128 research outputs found
Using the Tip of the Red Giant Branch as a Distance Indicator in the Near Infrared
The tip of the red giant branch (TRGB) is a well-established standard candle
used to measure distances to nearby galaxies. The TRGB luminosity is typically
measured in the I-band, where the luminosity has little dependency on stellar
age or stellar metallicity. As the TRGB is brighter at wavelengths redder than
the I-band, observational gains can be made if the TRGB luminosity can be
robustly calibrated at longer wavelengths. This is of particular interest given
the infrared capabilities that will be available with the James Webb Space
Telescope and an important calibration consideration for using TRGB distances
as part of an independent measurement of the Hubble constant. Here, we use
simulated photometry to investigate the dependency of the TRGB luminosity on
stellar age and metallicity as a function of wavelength (475 nm - 4.5 micron).
We find intrinsic variations in the TRGB magnitude to increase from a few
hundredths of a magnitude at 800-900 nm to ~0.6 mag by 1.5 micron. We show that
variations at the longer infrared wavelengths can be reduced to 0.02-0.05 mag
(1-2% accuracy in distance) with careful calibrations that account for changes
in age and metal content. These represent the minimum uncertainties;
observational uncertainties will be higher. Such calibration efforts may also
provide independent constraints of the age and metallicity of stellar halos
where TRGB distances are best measured. At 3.6 and 4.5 micron, the TRGB
magnitude is predicted to vary up to ~0.15 mag even after corrections for
stellar age and metallicity, making these wavelengths less suitable for
precision distances.Comment: 11 pages, 7 figures, 1 table, Accepted to the Astrophysical Journa
Observational Constraints on Red and Blue Helium Burning Sequences
We derive the optical luminosity, colors, and ratios of the blue and red
helium burning (HeB) stellar populations from archival Hubble Space Telescope
observations of nineteen starburst dwarf galaxies and compare them with
theoretical isochrones from Padova stellar evolution models across
metallicities from Z=0.001 to 0.009. We find that the observational data and
the theoretical isochrones for both blue and red HeB populations overlap in
optical luminosities and colors and the observed and predicted blue to red HeB
ratios agree for stars older than 50 Myr over the time bins studied. These
findings confirm the usefulness of applying isochrones to interpret
observations of HeB populations. However, there are significant differences,
especially for the red HeB population. Specifically we find: (1) offsets in
color between the observations and theoretical isochrones of order 0.15 mag
(0.5 mag) for the blue (red) HeB populations brighter than M_V ~ -4 mag, which
cannot be solely due to differential extinction; (2) blue HeB stars fainter
than M_V ~ -3 mag are bluer than predicted; (3) the slope of the red HeB
sequence is shallower than predicted by a factor of ~3; and (4) the models
overpredict the ratio of the most luminous blue to red HeB stars corresponding
to ages <50 Myr. Additionally, we find that for the more metal-rich galaxies in
our sample (Z> 0.5 Zsolar) the red HeB stars overlap with the red giant branch
stars in the color magnitude diagrams, thus reducing their usefulness as
indicators of star formation for ages >100 Myr.Comment: 18 pages, 11 figures, 3 table
Observational Constraints on the Molecular Gas Content in Nearby Starburst Dwarf Galaxies
Using star formation histories derived from optically resolved stellar
populations in nineteen nearby starburst dwarf galaxies observed with the
Hubble Space Telescope, we measure the stellar mass surface densities of stars
newly formed in the bursts. By assuming a star formation efficiency (SFE), we
then calculate the inferred gas surface densities present at the onset of the
starbursts. Assuming a SFE of 1%, as is often assumed in normal star-forming
galaxies, and assuming that the gas was purely atomic, translates to very high
HI surface densities (~10^2-10^3 Msun pc^-2), which are much higher than have
been observed in dwarf galaxies. This implies either higher values of SFE in
these dwarf starburst galaxies or the presence of significant amounts of H_2 in
dwarfs (or both). Raising the assumed SFEs to 10% or greater (in line with
observations of more massive starbursts associated with merging galaxies),
still results in HI surface densities higher than observed in 10 galaxies.
Thus, these observations appear to require that a significant fraction of the
gas in these dwarf starbursts galaxies was in the molecular form at the onset
of the bursts. Our results imply molecular gas column densities in the range
10^19-10^21 cm^-2 for the sample. In those galaxies where CO observations have
been made, these densities correspond to values of the CO-H_2 conversion factor
(X_CO) in the range >3-80x10^20 cm^-2 (K km s^-1)^-1, or up to 40x greater than
Galactic X_CO values.Comment: 8 pages, 4 figures, 2 table
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