Gas metallicities in the extended disks of NGC1512 and NGC3621: chemical signatures of metal mixing or enriched gas accretion?

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Title: Gas metallicities in the extended disks of NGC1512 and NGC3621: chemical signatures of metal mixing or enriched gas accretion?
Author(s): Bresolin, Fabio; Kennicutt, Robert C.; Ryan-Weber, Emma
Abstract: We have obtained spectra of 135 H II regions located in the inner and extended disks of the spiral galaxies NGC 1512 and NGC 3621, spanning the range of galactocentric distances 0.2-2 × R 25 (from ~2-3 kpc to ~18-25 kpc). We find that the excitation properties of nebulae in the outer (R > R 25) disks are similar to those of the inner disks, but on average younger H II regions tend to be selected in the bright inner disks. Reddening by dust is not negligible in the outer disks and subject to significant large-scale spatial variations. For both galaxies, the radial abundance gradient flattens to a constant value outside of the isophotal radius. The outer disk O/H abundance ratio is highly homogeneous, with a scatter of only ~0.06 dex. In the case of the interacting galaxy NGC 1512 we find a number of H II regions with peculiar metallicity for their radius, a result which can be interpreted by gas flows activated by the gravitational encounter with NGC 1510. Based on the excitation and chemical (N/O ratio) analysis, we find no compelling evidence for variations in the upper initial mass function of ionizing clusters of extended disks. The O/H abundance in the outer disks of the target galaxies corresponds to ~35% of the solar value (or higher, depending on the metallicity diagnostic). This agrees with our earlier measurements in M83 and NGC 4625, and conflicts with the notion that metallicities in extended disks of spiral galaxies are low and on the order of ~0.1 × Z ☉. We show that, in general, the observed metal enrichment cannot be produced with the current level of star formation, even if the latter extends over a Hubble time. We discuss the possibility that metal transport mechanisms from the inner disks lead to metal pollution of the outer disks. Gas accretion from the intergalactic medium, enriched by outflows, offers an alternative solution, justified within the framework of hydrodynamic simulations of galaxy evolution. Specific model predictions of the chemical enrichment and the flat gradients in extended disks of nearby galaxies will be valuable to discriminate between these different scenarios.
Publication type: Journal article
Research centre: Swinburne University of Technology. Faculty of Information and Communication Technologies. Centre for Astrophysics and Supercomputing
Source: Astrophysical Journal, Vol. 750, no. 2 (May 2012), article no. 122
Publication year: 2012
FOR Code(s): 0201 Astronomical and Space Sciences; 0305 Organic Chemistry; 0306 Physical Chemistry (Incl. Structural)
Keyword(s): Abundances; Galaxies; ISM; NGC 1512; NGC 3621
Publisher: Institute of Physics Publishing
ISSN: 0004-637X
Publisher URL: http://dx.doi.org/10.1088/0004-637x/750/2/122
Copyright: Copyright © 2012 The American Astronomical Society. All rights reserved. The American Astronomical Society does not allow Swinburne Research Bank to archive either the accepted manuscript or the published version of the article. However you can find an earlier version of the full text here: http://arxiv.org/abs/1203.0956
Research Projects: The intergalactic medium: from cosmic dawn to the local web, Australian Research Council grant number DP1095600
Peer reviewed