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Extragalactic Astrophysics and Cosmology

ALMA reveals an extremely compact starburst around a hidden black hole in the early Universe.

Most massive galaxies in the Universe host a giant black hole (BH) at their center, with mass ranging from one-million to one-billion solar masses. The formation of stars in a galaxy and the growth of its central supermassive BH are often thought to be correlated processes, which is conventionally referred to as galaxy-BH co-evolution. Active BH-galaxy systems, in which most of the stars and of the BH mass are being assembled at the same time, have been observed both in the local and distant Universe. During the early phases of this intense activity, the radiation from the active BH is thought to be depressed by large amount of dust and gas, as it is observed in a number of systems.

The ALMA 1.3mm image of the z=4.75 galaxy. The observed thermal emission is produced by dust heated by star formation, and is unresolved at a resolution of 0.7 arcsec. This suggests a compact distribution of dust and stars on scales of ~1kpc.

A striking example was discovered a few years ago in the 4Ms Chandra Deep Field South, the deepest X-ray survey ever performed. The X-ray data revealed the presence of an active but extremely obscured (Compton-thick) BH in a distant galaxy at z=4.75, when the Universe was at 10% of its present age. This galaxy was already known to form stars at the impressive rate of 1000 Msun/yr, as estimated from its bright FIR/submm thermal emission from heated dust.

In 2012, Roberto Gilli, astronomer at INAF – Bologna Observatory, and his collaborators observed this system with the Atacama Large Millimeter/submillimiter Array (ALMA) as part of Early Science Cycle 0. Their observations at 1.3mm (ALMA band 6) reached the unprecedented resolution of ~1 kpc at the source redshift, and allowed the research team to put strong constraints on the compactness of the dust distribution and of star formation, revealing that they are located on scales of ~1 kpc or even smaller.

The key results of this study have far reaching implications.

First, assuming that the gas reservoir is distributed as the dust, it would produce an absorption level comparable with what has been estimated from the X-ray data. In turn, this would imply that a significant amount of nuclear obscuration is produced in the host galaxy and not just by material in the immediate BH neighborhoods. Since the average gas content in galaxies has been found to increase with redshift, this finding, if confirmed, may explain why the fraction of heavily obscured QSOs has been observed to increase with redshift.

Observed spectral energy distribution (red circles and arrows) modeled with the sum (black curve) of stellar emission (red curve), QSO-heated dust (blue) and star formation heated dust (green).

Second, these observations show that, besides the mass, star formation rate and gas depletion timescale, this system has also the right size to be one of the progenitors of the compact quiescent massive galaxies seen at z ∼ 3. Compact systems represent a substantial fraction of all quiescent galaxies at z > 1 and have sizes a factor of 3-5 smaller than those of local quiescent galaxies of equal mass. One open question is how such old and compact remnants have formed. A recent idea is that AGN feedback may speed up the evolution of star formation, leaving compact remnants on short timescales (∼100 million yr). The possible evidence for a large-scale galactic outflow would perfectly fit within this scenario.

All these results are being published in a paper accepted for publication in Astronomy & Astrophysics on 04 Jan 2014: “ALMA reveals a warm and compact starburst around a heavily obscured supermassive black hole at z = 4.75”, by R. Gilli et al.

The paper features the contribution of 6 researchers from the Bologna Observatory and of a total of 14 researchers either hired by or associated to INAF. All the work has been constantly supported by the ALMA Regional Center (ARC) node hosted by IRA Bologna. The Italian ARC node followed the project from the original ALMA proposal writing, through the scheduling of the observations and analysis of the data, and finally to the paper writing.