95 research outputs found
Octahedron-based Projections as Intermediate Representations for Computer Imaging: TOAST, TEA, and More
This paper defines and discusses a set of rectangular all-sky projections that have no singular points, notably the Tesselated Octahedral Adaptive Spherical Transformation (or TOAST) developed initially for the WorldWide Telescope. These have proven to be useful as intermediate representations for imaging data where the application transforms dynamically from a standardized internal format to a specific format (projection, scaling, orientation, etc.) requested by the user. TOAST is strongly related to the Hierarchical Triangular Mesh pixelization and is particularly well adapted to situations where one wishes to traverse a hierarchy of images increasing in resolution. Because it can be recursively computed using a very simple algorithm it is particularly adaptable to use with graphical processing units
Self-consistent predictions for LIER-like emission lines from post-AGB stars
Early type galaxies (ETGs) frequently show emission from warm ionized gas.
These Low Ionization Emission Regions (LIERs) were originally attributed to a
central, low-luminosity active galactic nuclei. However, the recent discovery
of spatially-extended LIER emission suggests ionization by both a central
source and an extended component that follows a stellar-like radial
distribution. For passively-evolving galaxies with old stellar populations, hot
post-Asymptotic Giant Branch (AGB) stars are the only viable extended source of
ionizing photons. In this work, we present the first prediction of LIER-like
emission from post-AGB stars that is based on fully self-consistent stellar
evolution and photoionization models. We show that models where post-AGB stars
are the dominant source of ionizing photons reproduce the nebular emission
signatures observed in ETGs, including LIER-like emission line ratios in
standard optical diagnostic diagrams and H equivalent widths of order
0.1-3 angstroms. We test the sensitivity of LIER-like emission to the details
of post-AGB models, including the mass loss efficiency and convective mixing
efficiency, and show that line strengths are relatively insensitive to post-AGB
timescale variations. Finally, we examine the UV-optical colors of the models
and the stellar populations responsible for the UV-excess observed in some
ETGs. We find that allowing as little as 3% of the HB population to be
uniformly distributed to very hot temperatures (30,000 K) produces realistic UV
colors for old, quiescent ETGs.Comment: ApJ accepted. 20 pages, 8 figure
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
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