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UV Continuum Slope and Dust Obscuration from z ~ 6 to z ~ 2: The Star Formation Rate Density at High Redshift

Bouwens, R. J. and Illingworth, G. D. and Franx, M. and Chary, R.-R. and Meurer, G. R. and Conselice, C. J. and Ford, H. and Giavalisco, M. and van Dokkum, P. (2009) UV Continuum Slope and Dust Obscuration from z ~ 6 to z ~ 2: The Star Formation Rate Density at High Redshift. Astrophysical Journal, 705 (1). pp. 936-961. ISSN 0004-637X.

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We provide a systematic measurement of the rest-frame UV continuum slope β over a wide range in redshift (z ~ 2-6) and rest-frame UV luminosity (0.1 L^*_(z = 3) to 2 L^*_(z = 3)) to improve estimates of the star formation rate (SFR) density at high redshift. We utilize the deep optical and infrared data (Advanced Camera for Surveys/NICMOS) over the Chandra Deep Field-South and Hubble Deep Field-North Great Observatories Origins Deep Survey fields, as well as the UDF for our primary UBVi "dropout" Lyman Break Galaxy sample. We also use strong lensing clusters to identify a population of very low luminosity, high-redshift dropout galaxies. We correct the observed distributions for both selection biases and photometric scatter. We find that the UV-continuum slope of the most luminous galaxies is substantially redder at z ~ 2-4 than it is at z ~ 5-6 (from ~–2.4 at z ~ 6 to ~–1.5 at z ~ 2). Lower luminosity galaxies are also found to be bluer than higher luminosity galaxies at z ~ 2.5 and z ~ 4. We do not find a large number of galaxies with β's as red as –1 in our dropout selections at z ~ 4, and particularly at z ≳ 5, even though such sources could be readily selected from our data (and also from Balmer Break Galaxy searches at z ~ 4). This suggests that star-forming galaxies at z ≳ 5 almost universally have very blue UV-continuum slopes, and that there are not likely to be a substantial number of dust-obscured galaxies at z ≳ 5 that are missed in "dropout" searches. Using the same relation between UV-continuum slope and dust extinction as has been found to be appropriate at both z ~ 0 and z ~ 2, we estimate the average dust extinction of galaxies as a function of redshift and UV luminosity in a consistent way. As expected, we find that the estimated dust extinction increases substantially with cosmic time for the most UV luminous galaxies, but remains small (≾2 times) at all times for lower luminosity galaxies. Because these same lower luminosity galaxies dominate the luminosity density in the UV continuum, the overall dust extinction correction remains modest at all redshifts and the evolution of this correction with redshift is only modest. We include the contribution from ultra-luminous IR galaxies in our SFR density estimates at z ~ 2-6, but find that they contribute only ~20% of the total at z ~ 2.5 and ≾10% at z ≳ 4.

Item Type:Article
Related URLs:
URLURL TypeDescription
Bouwens, R. J.0000-0002-4989-2471
Illingworth, G. D.0000-0002-8096-2837
Franx, M.0000-0002-8871-3026
Chary, R.-R.0000-0001-7583-0621
Meurer, G. R.0000-0002-0163-2507
Conselice, C. J.0000-0003-1949-7638
Giavalisco, M.0000-0002-7831-8751
van Dokkum, P.0000-0002-8282-9888
Additional Information:© 2009 The American Astronomical Society. Received 2009 May 28; accepted 2009 September 23; published 2009 October 16. Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs 7235, 7817, 9425, 9575, 9797, 9803, 9978, 9979, 10189, 10339, 10340, 10403, 10504, 10530, 10632, 10872, 10874, 11082, and 11144. We thank Louis Bergeron, Susan Kassin, Dan Magee, Massimo Stiavelli, and Rodger Thompson for their assistance in the reduction of NICMOS data which have been essential for quantifying the UV-continuum slopes for faint star-forming galaxies at z ∼ 5–6. We thank Veronique Buat, Denis Burgarella, David Elbaz, Daniel Schaerer, and Naveen Reddy for stimulating conversations and Roderik Overzier and Naveen Reddy for helpful feedback on our submitted paper. Alice Shapley kindly sent us an electronic copy of her stacked spectrum of z ∼ 3 LBGs so we could compare it against the model spectra we use to estimate the UV-continuum slopes β. We acknowledge Roelof de Jong and other scientists at STScI for their efforts at characterizing the nonlinearity in the NICMOS detector. We are appreciative to our referee for detailed and insightful feedback which greatly improved this manuscript. Finally, we believe thanks are especially due to all those at NASA, STScI and throughout the community who have worked so diligently to make Hubble the remarkable observatory that it is today. The servicing missions, like the recent SM4, have rejuvenated HST and made it an extraordinarily productive scientific facility time and time again, and we greatly appreciate the support of policymakers, and all those in the flight and servicing programs who contributed to the repeated successes. We acknowledge support from NASA grants HST-GO09803.05-A and NAG5-7697.
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Subject Keywords:galaxies: evolution – galaxies: high-redshift
Issue or Number:1
Record Number:CaltechAUTHORS:20091104-085703945
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Official Citation:UV Continuum Slope and Dust Obscuration from z ~ 6 to z ~ 2: The Star Formation Rate Density at High Redshift R. J. Bouwens, G. D. Illingworth, M. Franx, R.-R. Chary, G. R. Meurer, C. J. Conselice, H. Ford, M. Giavalisco, and P. van Dokkum 2009 ApJ 705 936-961 doi: 10.1088/0004-637X/705/1/936
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16572
Deposited By: Tony Diaz
Deposited On:04 Nov 2009 23:10
Last Modified:09 Mar 2020 13:19

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