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Title:
Life in the last lane: star formation and chemical evolution in an extremely gas rich dwarf
Authors:
Begum, Ayesha; Chengalur, Jayaram N.; Kennicutt, Robert C.; Karachentsev, Igor D.; Lee, Janice C.
Affiliation:
AA(Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA), AB(National Centre for Radio Astrophysics, Post Bag 3, Ganeshkhind, Pune 411 007, India), AC(Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA), AD(Special Astrophysical Observatory, Nizhnii Arkhys 369167, Russia), AE(Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 383, Issue 2, pp. 809-816. (MNRAS Homepage)
Publication Date:
01/2008
Origin:
MNRAS
MNRAS Keywords:
galaxies: dwarf , galaxies: individual: NGC3741 , galaxies: kinematics and dynamics , radio lines: galaxies
DOI:
10.1111/j.1365-2966.2007.12592.x
Bibliographic Code:
2008MNRAS.383..809B

Abstract

We present an analysis of HI, Hα and oxygen abundance data for NGC3741. This galaxy has a very extended gas disc (~8.8 times the Holmberg radius), and a dark-to-luminous (i.e. stellar) mass ratio of ~149, which makes it one of the `darkest' dwarf irregular galaxies known. However, its ratio of baryon (i.e. gas + stellar) mass to dark mass is typical of that in galaxies. Our new high-resolution HI images of the galaxy show evidence for a large-scale (purely gaseous) spiral arm and central bar. From our HI data, a rotation curve can be derived out to ~37-44 disc scalelengths in the J and B bands, respectively. This is just slightly short of the radius at which one would expect a Navarro-Frenk-White type rotation curve to start falling. The galaxy has an integrated star formation rate (SFR) of ~0.0034Msolaryr-1, while the average SFR within the optical disc is ~0.0049Msolaryr-1kpc-2. Despite the gaseous spiral feature and the ongoing star formation, we find that the global gas density in NGC3741 is significantly lower than the Toomre instability criterion. This is consistent with the behaviour seen in other dwarf galaxies. We also find that the SFR is consistent with that expected from the observed correlations between HI mass and SFR and the global Kennicutt-Schmidt law, respectively. We measure the oxygen abundance to be 12 + log(O/H) = 7.66 +/- 0.10, which is consistent with that expected from the metallicity-luminosity relation, despite its extreme gas mass ratio. We also examine the issue of chemical evolution of NGC3741 in the context of the closed-box model of chemical evolution. The effective oxygen yield of NGC3741 is consistent with recent model estimates of closed-box yields, provided one assumes that the gas has been efficiently mixed all the way to the edge of the HI disc (i.e. greater than eight times the optical radius). This seems a priori unlikely. On the other hand, using a sample of galaxies with both interferometric HI maps and chemical abundance measurements, we find that the effective yield is anticorrelated with the total dynamical mass, as expected in leaky box models.
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