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UV-bright Nearby Early-type Galaxies Observed in the Mid-infrared: Evidence for a Multi-stage Formation History by Way of WISE and GALEX Imaging

Petty, S. M. and Neill, J. D. and Jarrett, T. H. and Blain, A. W. and Farrah, D. G. and Rich, R. M. and Tsai, C.-W. and Benford, D. J. and Bridge, C. R. and Lake, S. E. and Masci, F. J. and Wright, E. L. (2013) UV-bright Nearby Early-type Galaxies Observed in the Mid-infrared: Evidence for a Multi-stage Formation History by Way of WISE and GALEX Imaging. Astronomical Journal, 146 (4). Art. No. 77. ISSN 0004-6256. http://resolver.caltech.edu/CaltechAUTHORS:20131011-105108149

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Abstract

In the local universe, 10% of massive elliptical galaxies are observed to exhibit a peculiar property: a substantial excess of ultraviolet emission than what is expected from their old, red stellar populations. Several origins for this ultraviolet excess (UVX) have been proposed including a population of hot young stars and a population of old, blue horizontal branch or extended horizontal branch (BHB or EHB) stars that have undergone substantial mass loss from their outer atmospheres. We explore the radial distribution of UVX in a selection of 49 nearby E/S0-type galaxies by measuring their extended photometry in the UV through mid-infrared (mid-IR) with the Galaxy Evolution Explorer (GALEX), the Sloan Digital Sky Survey, and the Wide-field Infrared Survey Explorer (WISE). We compare UV/optical and UV/mid-IR colors with the Flexible Stellar Population Synthesis models, which allow for the inclusion of EHB stars. We find that combined WISE mid-IR and GALEX UV colors are more effective in distinguishing models than optical colors, and that the UV/mid-IR combination is sensitive to the EHB fraction. There are strong color gradients, with the outer radii bluer than the inner half-light radii by ~1 mag. This color difference is easily accounted for with an increase in the BHB fraction of 0.25 with radius. We estimated that the average ages for the inner and outer radii are 7.0 ± 0.3 Gyr, and 6.2 ± 0.2 Gyr, respectively, with the implication that the outer regions are likely to have formed ~1 Gyr after the inner regions. Additionally, we find that metallicity gradients are likely not a significant factor in the color difference. The separation of color between the inner and outer regions, which agrees with a specific stellar population difference (e.g., higher EHB populations), and the ~0.5–2 Gyr age difference suggests multi-stage formation. Our results are best explained by inside-out formation: rapid star formation within the core at early epochs (>4 Gyr ago) and at least one later stage starburst event coinciding with z ~ 1.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://arxiv.org/abs/1307.6282arXivDiscussion Paper
http://dx.doi.org/10.1088/0004-6256/146/4/77 DOIArticle
http://iopscience.iop.org/1538-3881/146/4/77/PublisherArticle
ORCID:
AuthorORCID
Neill, J. D.0000-0002-0466-1119
Farrah, D. G.0000-0003-1748-2010
Masci, F. J.0000-0002-8532-9395
Wright, E. L.0000-0001-5058-1593
Additional Information:© 2013 The American Astronomical Society. Received 2012 April 22; accepted 2013 July 22; published 2013 August 21. We thank the anonymous referee for thorough comments that greatly improved this paper. We thank Marcio Catelan for discussions on evolved stellar populations, and D. Stern for numerous insights into the discussion of galaxy evolution.We also thank R. Assef for his feedback on the analysis and early development of this project. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. The publication is based on observations made with the NASA Galaxy Evolution Explorer. GALEX is operated for NASA by the California Institute of Technology under NASA contract NAS5-98034. This publication makes use of data products from the Sloan Digital Sky Survey. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web site is http://www.sdss.org/. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research has made use of the NASA/ IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
Group:Space Astrophysics Laboratory
Funders:
Funding AgencyGrant Number
NASANAS5-98034
Alfred P. Sloan Foundation UNSPECIFIED
NSFUNSPECIFIED
Department of Energy (DOE)UNSPECIFIED
Japanese Monbukagakusho UNSPECIFIED
Max Planck Society UNSPECIFIED
Higher Education Funding Council for England UNSPECIFIED
Subject Keywords:galaxies: elliptical and lenticular, cD; galaxies: evolution; galaxies: formation; infrared: galaxies; ultraviolet: galaxies
Record Number:CaltechAUTHORS:20131011-105108149
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20131011-105108149
Official Citation:UV-bright Nearby Early-type Galaxies Observed in the Mid-infrared: Evidence for a Multi-stage Formation History by Way of WISE and GALEX Imaging S. M. Petty et al. 2013 The Astronomical Journal 146 77
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:41891
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:11 Oct 2013 20:54
Last Modified:24 Oct 2017 22:16

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