Significant Dust-obscured Star Formation in Luminous Lyman-break Galaxies at z ∼ 7–8
Sander Schouws, Mauro Stefanon, R. J. Bouwens, Renske Smit, Jacqueline Hodge, Ivo Labbé, Hiddo Algera, Leindert Boogaard, Stefano Carniani, Yoshinobu Fudamoto, Benne W. Holwerda, G. D. Illingworth, R. Maiolino, Michael V. Maseda, Pascal A. Oesch, P. van der Werf
Abstract
Abstract We make use of Atacama Large Millimeter/submillimeter Array continuum observations of 15 luminous Lyman-break galaxies at z ∼ 7–8 to probe their dust-obscured star formation. These observations are sensitive enough to probe obscured star formation rates (SFRs) of 20 M ⊙ yr −1 (3 σ ). Six of the targeted galaxies show significant (≥3 σ ) dust-continuum detections, more than doubling the number of known dust-detected galaxies at z > 6.5. Their IR luminosities range from 2.7 × 10 11 L ⊙ to 1.1 × 10 12 L ⊙ , equivalent to obscured SFRs of 25 to 101 M ⊙ yr −1 . We use our results to quantify the correlation of the infrared excess (IRX) on the UV-continuum slope β UV and stellar mass. Our results are most consistent with a Small Magellanic Cloud (SMC) attenuation curve for intrinsic UV-slopes <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>β</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>UV</mml:mi> <mml:mo>,</mml:mo> <mml:mi>intr</mml:mi> </mml:mrow> </mml:msub> </mml:math> of −2.63 and most consistent with an attenuation curve in between SMC and Calzetti for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>β</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>UV</mml:mi> <mml:mo>,</mml:mo> <mml:mi>intr</mml:mi> </mml:mrow> </mml:msub> </mml:math> slopes of −2.23, assuming a dust temperature T d of 50 K. Our fiducial IRX–stellar mass results at z ∼ 7–8 are consistent with marginal evolution from z ∼ 0. We then show how both results depend on T d . For our six dust-detected sources, we estimate their dust masses and find that they are consistent with dust production from supernovae if the dust destruction is low (<90%). Finally we determine the contribution of dust-obscured star formation to the SFR density for UV luminous ( H <−21.5 mag: ≳1.7 L * UV ) z ∼ 7–8 galaxies, finding that the total SFR density at z ∼ 7 and z ∼ 8 from bright galaxies is <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.20</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.10</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> dex and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.23</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.09</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.06</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> dex higher, respectively; i.e., ∼ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mstyle displaystyle="false"> <mml:mfrac> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:mfrac> </mml:mstyle> </mml:math> of the star formation in ≳1.7 L * UV galaxies at z ∼ 7–8 is obscured by dust.