Michel Fioc

The 12 μm ISO-ESO-Sculptor and 24 μm Spitzer faint counts reveal a population of ULIRGs as dusty massive ellipticals: Evolution by types and cosmic star formation

Context. Multi-wavelength galaxy number counts provide clues to the nature of galaxy evolution. The interpretation per galaxy type of the mid-IR faint counts obtained with ISO and Spitzer, consistent with the analysis of deep UV-optical-near IR galaxy counts, provide new constraints on the dust and stellar emission. Discovering the nature of new populations, such as high redshift ultra-luminous (≥10^(12) L_⊙) infrared galaxies (ULIRGs), is also crucial for understanding galaxy evolution at high redshifts. Aims. We first present the faint galaxy counts at 12 μm from the catalogue of the ISO-ESO-Sculptor Survey (ISO-ESS) published in a companion article (Seymour et al. 2007a, A&A, 475, 791). They go down to 0.31 mJy after corrections for incompleteness. We verify the consistency with the existing ISO number counts at 15 μm. Then we analyse the 12 μm (ISO-ESS) and the 24 μm (Spitzer) faint counts, to constrain the nature of ULIRGs, the cosmic star formation history and time scales for mass buildup. Methods. We show that the “normal” scenarios in our evolutionary code PEGASE, which had previously fitted the deep UV-opticalnear IR counts, are unsuccessful at 12 μm and 24 μm. We thus propose a new ULIRG scenario adjusted to the observed cumulative and differential 12 μm and 24 μm counts and based on observed 12 μm and 25 μm IRAS luminosity functions and evolutionary optical/mid-IR colours from PEGASE. Results. We succeed in simultaneously modelling the typical excess observed at 12 μm, 15 μm (ISO), and 24 μm (Spitzer) in the cumulative and differential counts by only changing 9% of normal galaxies (1/3 of the ellipticals) into ultra-bright dusty galaxies evolving as ellipticals, and interpreted as distant ULIRGs. These objects present similarities with the population of radio-galaxy hosts at high redshift. No number density evolution is included in our models even if minor starbursts due to galaxy interactions remain compatible with our results. Conclusions. Higher spectral and spatial resolution in the mid-IR, together with submillimeter observations using the future Herschel observatory, will be useful to confirm these results.

Disentangling star formation and AGN activity in powerful infrared luminous radio galaxies at 1 < z < 4

High-redshift radio galaxies present signs of both star formation and AGN activity, making them ideal candidates to investigate the connection and coevolution of AGN and star formation in the progenitors of present-day massive galaxies. We make use of a sample of 11 powerful radio galaxies spanning 1 < z < 4 which have complete coverage of their spectral energy distribution (SED) from UV to FIR wavelengths. Using Herschel data, we disentangle the relative contribution of the AGN and star formation by combining the galaxy evolution code PEGASE.3 with an AGN torus model. We find that three components are necessary to reproduce the observed SEDs: an evolved and massive stellar component, a submm bright young starburst, and an AGN torus. We find that powerful radio galaxies form at very high-redshift, but experience episodic and important growth at 1 < z < 4 as the mass of the associated starburst varies from 5 to 50% of the total mass of the system. The properties of star formation differ from source to source, indicating no general trend of the star formation properties in the most infrared luminous high-redshift radio galaxies and no correlation with the AGN bolometric luminosity. Moreover, we find that AGN scattered light have a very limited impact on broad-band SED fitting on our sample. Finally, our analysis also suggests a wide range in origins for the observed star formation, which we partially constrain for some sources.

Starbursts and dusty tori in distant 3CR radio galaxies

We present a study of the complete ultraviolet to submillimetre spectral energy distributions (SEDs) of 12 3CR radio galaxy hosts in the redshift range 1.0 13 L⊙) young stellar populations with ages of several hundred million years in hosts with masses exceeding 1011 M⊙. Dust masses are seen to increase with redshift, and a surprising correlation - or better upper envelope behaviour - is found between the AGN torus luminosity and the starburst luminosity, as revealed by their associated dust components. The latter consistently exceeds the former by a constant factor, over a range of one order of magnitude in both quantities.

Galaxy evolution and faint counts in the near‐infrared

The contribution of distant galaxies to the diffuse near‐infrared and submillimetric extragalactic backgrounds can be predicted with the help of a multispectral modelling of the faint galaxy counts. In particular, star‐forming galaxies have to be taken into account as well as old evolved galaxies implying a coherent simulation of the stellar emission from the blue to the near‐infrared with gas and dust contributions. For this reason, an extension of our previous UV and visible models is worked out with a detailed synthesis population model for strong, short timescale starbursts in a dusty medium in the near IR (Lancon and Rocca‐Volmerange, 1995, hereafter LRV95) by using a spectral library of stars from 1.4 to 2.5 μm observed with the FTS instrument at the 3.60m/CFHT and fitted on starburst spectra observed with the instrument. Then an extension of our atlas of synthetic galaxies was carried out, the near‐IR emission (K band) being carefully normalised to the blue (B, V or J+ bands) emission. Galaxy count...

High Resolution Spectra of Galaxies

We present inversion techniques which aim at recovering the composite nature and the kinematics of a stellar population from its high resolution absorption line spectrum. The originality of the combined inversion is its potential to recover both the stellar content and the kinematics simultaneously. These techniques use new synthetic high resolution spectra produced by PEGASE and minimization algorithms. We apply them to mock data representing the bulge and disk population of the inner region of spiral galaxies.

Modelling and interpreting optical spectra of galaxies at R=10 000

One way to extract more information from the integrated light of galaxies is to improve the spectral resolution at which observations and analysis are carried out. The population synthesis code currently providing the highest spectral resolution is PEGASE‐HR, which was made available by D. Le Borgne et al. in 2004. Based on an empirical stellar library, it provides synthetic spectra between 4000 and 6800 A at λ/δλ = 10 000 for any star formation history, with or without chemical evolution. Such a resolution is particularly useful for the study of low mass galaxies, massive star clusters, or other galaxy regions with low internal velocity dispersions.After a summary of the main features of PEGASE‐HR and comparisons with other population synthesis codes, this paper focuses on the inversion of optical galaxy spectra. We explore the limits of what information can or can not be recovered, based on theoretical principles and extensive simulations. First applications to extragalactic objects are shown.

An Extremely Blue Population in Multispectral Galaxy Surveys

The nature of the blue galaxy excess, discovered in faint counts is tentatively analyzed in the frame of a multispectral approach including far-UV counts. The far-UV (2000A) counts observed from the balloon experiment FOCA20000 bring conclusive constraints on star-forming populations. The evolution is followed with the help of our new model of galaxy evolution PEGASE. From Fioc and Rocca-Volmerange, 1997b (FRV97b), a population of dwarfs, periodically bursting, is needed to interpret the 2000A data, in agreement with the optical and near-IR faint counts. However, this near UV-bright population is unable to solve the debated question of the high-z blue galaxy excess in a flat universe.

A New Model of Spectral Evolution: PEGASE Application to Galaxy Counts

A new model of spectral evolution of galaxies (Fioc & Rocca-Volmerange 1996) named PEGASE (in french, Projet d’Etude des GAlaxies par Synthese Evolutive) has been developped. It uses the principle of isochrone synthesis and allows to compute synthetic spectra of galaxies on timescales as short as 1 Myr for starbursts or as long as 20 Gyrs for Hubble sequence galaxies. Its wavelength range extends from the far UV (200 A) to the NIR (3 μm). A new library of stellar spectra, mainly observational, has been built. Isochrones are computed for 2 sets of tracks with solar metallicity (Padova and Genova) up to the EAGB completed by the TPAGB phase (Groenewegen et al., 1993) and the PAGB phase from Schonberner (1983) and Blocker (1995). Nebular emission (continuum and lines), extinction in disks and in spheroids computed from the metallicity of the ISM and ejecta from stars are also considered.