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Issue A&A
Volume 508, Number 2, December III 2009
Page(s) L21 - L25
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361/200913407
Published online 19 November 2009

A&A 508, L21-L25 (2009)
DOI: 10.1051/0004-6361/200913407

Letter

Ly$\alpha$ emitters: blue dwarfs or supermassive ULIRGs? Evidence for a transition with redshift

K. K. Nilsson1 and P. Møller2

1  ST-ECF, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
    e-mail: knilsson@eso.org
2  European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany

Received 5 October 2009 / Accepted 16 November 2009

Abstract
The traditional view that Ly$\alpha$ emission and dust should be mutually exclusive has been questioned more and more often; most notably, the observations of Ly$\alpha$ emission from ULIRGs seem to counter this view. In this paper we seek to address the reverse question. How large a fraction of Ly$\alpha$ selected galaxies are ULIRGs? Using two samples of 24/25 Ly$\alpha$ emitting galaxies at z = 0.3/2.3, we perform this test, including results at z = 3.1, and find that, whereas the ULIRG fraction at z = 3.1 is very small, it systematically increases towards lower redshifts. There is a hint that this evolution may be quite sudden and that it happens around a redshift of z ~ 2.5. After measuring the infrared luminosities of the Ly$\alpha$ emitters, we find that they are in the normal to ULIRG range in the lower redshift sample, while the higher redshift galaxies all have luminosities in the ULIRG category. The Ly$\alpha$ escape fractions for these infrared bright galaxies are in the range 1-100% in the z = 0.3 galaxies, but are very low in the z = 2.3 galaxies, 0.4% on average. The unobscured star formation rates are very high, ranging from 500 to more than 5000 $M_{\odot}$ yr-1, and the dust attenuation derived are in the range 0.0 < AV < 3.5.


Key words: cosmology: observations -- galaxies: high- redshift



© ESO 2009

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