arXiv subjects

A. C. Seth

Publications and source records attributed to A. C. Seth.

22 records · Page 2Linked to original sources

arXiv2012

The Panchromatic Hubble Andromeda Treasury

The Panchromatic Hubble Andromeda Treasury (PHAT) is an on-going HST Multicycle Treasury program to image ~1/3 of M31's star forming disk in 6 filters, from the UV to the NIR. The full survey will resolve the galaxy into more than 100 million stars with projected radii from 0-20 kpc over a contiguous 0.5 square degree area in 828 orbits, producing imaging in the F275W and F336W filters with WFC3/UVIS, F475W and F814W with ACS/WFC, and F110W and F160W with WFC3/IR. The resulting wavelength coverage gives excellent constraints on stellar temperature, bolometric luminosity, and extinction for most spectral types. The photometry reaches SNR=4 at F275W=25.1, F336W=24.9, F475W=27.9, F814W=27.1, F110W=25.5, and F160W=24.6 for single pointings in the uncrowded outer disk; however, the optical and NIR data are crowding limited, and the deepest reliable magnitudes are up to 5 magnitudes brighter in the inner bulge. All pointings are dithered and produce Nyquist-sampled images in F475W, F814W, and F160W. We describe the observing strategy, photometry, astrometry, and data products, along with extensive tests of photometric stability, crowding errors, spatially-dependent photometric biases, and telescope pointing control. We report on initial fits to the structure of M31's disk, derived from the density of RGB stars, in a way that is independent of the assumed M/L and is robust to variations in dust extinction. These fits also show that the 10 kpc ring is not just a region of enhanced recent star formation, but is instead a dynamical structure containing a significant overdensity of stars with ages >1 Gyr. (Abridged)

astro-ph.CO↗

arXiv2008

Revised bolometric corrections and interstellar extinction coefficients for the ACS and WFPC2 photometric systems

We present extensive tables of bolometric corrections and interstellar extinction coefficients for the WFPC2 and ACS (both WFC and HRC) photometric systems. They are derived from synthetic photometry applied to a database of spectral energy distributions covering a large range of effective temperatures, surface gravity, and metal content. Carbon stars are also considered. The zero-points take into consideration the new high-accuracy Vega fluxes from Bohlin. These tables are employed to transform Padova isochrones into WFPC2 and ACS photometric systems using interstellar extinction coefficients on a star-to-star basis. All data are available either in tabular form or via an interactive web interface in the case of the isochrones. Preliminary tables for the WFC3 camera are also included in the database.

astro-ph↗

arXiv2007

Stellar populations across the NGC4244 truncated galactic disk

We use HST/ACS to study the resolved stellar populations of the nearby, nearly edge-on galaxy NGC4244 across its outer disk surface density break. The stellar photometry allows us to study the distribution of different stellar populations and reach very low equivalent surface brightnesses. We find that the break occurs at the same radius for young, intermediate age, and old stars. The stellar density beyond the break drops sharply by a factor of at least 600 in 5 kpc. The break occurs at the same radius independent of height above the disk, but is sharpest in the midplane and nearly disappears at large heights. These results make it unlikely that truncations are caused by a star formation threshold alone: the threshold would have to keep the same radial position from less than 100 Myr to 10 Gyr ago, in spite of potential disturbances such as infall and redistribution of gas by internal processes. A dynamical interpretation of truncation formation is more likely such as due to angular momentum redistribution by bars or density waves, or heating and stripping of stars caused by the bombardment of dark matter sub-halos. The latter explanation is also in quantitative agreement with the small diffuse component we see around the galaxy.

astro-ph↗

arXiv2007

GHOSTS: The Resolved Stellar Outskirts of Massive Disk Galaxies

We show initial results from our ongoing HST GHOSTS survey of the resolved stellar envelopes of 14 nearby, massive disk galaxies. In hierarchical galaxy formation the stellar halos and thick disks of galaxies are formed by accretion of minor satellites and therefore contain valuable information about the (early) assembly process of galaxies. We detect for the first time the very small halo of NGC4244, a low mass edge-on galaxy. We find that massive galaxies have very extended halos, with equivalent surface brightnesses of 28-29 V-mag/arcsec^2 at 20-30 kpc from the disk. The old RGB stars of the thick disk in the NGC891 and NGC4244 edge-on galaxies truncate at the same radius as the young thin disk stars, providing insights into the formation of both disk truncations and thick disks. We furthermore present the stellar populations of a very low surface brightness stream around M83, the first such a stream resolved into stars beyond those of the Milky Way and M31.

astro-ph↗