Journal Article

The <i>Swift</i>/UVOT catalogue of NGC4321 star-forming sources: a case against density wave theory

Ignacio Ferreras, Mark Cropper, Daisuke Kawata, Mat Page and Erik A. Hoversten

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 424, issue 3, pages 1636-1646
Published in print August 2012 | ISSN: 0035-8711
Published online August 2012 | e-ISSN: 1365-2966 | DOI:
The Swift/UVOT catalogue of NGC4321 star-forming sources: a case against density wave theory

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We study the star-forming regions in the spiral galaxy NGC 4321 (M100). We take advantage of the spatial resolution (2.5 arcsec full width at half-maximum) of the Swift/Ultraviolet/Optical Telescope camera and the availability of three ultraviolet (UV) passbands in the region 1600 < λ < 3000 Å, in combination with optical and infrared (IR) imaging from Sloan Digital Sky Survey, KPNO/Hα and Spitzer/IRAC, to obtain a catalogue of 787 star-forming regions out to three disc scalelengths. We use a large volume of star formation histories, combined with stellar population synthesis, to determine the properties of the young stellar component and its relationship with the spiral arms. The Hα luminosities of the sources have a strong decreasing radial trend, suggesting more massive star-forming regions in the central part of the galaxy. When segregated with respect to near-UV (NUV)–optical colour, blue sources have a significant excess of flux in the IR at 8 , revealing the contribution from polycyclic aromatic hydrocarbons, although the overall reddening of these sources stays below E(BV) = 0.2 mag. The distribution of distances to the spiral arms is compared for subsamples selected according to Hα luminosity, NUV–optical colour or ages derived from a population synthesis model. An offset would be expected between these subsamples as a function of radius if the pattern speed of the spiral arm were constant – as predicted by classic density wave theory. No significant offsets are found, favouring instead a mechanism where the pattern speed has a radial dependence.

Keywords: galaxies: general; galaxies: individual: NGC 4321; galaxies: spiral; galaxies: star formation

Journal Article.  7347 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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