857 research outputs found
Associations of Dwarf Galaxies
Hubble Space Telescope Advanced Cameras for Surveys has been used to
determine accurate distances for 20 galaxies from measurements of the
luminosity of the brightest red giant branch stars. Five associations of dwarf
galaxies that had originally been identified based on strong correlations on
the plane of the sky and in velocity are shown to be equally well correlated in
distance. Two more associations with similar properties have been discovered.
Another association is identified that is suggested to be unbound through tidal
disruption. The associations have the spatial and kinematic properties expected
of bound structures with 1 - 10 x 10^11 solar mass. However, these entities
have little light with the consequence that mass-to-light ratios are in the
range 100 - 1000 in solar units. Within a well surveyed volume extending to 3
Mpc, all but one known galaxy lies within one of the groups or associations
that have been identified.Comment: 50 pages, 2 tables, 15 encapsulated figures, 1 (3 part) jpg figure.
Submitted to Astronomical Journa
An inverse method to interpret colour-magnitude diagrams
An inverse method is developed to determine the star formation history, the
age-metallicity relation, and the IMF slope from a colour-magnitude diagram.
The method is applied to the Hipparcos HR diagram. We found that the thin
disk of our Galaxy shows a peak of stellar formation 1.6 Gyr ago. The stars
close to the Sun have a solar metallicity and a mean IMF index equal to 3.2.
However, the model and the evolutionary tracks do not correctly reproduce the
horizontal giant branch.Comment: 14 pages, 11 figures. To be published in Astronomy & Astrophysic
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
The Contribution of TP-AGB and RHeB Stars to the Near-IR Luminosity of Local Galaxies: Implications for Stellar Mass Measurements of High Redshift Galaxies
Using high spatial resolution HST WFC3 and ACS imaging of resolved stellar
populations, we constrain the contribution of thermally-pulsing asymptotic
giant branch (TP-AGB) stars and red helium burning (RHeB) stars to the 1.6 um
near-infrared (NIR) luminosities of 23 nearby galaxies. The TP-AGB phase
contributes as much as 17% of the integrated F160W flux, even when the red
giant branch is well populated. The RHeB population contribution can match or
even exceed the TP-AGB contribution, providing as much as 21% of the integrated
F160W light. The NIR mass-to-light (M/L) ratio should therefore be expected to
vary significantly due to fluctuations in the star formation rate over
timescales from 25 Myr to several Gyr. We compare our observational results to
predictions based on optically derived star formation histories and stellar
population synthesis (SPS) models, including models based on the Padova
isochrones (used in popular SPS programs). The SPS models generally reproduce
the expected numbers of TP-AGB stars in the sample. The same SPS models,
however, give a larger discrepancy in the F160W flux contribution from the
TP-AGB stars, over-predicting the flux by a weighted mean factor of 2.3 +/-0.8.
This larger offset is driven by the prediction of modest numbers of high
luminosity TP-AGB stars at young (<300 Myrs) ages. The best-fit SPS models
simultaneously tend to under-predict the numbers and fluxes of stars on the
RHeB sequence, typically by a factor of 2.0+/-0.6 for galaxies with significant
numbers of RHeBs. Coincidentally, over-prediction of the TP-AGB and
under-prediction of the RHeBs result in a NIR M/L ratio largely unchanged for a
rapid star formation rate. However, the NIR-to-optical flux ratio of galaxies
could be significantly smaller than AGB-rich models would predict, an outcome
that has been observed in some intermediate redshift post-starburst galaxies.
(Abridged)Comment: 28 Pages, 12 Figures, 5 Tables, Accepted for Publication in the
Astrophysical Journa
The Star-forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations. II. Photometric Study of the Intermediate-Age Star Cluster BS 90
We present the results of our investigation of the intermediate-age star
cluster BS 90, located in the vicinity of the HII region N 66 in the SMC,
observed with HST/ACS. The high-resolution data provide a unique opportunity
for a very detailed photometric study performed on one of the rare
intermediate-age rich SMC clusters. The complete set of observations is
centered on the association NGC 346 and contains almost 100,000 stars down to V
~28 mag. In this study we focus on the northern part of the region, which
covers almost the whole stellar content of BS 90. We construct its stellar
surface density profile and derive structural parameters. Isochrone fits on the
CMD of the cluster results in an age of about 4.5 Gyr. The luminosity function
is constructed and the present-day mass function of BS 90 has been obtained
using the mass-luminosity relation, derived from the isochrone models. We found
a slope between -1.30 and -0.95, comparable or somewhat shallower than a
typical Salpeter IMF. Examination of the radial dependence of the mass function
shows a steeper slope at larger radial distances, indicating mass segregation
in the cluster. The derived half-mass relaxation time of 0.95 Gyr suggests that
the cluster is mass segregated due to its dynamical evolution. From the
isochrone model fits we derive a metallicity for BS 90 of [Fe/H]=-0.72, which
adds an important point to the age-metallicity relation of the SMC. We discuss
our findings on this relation in comparison to other SMC clusters.Comment: Accepted for Publication in ApJ, 12 pages emulateapj TeX style, 10
figure
Resolved Near-infrared Stellar Populations in Nearby Galaxies
We present near-infrared (NIR) color-magnitude diagrams (CMDs) for the resolved stellar populations within 26 fields of 23 nearby galaxies (âČ 4 Mpc), based on images in the F110W and F160W filters taken with the Wide-Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). The CMDs are measured in regions spanning a wide range of star formation histories, including both old dormant and young star-forming populations. We match key NIR CMD features with their counterparts in more familiar optical CMDs, and identify the red core helium-burning (RHeB) sequence as a significant contributor to the NIR flux in stellar populations younger than a few 100 Myr old. The strength of this feature suggests that the NIR mass-to-light ratio can vary significantly on short timescales in star-forming systems. The NIR luminosity of star-forming galaxies is therefore not necessarily proportional to the stellar mass. We note that these individual RHeB stars may also be misidentified as old stellar clusters in images of nearby galaxies. For older stellar populations, we discuss the CMD location of asymptotic giant branch (AGB) stars in the HST filter set and explore the separation of AGB subpopulations using a combination of optical and NIR colors. We empirically calibrate the magnitude of the NIR tip of the red giant branch in F160W as a function of color, allowing future observations in this widely adopted filter set to be used for distance measurements. We also analyze the properties of the NIR red giant branch (RGB) as a function of metallicity, showing a clear trend between NIR RGB color and metallicity. However, based on the current study, it appears unlikely that the slope of the NIR RGB can be used as an effective metallicity indicator in extragalactic systems with comparable data. Finally, we highlight issues with scattered light in the WFC3, which becomes significant for exposures taken close to a bright Earth limb
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