The assembly of ‘normal’ galaxies at<i>z</i> ∼ 7 probed by ALMA

Abstract

We report new deep observations obtained with the Atacama Large Millimetre Array (ALMA) aimed at investigating the [C &lt;scp&gt;ii&lt;/scp&gt;]158 μm line and continuum emission in three spectroscopically confirmed Lyman break galaxies at 6.8 &lt; &lt;it&gt;z&lt;/it&gt; ≤ 7.1, i.e. well within the re-ionization epoch. With star formation rates of SFR ∼ 5–15M&lt;inf&gt;&amp;odot;&lt;/inf&gt; yr− 1 these systems are much more representative of the high-&lt;it&gt;z&lt;/it&gt; galaxy population than other systems targeted in the past by millimetre observations. For the galaxy with the deepest observation we detect [C &lt;scp&gt;ii&lt;/scp&gt;] emission at redshift &lt;it&gt;z&lt;/it&gt; = 7.107, fully consistent with the Ly α redshift, but spatially offset by 0.7 arcsec (4 kpc) from the optical emission. At the location of the optical emission, tracing both the Ly α line and the far-UV continuum, no [C &lt;scp&gt;ii&lt;/scp&gt;] emission is detected in any of the three galaxies, with 3σ upper limits significantly lower than the [C &lt;scp&gt;ii&lt;/scp&gt;] emission observed in lower redshift galaxies. These results suggest that molecular clouds in the central parts of primordial galaxies are rapidly disrupted by stellar feedback. As a result, [C &lt;scp&gt;ii&lt;/scp&gt;] emission mostly arises from more external accreting/satellite clumps of neutral gas. These findings are in agreement with recent models of galaxy formation. Thermal far-infrared continuum is not detected in any of the three galaxies. However, the upper limits on the infrared-to-UV emission ratio do not exceed those derived in metal- and dust-poor galaxies.