Institute of research into the fundamental laws of the Universe

Observations carried out these last decades brought stringent constraints on the extreme conditions that prevail at the center of galaxies. In particular, astronomers think that each of them hosts in its core a gigantic black hole, which mass can reach several billions that of the Sun for the most massive galaxies of the Universe. These are referred to as “super-massive” black holes, as opposed to those of stellar origin that only weight a few dozens of solar masses at most. While they can not be directly observed, it is the impact they have on their close environment that betrays their presence. For instance, the accretion disk that forms around these black holes from the absorbed nearby material produces characteristic radiations (we then call them “active galactic nuclei”) that allows probing the nuclear activity of galaxies throughout cosmic times. However, the genuine origin of these giant black holes is still a mystery, and the physical mechanisms funneling to the central region enough material to feed them and make them growing up to such masses have not been elucidated yet.

Active nuclei buried within ultra-compact galaxies

Using deep imaging obtained with the Hubble Space Telescope, a team of astronomers led by the Sap has opened a new window on this topic. The researchers analyzed a population of galaxies with active nuclei selected in the cosmological field COSMOS at an epoch of cosmic history corresponding to the peak of the growth of structures (spectral redshift z~1, equivalent to a lookback time of ~8 billion years). The COSMOS field covers a region of 2 square degrees on the sky, and benefits from a wealth of multi-wavelength imaging and spectroscopic observations, in particular with deep data at high angular resolution obtained by the Hubble Space Telescope. Thanks to these observations, the astronomers compared the morphological properties of these sources with the morphology of  “normal” galaxies (galaxies with no sign of nuclear activity), chosen at comparable distances, stellar masses and star formation rates. Contrary to most of the previous studies, they focused more specifically on active nuclei buried within large amount of dust. These sources can be identified thanks to their X-ray and Infrared emission, but they are almost undetected at optical wavelengths. This feature allows studying the morphology of their host galaxy in the visible domain, without suffering from the emission of the nucleus that would otherwise severely hampers the analysis if it were accessible at such wavelengths. This new approach and these methods thus make their work original. The astronomers were thus able to measure the size of the sources in their sample, by fitting for each object a physical galaxy model accounting for the spatial light distribution as constrained by the images obtained with the Hubble Space Telescope. They found that galaxies hosting these active nuclei have systematically smaller sizes and they are typically more compact than galaxies of their reference sample.

The “stacking” method

Measuring the individual size of distant galaxies is however not straight forward, as the techniques commonly used to constrain their light profile do not always account for the complexity of their morphology. Moreover, the external regions of the disk of high-redshift galaxies are difficult to detect, because of their very low surface brightness. To test the robustness of their results, the astronomers then used the so-called “stacking” method.  This technique aims at determining the average image of a given population of sources, by stacking together the images of these objects and by measuring for each pixel of the final image the mean value of the individual measurements within this pixel. The result that they obtained is obvious: the average image of galaxies hosting an active nucleus appears much more compact than that of the reference galaxies, as also revealed by the comparison between their average radial light profiles.

A dynamical contraction of galaxies hosting an active nucleus ?

This more compact morphology characterizing the galaxies hosting an active nucleus can not be due to the presence of an old stellar bulge that would be larger in these galaxies than in the reference sample. The evolved stellar populations being characterized by colors redder than those of young and massive stars, the hosts of active nuclei would show a bias in color, which does not appear in the observations. The researchers hence believe that these galaxies have rather experienced an episode of dynamical contraction, similar to the mechanisms proposed to explain the formation of blue compact galaxies. According to high resolution hydrodynamic simulations carried out these past few years, this compactification could follow the violent gravitational instabilities that are naturally triggered within the disk of gas-rich galaxies in the distant Universe. This mechanism could thus favor the funneling of gas and material from the interstellar medium toward the core of galaxies, feeding at the same time the accretion of matter around their central super-massive black hole.

Contacts : Yu-Yen Chang, Emeric Le Floc'h

Publication :

“Obscured active galactic nuclei triggered in compact star-forming galaxies”
Y.-Y. Chang, E. Le Floc’h, S. Juneau, E. da Cunha, M. Salvato, F. Civano, S. Marchesi, J.M. Gabor, O. Ilbert, C. Laigle, H.J. McCracken, B.-D. Hsieh, P. Capak
published in the MNRAS Journal, March 2017

DOI: https://doi.org/10.1093/mnrasl/slw247

Access to the electronic version of the publication

See also : Comment nourrir les trous noirs géants ? (11 novembre 2011)
                 Les clés de l'évolution des galaxies (Point Presse - 7 juin 2013)

Rédaction : Emeric Le Floc'h