A dusty, normal galaxy in the epoch of reionization

Abstract

Candidates for the modest galaxies that formed most of the stars in the early universe, at redshifts z > 7, have been found in large numbers with extremely deep restframe-UV imaging 1 . But it has proved difficult for existing spectrographs to characterise them in the UV 2,3,4 . The detailed properties of these galaxies could be measured from dust and cool gas emission at far-infrared wavelengths if the galaxies have become sufficiently enriched in dust and metals. So far, however, the most distant UV-selected galaxy detected in dust emission is only at z = 3.2 5 , and recent results have cast doubt on whether dust and molecules can be found in typical galaxies at this early epoch 6,7,8 . Here we report thermal dust emission from an archetypal early universe starforming galaxy, A1689-zD1. We detect its stellar continuum in spectroscopy and determine its redshift to be z = 7.5±0.2 from a spectroscopic detection of the Lyα break. A1689-zD1 is representative of the star-forming population during reionisation 9 , with a total star-formation rate of about 12 M ⊙ yr -1 . The galaxy is highly evolved: it has a large stellar mass, and is heavily enriched in dust, with a dust-to-gas ratio close to that of the Milky Way. Dusty, evolved galaxies are thus present among the fainter star-forming population at z > 7, in spite of the very short time since they first appeared.As part of a programme to investigate galaxies at z > 7 with the X-shooter spectrograph on the Very Large Telescope, we observed the candidate high-redshift galaxy, A1689-zD1, behind the lensing galaxy cluster, Abell 1689 (Fig. 1). The source was originally identified 10 as a candidate z > 7 system from deep imaging with the Hubble and Spitzer Space Telescopes. Suggested to be at z = 7.6±0.4 from photometry fitting, it is gravitationally magnified by a factor of 9.3 by the galaxy cluster 10 , and though intrinsically faint, because of the gravitational amplification, is one of the brightest candidate z > 7 galaxies known. The X-shooter observations were carried out on several nights between March 2010 and March 2012 with a total time of 16 hours on target.The galaxy continuum is detected and can be seen in the binned spectrum (Fig. 2). The Lya cutoff is at 1035±24 nm and defines the redshift, z = 7.5±0.2. It is thus one of the most distant galaxies known to date to be confirmed via spectroscopy, and the only galaxy at z > 7 where the redshift is determined from spectroscopy of its stellar continuum. The spectral slope is blue; using a power-law fit, F λ ∝ λ -β , β = 2.0 ± 0.1. The flux break is sharp, and greater than a factor of ten in depth. In addition, no line emission is detected, ruling out a different redshift solution for the galaxy. Line emission is excluded to lensing-corrected depths of 3×10 -19 erg cm -2 s -1 (3σ) in the absence of sky emission lines, making this by far the deepest intrinsic spectrum published of a reionization era object, highlighting the difficulty of obtaining UV redshifts for objects at this epoch that are not strongly domin...

The infrared-dark dust content of high redshift galaxies

“…An exception to the above scenario is the puzzling case of A1689-zD1 (Watson et al 2015;Knudsen et al 2016), a z = 7.5 ± 0.2 gravitationally-lensed LBG for which the thermal dust emission has been detected by ALMA. The large FIR flux LFIR = (6.2 ± 0.8) × 10 10 L is indicative of considerable amounts of dust, consistent with a Milky Way dust-to-gas ratio.…”

The dust content of galaxies from z = 0 to z = 9

“…All model variants predict dust masses approximately 0.5 dex larger than those predicted by our fiducial model for galaxies in this stellar mass range. This is driven by higher efficiencies for the condensation of dust in stellar ejecta in the 'no-acc' and 'high-cond' (2015) at z = 3, 4, and 5, and a compilation of data in Mancini et al (2015) at z = 6 and 7, taken from Kanekar et al (2013), Ouchi et al (2013), Ota et al (2014), Maiolino et al (2015), Schaerer et al (2015), and Watson et al (2015).…”

“…Watson et al (2015) found a galaxy at z = 7.5 ± 0.2 with a dust mass of 4 × 10 7 M and a DTG ratio that is half of the Milky Way value. Although these dusty examples may not be representative of typical high-redshift galaxies, they set strong constraints on our understanding of dust formation and growth in galaxies in the early Universe.…”

Dust production 680–850 million years after the Big Bang

“…The sample includes HFLS3, a red Herschel-selected z = 6.34 galaxy (Riechers et al 2013); ULAS J1120+0641, a colour-selected z = 7.085 quasar (Mortlock et al 2011;Venemans et al 2012); and A1689-zD1, a lensed z = 7.5 Lyman break galaxy (Watson et al 2015).…”