Morphological and multi-wavelength constraints on the coeval growth of galaxies and their super-massive black holes – MORPHOSTAR

MORPHOSTAR was carried out thanks to complete samples of galaxies selected in two of the so far best covered cosmological fields: the COSMOS field and the Hubble Ultra Deep Field (HUDF). We developed new analyzing techniques and new galaxy selection methods, which enabled major progress in our understanding of the modes of distant galaxy formation and the growth of their central black hole:

- the analysis of galaxies in the HUDF was pursued using high resolution images obtained with the Hubble space telescope. We reconstructed, pixel-to-pixel, their spectral energy distribution at optical and near-infrared wavelengths, and their fit with stellar emission models allowed us to infer spatially resolved maps of stellar mass and star formation rate surface density. The physical constraints inferred from these new maps were validated with hydro-dynamical simulations of galaxy formation prepared within our group.

- the analysis of the spectral energy distributions of AGN-dominated galaxies in the COSMOS field was led thanks to an extension of an SED fitting code publicly available, an extension that we developed jointly with the authors of this code. This new version, which will also be publicly released soon, allows disentangling the contributions from the stellar emission and the AGN, over a wavelength window ranging from the UV to the far-infrared.

- finally, we focused part of our work on star-forming regions not selected from broad-band imaging but observed from some of their ionized gas emission lines. This technique yields a selection of sources more closely correlated to their instantaneous activity of star formation, since it depends much less on the underlying continuum of stellar emission.

Our analysis of the Hubble Space Telescope data led to the discovery of star-forming activity in a giant (mass > 10^9 Msun) and extremely young (age lower than ~10 million years) clump in the disk of a distant galaxy (see illustration). These clumps are typical of the morphology of galaxies in the early Universe, but the very early stages of their formation had never been observed so far. The constraints that we obtained with respect to their efficiency of star formation during such phases reveal that they experience a starburst-like activity at their birth, during which stars can form more efficiently than what is typically observed in galaxy disks over longer time scales. Our results also show that they can survive stellar winds for several hundreds of million years. This rather long «life expectancy« could make them migrating toward the central region of the galaxy where they formed, hence contributing to the growth of bulges in the distant Universe.

Besides, the studies that we carried out in the COSMOS field were focused on the identification of highly-obscured active nuclei and the morphological characterization of their host galaxies. We demonstrated that the average image of such galaxies appears much more compact than observed in other galaxies of the field. We suggest that these sources recently experienced a phase of dynamical contraction. According to hydro-dynamical simulations at high resolution developed in the past few years, this compaction could follow the violent gravitational instabilities that are naturally triggered in gas-rich disk galaxies of the distant Universe. This mechanism could then help funneling large amounts of gas toward their central region, feeding at the same time the nuclear activity of their super-massive black hole.

The results obtained as part of MORPHOSTAR are now at the basis of several follow-up (supplementary observations, use of diagnostics developed within the project), in which we are still highly involved. The discovery of very young star-forming regions in distant galaxies led to a more detailed follow-up of their physical properties, especially with the millimeter interferometer ALMA and the MUSE integral field spectrograph installed on the Very Large Telescope in Chile (the data are curently being analyzed). The diagnostics of morphological classification that we developed as part of MORPHOSTAR using stellar mass surface density maps for distant galaxies have recently been applied for determining the rate of mergers and galaxy interactions at different epochs of cosmic history, more specially with the goal of constraining the nature of galaxies experiencing a «starburst« episode. Finally, the constraints on the size of galaxies hosting a dust-obscured active nucleus resulted in new observing programs with the ALMA interferometer, so as to constrain more precisely the spatial distribution of the star-forming activity in these sources.

1. « An extremely young massive clump forming by gravitational collapse in a primordial galaxy », Zanella, A. et al., 2015, Nature 521, 54

2. « A Physical Approach to the Identification of High-z Mergers: Morphological Classification in the Stellar Mass Domain », Cibinel, A., Le Floc’h, E., Perret, V., et al., 2015, ApJ 805, 181

3. « Obscured active galactic nuclei triggered in compact star-forming galaxies », Chang, Y.-Y., Le Floc’h, E., Juneau, S., et al., 2017, MNRAS 466, L103

4. « ALMA constraints on star-forming gas in a prototypical z=1.5 clumpy galaxy: the dearth of CO(5-4) emission from UV-bright clumps », Cibinel, A., et al., 2017, MMRAS 469, 4683

5. «Infrared Selection of Obscured Active Galactic Nuclei in the COSMOS Field«, Chang, Y.-Y., Le Floc’h, E., Juneau, S., et al., 2017, ApJS 233, 19

6. «Unveiling sizes of compact AGN hosts by ALMA«, Chang, Y.-Y., Le Floc’h, E., Juneau, S., et al., article submitted to the Astrophysical Journal

7. «Early- and late-stage mergers among main sequence and starburst galaxies at 0.2 < z < 2«, Cibinel, A., et al., 2018, article submitted to MNRAS

8. « Statistical properties of young star-forming clumps at z~2 », Zanella, A., Le Floc’h, E., Bournaud, F., et al., in preparation.

The existence of massive ellipticals, large rotating disks and super-massive black holes at high redshifts shows that a rapid growth and structural evolution of galaxies occurred as soon as the first few billion years of cosmic history, thanks not only to “external” phenomena like galaxy merging but also internal mechanisms such as disk instabilities, turbulence and energy feedback. Yet, the exact role that each of these different processes played in the cosmic growth of structures remains poorly constrained by the observations. We propose an original program that will aim at quantifying the contribution of the different modes that governed the assembly of galaxies, with some specific emphasis on the mechanisms that drove the parallel growth of bulges and their central black holes. To achieve this goal, we will combine unique observations of the deep Universe at 1<z<3 with state-of-the-art hydrodynamic simulations, in order to relate the evolution of galaxy morphologies with simultaneously their stellar mass, their star formation rate and their nuclear activity. Such a 3-dimentional understanding of the morphological evolution of galaxies across cosmic times has never been explored so far and should yield substantial improvements of our understanding of structure formation. In particular, our project will aim at determining:<br />
(i) At the scale of individual galaxies, how morphologies varied with galaxy stellar mass, star formation rate, AGN activity as well as with cosmic time;
(ii) At cosmological scale, how the different processes that drove galaxy evolution contributed to the cosmic history of stellar mass assembly and black hole growth;
(iii) The history of the morphological transformations of galaxies throughout cosmic time.

Our team at CEA has a privileged access to some of the most exquisite multi-wavelengths data sets of the distant Universe, including new high-resolution imaging from HST/WFC3 carried out in the GOODS and COSMOS fields, ultra-deep far-IR & X-ray coverage obtained by the Chandra and Herschel satellites, as well as extensive ground-based spectroscopy and large collections of redshifts. Our expertise also includes detailed characterization of galaxy physical properties (SEDs, activity diagnostics, gas dynamics, ...), and we are now developing extensive hydrodynamic simulations of structure formation to interpret the observed structural evolution of galaxies in the full cosmological context. In our group though, the characterization of galaxy morphologies has not been pushed as a priority so far and we are missing a leadership in this topic. We thus request the support from the ANR to fund a 3-year long position for an external researcher with high qualification in the morphology of distant galaxies. Filling this position will supplement our current expertise and will thus enable the synergy needed for bringing our program to completion. The total cost of our project is 1,271,603 Euros and the financial help that we request to the ANR reaches 205,920 Euros (16%).