations at the Outer Edge of the Large Magellanic Cloud: No Stellar Halo Yet
We present a high quality CMD for a 36'x 36' field located 8 degrees (7 kpc)
from the LMC center, as well as a precise determination of the LMC surface
brightness derived from the resolved stellar population out to this large
galactocentric radius. This deep CMD shows for the first time the detailed age
distribution at this position, where the surface brightness is V=26.5 mag/sq".
At a radius R=474' the main sequence is well populated from the oldest turnoff
at I=21.5 to the 2.5 Gyr turnoff at I=19.5. Beyond this radius, a relatively
strong gradient in the density of stars with ages in the 2.5-4 Gyr range is
apparent. There are some stars brighter and bluer than the main population,
quite uniformly distributed over the whole area surveyed, which are well
matched by a 1.5 Gyr isochrone and may be indicative of a relatively recent
star formation, or merger, event. The surface brightness profile of the LMC
remains exponential to this large galactocentric radius and shows no evidence
of disk truncation. Combining the information on surface brightness and stellar
population we conclude that the LMC disk extends (and dominates over a possible
stellar halo) out to a distance of at least 7 kpc. These results confirm that
the absence of blue stars in the relatively shallow off-center CMDs of dIrr
galaxies is not necessarily evidence for an exclusively old stellar population
resembling the halo of the Milky Way.Comment: ApJLett, in press 13 pages including 3 color figure
The origin of the LMC stellar bar: clues from the SFH of the bar and inner disk
We discuss the origin of the LMC stellar bar by comparing the star formation
histories (SFH) obtained from deep color-magnitude diagrams (CMDs) in the bar
and in a number of fields in different directions within the inner disk. The
CMDs, reaching the oldest main sequence turnoffs in these very crowded fields,
have been obtained with VIMOS on the VLT in service mode, under very good
seeing conditions. We show that the SFHs of all fields share the same patterns,
with consistent variations of the star formation rate as a function of time in
all of them. We therefore conclude that no specific event of star formation can
be identified with the formation of the LMC bar, which instead likely formed
from a redistribution of disk material that occurred when the LMC disk became
bar unstable, and shared a common SFH with the inner disk thereafter. The
strong similarity between the SFH of the center and edge of the bar rules out
significant spatial variations of the SFH across the bar, which are predicted
by scenarios of classic bar formation through buckling mechanisms.Comment: MNRAS Letters, accepte
The Star Formation History of the Pegasus Dwarf Irregular Galaxy
The star formation history (SFH) of the Pegasus dIr, a likely Local Group
member at 0.95 Mpc from the Milky Way, is investigated. We characterize the SFH
by two basic functions: the star formation rate, psi(t), and the chemical
enrichment law, Z(t). It has been derived by comparing the color-magnitude
diagram of the resolved stars in Pegasus, with a total of 189 model diagrams
produced with different psi(t) and Z(t) laws.
Star formation began in Pegasus about 15 Gyr ago and was larger, on average,
during the first half than during the second half of the galaxy's life. During
the most recent epoch, the star formation seems to be produced in a bursting
mode. This may have been the case for the whole life of the galaxy, although
the resolution in time towards older epochs is not good enough to actually
detect it. As for the chemical enrichment law, the best way to account for the
observed metallicity of the galaxy (Zf=0.002(+0.002/-0.001) is that it suffered
a prompt initial chemical enrichment. This would be the case if infall was
important and points to a picture in which the galaxy began forming stars and
enriching its interstellar medium in an early phase of collapse.
Pegasus and NGC 6822 are the only dIrs for which the kind of analysis
presented here has been done. The fact that both galaxies show an important old
to intermediate-age stellar population indicates that the Baade's sheet
observed in most dIr, may in fact be the signature of an important population
of old stars and suggest that dIr actually are old objects populated by large
numbers of old stars. The percentage of dark matter in Pegasus turns out to be
92%.Comment: 33 pages, 1 table, 11 figures Astronomical Journal, in pres
Spatially resolved LMC star formation history: I. Outside in evolution of the outer LMC disk
We study the evolution of three fields in the outer LMC disk Rgc=3.5-6.2 Kpc.
Their star formation history indicates a stellar populations gradient such that
younger stellar populations are more centrally concentrated. We identify two
main star forming epochs, separated by a period of lower activity between ~7
and ~4 Gyr ago. Their relative importance varies from a similar amount of stars
formed in the two epochs in the innermost field, to only 40% of the stars
formed in the more recent epoch in the outermost field. The young star forming
epoch continues to the present time in the innermost field, but lasted only
till ~0.8 and 1.3 Gyr ago at Rgc=5.5 degrees and 7.1 degrees, respectively.
This gradient is correlated with the measured HI column density and implies an
outside-in quenching of the star formation, possibly related to a variation of
the size of the HI disk. This could either result from gas depletion due to
star formation or ram-pressure stripping, or from to the compression of the gas
disk as ram-pressure from the Milky Way halo acted on the LMC interstellar
medium. The latter two situations may have occurred when the LMC first
approached the Milky Way.Comment: 15 pages, 13 figures, 4 tables. MNRAS, in pres
- …