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Title: THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY. II. TRACING THE INNER M31 HALO WITH BLUE HORIZONTAL BRANCH STARS

Abstract

We attempt to constrain the shape of M31's inner stellar halo by tracing the surface density of blue horizontal branch (BHB) stars at galactocentric distances ranging from 2 kpc to 35 kpc. Our measurements make use of resolved stellar photometry from a section of the Panchromatic Hubble Andromeda Treasury survey, supplemented by several archival Hubble Space Telescope observations. We find that the ratio of BHB to red giant stars is relatively constant outside of 10 kpc, suggesting that the BHB is as reliable a tracer of the halo population as the red giant branch. In the inner halo, we do not expect BHB stars to be produced by the high-metallicity bulge and disk, making BHB stars a good candidate to be a reliable tracer of the stellar halo to much smaller galactocentric distances. If we assume a power-law profile r {sup -{alpha}} for the two-dimensional (2D) projected surface density BHB distribution, we obtain a high-quality fit with a 2D power-law index of {alpha} = 2.6{sup +0.3} {sub -0.2} outside of 3 kpc, which flattens to {alpha} < 1.2 inside of 3 kpc. This slope is consistent with previous measurements but is anchored to a radial baseline that extends much farthermore » inward. Finally, assuming azimuthal symmetry and a constant mass-to-light ratio, the best-fitting profile yields a total halo stellar mass of 2.1{sup +1.7} {sub -0.4} Multiplication-Sign 10{sup 9} M {sub Sun }. These properties are comparable with both simulations of stellar halo formation by satellite disruption alone and simulations that include some in situ formation of halo stars.« less

Authors:
; ; ;  [1];  [2];  [3];  [4];  [5];  [6];  [7]
  1. Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195 (United States)
  2. Department of Astronomy, University of Michigan, 550 Church St., Ann Arbor MI 48109 (United States)
  3. UCO/Lick Observatory, Astronomy and Astrophysics Department, University of California, Santa Cruz, CA 95064 (United States)
  4. National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719 (United States)
  5. Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112 (United States)
  6. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
  7. Osservatorio Astronomico di Padova-INAF, Vicolo dell'Osservatorio 5, I-35122 Padova (Italy)
Publication Date:
OSTI Identifier:
22086460
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 759; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTRONOMY; ASTROPHYSICS; COMPUTERIZED SIMULATION; DENSITY; DISTANCE; GALACTIC EVOLUTION; GALAXIES; INDEXES; MASS; PHOTOMETRY; RED GIANT STARS; SATELLITES; SURFACES; SYMMETRY; TELESCOPES; VISIBLE RADIATION

Citation Formats

Williams, Benjamin F., Dalcanton, Julianne J., Gilbert, Karoline M., Rosenfield, Philip, Bell, Eric F., Guhathakurta, Puragra, Lauer, Tod R., Seth, Anil C., Kalirai, Jason S., and Girardi, Leo, E-mail: ben@astro.washington.edu, E-mail: jd@astro.washington.edu, E-mail: kgilbert@astro.washington.edu, E-mail: philrose@astro.washington.edu, E-mail: ericbell@umich.edu, E-mail: raja@uco.lick.org, E-mail: lauer@noao.edu, E-mail: aseth@astro.utah.edu, E-mail: jkalirai@stsci.edu, E-mail: lgirardi@pd.astro.it. THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY. II. TRACING THE INNER M31 HALO WITH BLUE HORIZONTAL BRANCH STARS. United States: N. p., 2012. Web. doi:10.1088/0004-637X/759/1/46.
Williams, Benjamin F., Dalcanton, Julianne J., Gilbert, Karoline M., Rosenfield, Philip, Bell, Eric F., Guhathakurta, Puragra, Lauer, Tod R., Seth, Anil C., Kalirai, Jason S., & Girardi, Leo, E-mail: ben@astro.washington.edu, E-mail: jd@astro.washington.edu, E-mail: kgilbert@astro.washington.edu, E-mail: philrose@astro.washington.edu, E-mail: ericbell@umich.edu, E-mail: raja@uco.lick.org, E-mail: lauer@noao.edu, E-mail: aseth@astro.utah.edu, E-mail: jkalirai@stsci.edu, E-mail: lgirardi@pd.astro.it. THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY. II. TRACING THE INNER M31 HALO WITH BLUE HORIZONTAL BRANCH STARS. United States. doi:10.1088/0004-637X/759/1/46.
Williams, Benjamin F., Dalcanton, Julianne J., Gilbert, Karoline M., Rosenfield, Philip, Bell, Eric F., Guhathakurta, Puragra, Lauer, Tod R., Seth, Anil C., Kalirai, Jason S., and Girardi, Leo, E-mail: ben@astro.washington.edu, E-mail: jd@astro.washington.edu, E-mail: kgilbert@astro.washington.edu, E-mail: philrose@astro.washington.edu, E-mail: ericbell@umich.edu, E-mail: raja@uco.lick.org, E-mail: lauer@noao.edu, E-mail: aseth@astro.utah.edu, E-mail: jkalirai@stsci.edu, E-mail: lgirardi@pd.astro.it. Thu . "THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY. II. TRACING THE INNER M31 HALO WITH BLUE HORIZONTAL BRANCH STARS". United States. doi:10.1088/0004-637X/759/1/46.
@article{osti_22086460,
title = {THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY. II. TRACING THE INNER M31 HALO WITH BLUE HORIZONTAL BRANCH STARS},
author = {Williams, Benjamin F. and Dalcanton, Julianne J. and Gilbert, Karoline M. and Rosenfield, Philip and Bell, Eric F. and Guhathakurta, Puragra and Lauer, Tod R. and Seth, Anil C. and Kalirai, Jason S. and Girardi, Leo, E-mail: ben@astro.washington.edu, E-mail: jd@astro.washington.edu, E-mail: kgilbert@astro.washington.edu, E-mail: philrose@astro.washington.edu, E-mail: ericbell@umich.edu, E-mail: raja@uco.lick.org, E-mail: lauer@noao.edu, E-mail: aseth@astro.utah.edu, E-mail: jkalirai@stsci.edu, E-mail: lgirardi@pd.astro.it},
abstractNote = {We attempt to constrain the shape of M31's inner stellar halo by tracing the surface density of blue horizontal branch (BHB) stars at galactocentric distances ranging from 2 kpc to 35 kpc. Our measurements make use of resolved stellar photometry from a section of the Panchromatic Hubble Andromeda Treasury survey, supplemented by several archival Hubble Space Telescope observations. We find that the ratio of BHB to red giant stars is relatively constant outside of 10 kpc, suggesting that the BHB is as reliable a tracer of the halo population as the red giant branch. In the inner halo, we do not expect BHB stars to be produced by the high-metallicity bulge and disk, making BHB stars a good candidate to be a reliable tracer of the stellar halo to much smaller galactocentric distances. If we assume a power-law profile r {sup -{alpha}} for the two-dimensional (2D) projected surface density BHB distribution, we obtain a high-quality fit with a 2D power-law index of {alpha} = 2.6{sup +0.3} {sub -0.2} outside of 3 kpc, which flattens to {alpha} < 1.2 inside of 3 kpc. This slope is consistent with previous measurements but is anchored to a radial baseline that extends much farther inward. Finally, assuming azimuthal symmetry and a constant mass-to-light ratio, the best-fitting profile yields a total halo stellar mass of 2.1{sup +1.7} {sub -0.4} Multiplication-Sign 10{sup 9} M {sub Sun }. These properties are comparable with both simulations of stellar halo formation by satellite disruption alone and simulations that include some in situ formation of halo stars.},
doi = {10.1088/0004-637X/759/1/46},
journal = {Astrophysical Journal},
issn = {0004-637X},
number = 1,
volume = 759,
place = {United States},
year = {2012},
month = {11}
}