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Research Interests

Herschel PACS surveys

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I am currently a post-doc in the IR group at the Max Planck Institute fuer Extraterrestrische Physik (MPE, Munich), working in the preparation of far-IR surveys with the Herschel PACS camera.

Our group is involved in the PACS Evolutionary Probe (PEP) survey. PEP is a Herschel guaranteed time key programme survey of the extragalactic sky, aimed to study the restframe far-infrared emission of galaxies up to redshift ~3, as a function of environment. The survey will shed new light on the constituents of the cosmic IR background and their nature, as well as on the co-evolution of AGN and starbursts.

The PEP survey is driven by science goals addressing a number of key open topics in galaxy evolution:

Please, visit the PEP homepage for futher details.

Observing in the Infrared

The development of new efficient infrared detectors, operating at the focal plane of Space Observatories has opened a new window to the exploration of the distant Universe. The extension of cosmological observations to longer wavelengths, with respect to the traditional optical spectral domain, opened new frontiers to the study of galaxy formation and evolution.

The IRAS (1984), ISO (1995), Spitzer (2003) and, later, ASTRO-F, Herschel, JWST satellites constitute a logical sequence in the study of galaxy infrared properties, with a well modulated improvement of observing capabilities, modern instruments detecting more distant sources.

The Infrared Astronomical Satellite (IRAS) mission enjoyed huge success, including the sensational discoveries of ultra- and hyper-luminous infrared galaxies and of a substantial population of evolving starbursts. However, the bright limiting flux densities restricted the samples to low redshifts (z < 0.3) for all but a few ultraluminous objects (see Sanders & Mirabel, 1996, for a review).

The Infrared Space Observatory (ISO) offered a ~1000 time improvement in sensitivity in the mid-IR over IRAS. Ambitious deep mid-IR (at 7 and 15 microns) observations and shallow wide-area surveys (e.g. Oliver et al. 2000; Aussel et al., 1999), aimed to trace the extinguished star formation history of the Universe to z~1-2, to select dust-shrouded quasars independently of orientation and to discovery hyper-luminous galaxies out to redshifts z<5 (see Genzel & Cesarsky 2000, for a review).

The NASA Great Observatory Spitzer (SST) was launched in August 2003. Its Infrared Array Camera (IRAC) was specifically designed for probing the assembly of stellar mass in galaxies at redshift >2, by observing in the 3-8 microns spectral domain. At the same time deep sky imaging with the Multiband Imaging Photometer (MIPS) at 24, 70 and 160 microns is detecting dust re-radiation from distant actively star-forming galaxies. The cosmic rate of stellar formation is being measured with high accuracy, in a way completely independent from UV-optical estimates, subject to extinction uncertainties.

The Herschel Space Observatory (simply Herschel), to be lauched in 2008, is the fourth `cornerstone' mission in the ESA science programme. It will perform photometry and spectroscopy in approximately the 55-672 microns range, with a primary mirror of 3.6 meters and three instruments aboard: the Photodetector Array Camera and Spectrometer (PACS), the Spectral and Photometric Imaging REceiver (SPIRE), and the Heterodyne Instrument for the Far Infrared (HIFI). Herschel was designed to observe the `cool universe'; the main targets of the mission are the earliest epoch proto-galaxies, the AGN/starburst co-evolution, and the mechanisms ruling the formation of stars and planetary systems.

The SWIRE survey and ground-based follow-up

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The Tadpole galaxy as seen by SWIRE/Spitzer.

The first year of Spitzer in-flight operations has been mostly devoted to six different Legacy science Programs, representing projects of general and lasting importance to the broad astronomical community. Two of these are dedicated to deep cosmological surveys: the Great Observatory Origins Deep Survey (GOODS) and the Spitzer Wide-Area Infrared Extragalactic (SWIRE, Lonsdale et al., 2003) survey, of which I am a collaborator. SWIRE is the largest Spitzer Legacy Program. It includes deep imaging of 6 high-latitude fields, totaling 49 square degrees in all the seven Spitzer bands. The main aim is to trace the evolution of dusty, star-forming galaxies, evolved stellar populations, and AGN as a function of environment, from redshifts z~3, down to the current epoch.

We make wide use of Spitzer/SWIRE data, as well as complementary multiwavelength observations, which are already available and will be scheduled in future. The best existing instruments and largest telescopes are being exploited. Some examples include --- among others --- spectroscopy and imaging from ESO Very Large Telescope (VLT), Gemini North and South, Canada France-Hawaii Telescope (CFHT), Kitt Peak National Observatory (KPNO) and Cerro Tololo Inter-American Observatory (CTIO), Palomar, Isaac Newton Telescope (INT), Galaxy Evolution Explorer (GALEX) ultra-violet imaging, XMM x-ray imaging, Very Large Array (VLA) ultradeep radio imaging, and so on.

A deep optical follow-up of SWIRE ELAIS-S1 field is being carried out at the Padova Astronomy Department, as an ESO Large Programme (P.I. A. Franceschini) covering more than 5 sq.deg. in five photometric bands. The ESO-Spitzer wide-area Imaging Survey (ESIS) includes B,V,R WFI@2.2m and I,z VIMOS@VLT imaging down to B=26 and V,R=25.5. I have taken the responsibility for observations, data reduction, analysis of the ESIS survey, and coordination with other ancillary multiwavelength programs in ELAIS-S1 since its beginning. This project is providing optical identifications, colors, rough morphologies, photometric redshifts, for the ~200000 IR sources detected by Spitzer in this area. Other ancillary data include observations in the X-ray, ultra-violet, near-IR, and radio spectral domains. See Berta et al. (2006) for more details.

Such a comprehensive wavelength coverage allows us to: study the complete SEDs of Spitzer sources, derive the physical properties of distant galaxies in the ELAIS-S1 area, up to z~3, search for distant (z>1) galaxy clusters, study the statistical properties (e.g. number counts) of starburst, evolved galaxies, AGNs and compare them to theoretical models, study the cosmic star formation density as a function of redshift, build the luminosity and mass functions of galaxies and study their dependance on redshift and environment.

The sources of mid- and far-IR background

24 microns number counts
24 microns source counts (from Berta PhD thesis).

The cosmic Infra-Red Background (CIRB, Puget et al. 1996, Hauser et al. 1998), discovered by the COBE satellite in the mid 90's, is the most relevant diffuse radiation after the CMB. Various analyses have indicated that this background may be explained as due to forming galaxies and quasars (e.g. Franceschini et al. 2003, Elbaz et al. 2002).

Since its discovery, one of the important themes in cosmology has been the detection and characterization of the sources contributing to CIRB energy budget. The combined use of deep observations from space by the ISO, for selecting high-z active galaxies (both starbursts and AGNs), and the VLT, for high-resolution optical studies, turned out to be particularly powerful (Franceschini et al. 2003) to physically characterize them. These sources constitute a population of luminous and ultra-luminous massive star-forming galaxies, at redshift z=0.5-1.5, powering at least 50% of the CIRB emission and strongly evolving in cosmic time.

While only ~30% of the light from normal galaxies is absorbed by dust, this fraction becomes much higher, when the most active star-forming regions in galaxies are taken into account. The universe seems to have experienced a phase of enhanced activity of star-formation and gravitational accretion in the past, mostly visible in the infrared.

The Spitzer MIPS camera provides the observations needed to push the study of mid-IR sources to fainter luminosities and larger distances. Policyclic Aromatic Hydrocarbons emission, typical of starburst 7-13 microns restframe spectra, will be sampled up to redshift ~3, corresponding to an epoch when the universe had only the ~15% of today's age.

As an extension of the work presented by Franceschini et al. (2001), the Padova group is developing a new cosmic evolutionary model, aimed at reproducing the observed number counts from mid-IR to sub-mm wavelengths, including early SWIRE data. The backward model k-corrects the LF observed at a reference redshift, convolves it to volume density and integrates over cosmic time. A simple power-law evolution of luminosity and density is considered, in order to reproduce the observations.

The availability of both old ISO 15 microns data and Spitzer/MIPS 24 microns observations in SWIRE fields, as well as the far-IR MIPS coverage to 70 and 160 microns, allows us also to build a direct relation between MIPS fluxes and the bolometric IR luminosity of the distant starbursts detected in SWIRE. Thanks to the good sensitivity and large area targeted, such a relation will be studied as a function of redshift and environment, investigating how starburst luminosity depends of cosmic distance and density.

The assembly of massive galaxies

IR-peakers mass function
Mass function of IR-peakers (from Berta et al. 2007).

Tracing the formation of galaxies and understanding the epoch when the bulk of their baryonic mass was assembled represents one of the major problems of modern cosmology, particularly controversial when dealing with massive (M[stars]>1e11 Msun) objects.

The assembly of massive galaxies is one of the critical questions in the cosmic evolutionary scenario. The uniform properties of local early-type galaxies and of the fundamental plane have inspired the so called ``monolithic collapse'' scenario (e.g Eggen et al., 1962, Chiosi & Carraro 2002}, in which galaxies formed in the remote past through huge events of star formation and subsequently evolved passively across cosmic time. On the other hand, in the more recent ``hierarchical'' scenario (e.g. White & Rees 1978, Kauffmann et al. 1996, Kauffmann & Charlot 1998), massive galaxies assemble by mergers of lower-mass units, with the most massive objects being born in the latest stages of evolution, at z<1.

The evolution of the global star formation rate provides a sensitive probe of galaxy formation and evolution. Complementary insights in the star formation history can be gained from other physical properties of galaxies. Stellar mass is the key parameter. The integral of the past stellar formation activity in galaxies provides the logical completion to the current view of galaxy evolution.

Several observations attempted to investigate the formation and evolution of massive systems and test model predictions (see, e.g., the recent works by Treu 2004, Fontana et al. 2004, 2006, and Cimatti et al. 2003), but a clear and coherent picture has not yet emerged. Very little is known about galaxy stellar mass assembly at redshift z > 1.5. This is a crucial redshift range because evidences exist that most massive galaxy assembly occurred at that cosmological epoch (e.g. Cimatti et al. 2003, Daddi et al. 2004). This results, for example, in the existence of a substantial population of old, massive and passive E/S0 galaxies at z>1, implying a minimum formation redshift of z~2 (e.g. Daddi et al. 2004 among many others).

The Infrared Array Camera onboard Spitzer is observing in the 3.6-8.0 microns wavelength range. The instrument was specifically designed for detecting galaxies' restframe near-IR emission, up to redshift z~3, hence directly probing their stellar mass assembly. The spectral energy distributions of IRAC/SWIRE sources, extended to the optical-UV thanks to ground-based and GALEX ancillary data, and to the far-IR by means of MIPS observations, are the subject of an extensive spectro-photometric synthesis analysis. We take advantage of the shape of near-IR spectral energy distribution (SEDs) of galaxies to identify high-redshift objects on the basis of IRAC colors. Our selection is based on the detection of the 1.6 microns stellar peak in galaxies, redshifted to the IRAC domain. We called these galaxies "IR-peakers"

We have derived the stellar mass function of these galaxies in the central square degree of the ELAIS-S1 SWIRE field, where optical, near-IR (J, Ks) photometry and optical spectroscopy are available (Berta et al. 2006 Dias et al., in prep., La Franca et al., in prep.). This area, and the sampled volume, are bigger than any other previously explored for studying very massive galaxies at z=1-3, which were limited to very deep, pencil-beam surveys (e.g. GOODS). Please, read Berta et al. (2007) for all the details!!!

Nowadays, no information is yet available on the nature of the environment hosting these high-redshift, very massive, evolved galaxies. Further investigations into this kind of objects, need to explore the neighbouring environments, in order to correctly interpret their nature and their contribution to the stellar mass function and its evolution.

Other research branches I was/am involved in