ALMA Reveals a Compact Starburst Around a Hidden QSO at z ∼ 5

We present ALMA 1.3mm observations of XID403, an SMG at z = 4.75 in the Chandra Deep Field South hosting a heavily obscured, Compton-thick QSO. The ALMA data show that the dust heated by star formation is distributed within similar to 0.9 kpc from the nucleus (effective radius). The SFR and dust temperature obtained from the Herschel+ALMA far-IR SED, reveal a warm and compact starburst with surface density of 200 M-circle dot yr(-1) kpc(-2). Our analysis suggest that, besides the mass, SFR and gas consumption timescale, objects like XID403 have also the right size to be the progenitors of the compact quiescent massive galaxies seen at z similar to 3. It is finally shown that the density of the gas co-spatial with the dust provides a substantial contribution to the absorbing column density towards the QSO as measured from the X-rays.