Letter to the Editor

Enhanced dust heating in the bulges of early-type spiral galaxies^*

¹ Steward Observatory, Univ. of Arizona, Tucson, AZ 85721, USA e-mail: cengelbracht@as.arizona.edu
² INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
⁶ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁷ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁸ NASA Herschel Science Center, IPAC, California Institute of Technology, Pasadena, CA 91125, USA
⁹ Spitzer Science Center, California Institute of Technology, MC 314-6, Pasadena, CA 91125, USA
¹⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹¹ Department of Physics and Astronomy, University of Toledo, 2801 West Bancroft Street, Toledo, OH 43606, USA

¹² Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹³ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
¹⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁵ Departamento de Astrofisica, Facultad de Ciencias Fisicas, Universidad Complutense Madrid, Ciudad Universitaria, Madrid, 28040, Spain
¹⁶ Tianjin Astrophysics Center, Tianjin Normal University, Tianjin 300387, PR China
¹⁷ Department of Physics and Astronomy, SUNY Stony Brook, Stony Brook, NY 11794-3800, USA
¹⁸ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹⁹ Department of Physics & Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada
²⁰ CEA/DSM/Irfu/Service d'Astrophysique, UMR AIM, CE Saclay, 91191 Gif-sur-Yvette Cedex, France
²¹ ICRAR, M468, University of Western Australia, 35 Stirling Hwy, Crawley, WA, 6009, Australia

Received: 31 March 2010
Accepted: 23 April 2010

Abstract

Stellar density and bar strength should affect the temperatures of the cool (T ~ 20–30 K) dust component in the inner regions of galaxies, which implies that the ratio of temperatures in the circumnuclear regions to the disk should depend on Hubble type. We investigate the differences between cool dust temperatures in the central 3 kpc and disk of 13 nearby galaxies by fitting models to measurements between 70 and 500 μm. We attempt to quantify temperature trends in nearby disk galaxies, with archival data from Spitzer/MIPS and new observations with Herschel/SPIRE, which were acquired during the first phases of the Herschel observations for the KINGFISH (Key Insights on Nearby Galaxies: a Far-Infrared Survey with Herschel) sample. We fit single-temperature modified blackbodies to far-infrared and submillimeter measurements of the central and disk regions of galaxies to determine the temperature of the component(s) emitting at those wavelengths. We present the ratio of central-region-to-disk-temperatures of the cool dust component of 13 nearby galaxies as a function of morphological type. We find a significant temperature gradient in the cool dust component in all galaxies, with a mean center-to-disk temperature ratio of 1.15 ± 0.03. The cool dust temperatures in the central ~3 kpc of nearby galaxies are 23 (±3)% hotter for morphological types earlier than Sc, and only 9 (±3)% hotter for later types. The temperature ratio is also correlated with bar strength, with only strongly barred galaxies having a ratio over 1.2. The strong radiation field in the high stellar density of a galactic bulge tends to heat the cool dust component to higher temperatures, at least in early-type spirals with relatively large bulges, especially when paired with a strong bar.