Research

(Latest update: September 28th, 2017).

Here follows an (incomplete) overview of the main science topics I have worked on in the past years, and my research interests.

The spectral energy distributions of galaxies

SED library (Berta et al. 2013a)

I have been working to galaxies spectral energy distributions (SEDs) since my second publication in far 2003. It is - so to say - one of the fil rouges in my research, an important and versatile tool for the study of galaxy evolution and the physical components of distant galaxies.

My goals have been diverse: measure stellar masses, study a possible AGN component, derive dust and gas masses, and combinations of these. I started with a very simple templates fitting, improved using multi-population synthesis, steered to dust modeling with different approaches, and landed into a full 3-components description of SEDs including stars, dust and AGN/torus (the SED3FIT code).

One of my latest achievements is the definition of a SED templates library based on Herschel PEP+HerMES data (see below, Berta et al. 2013a). Using this rich dataset, we reproduced the distribution of galaxies in a carefully selected restframe ten-colors space, using a superposition of multivariate Gaussian modes. For each class median SEDs were built and modeled with the precursor of SED3FIT.

The defined Gaussian grouping is used to identify also rare or odd sources. The zoology of outliers includes Herschel-detected ellipticals, very blue z ∼ 1 Ly-break galaxies, quiescent spirals, and torus-dominated AGN with star formation.

Out of the main groups and outliers, the new template library was assembled (and subsequently tested), consisting of 32 SEDs describing the intrinsic scatter in the restframe UV-to-submm colors of infrared galaxies (Berta et al. 2013a).

The AGN component

Black Hole accretion rate (Delvecchio et al. 2015)

As part of the SED analysis, I have gradually developed a fitting code, reproducing the UV-to-submm emission of galaxies with a combination of three components: stars; dust heated by star formation; an AGN (torus and central engine). This code is named SED3FIT and is described in a dedicated page.

SED3FIT has been adopted for different pieces of analysis and publications: from local ULIRGs with mid-IR spectroscopy (Gruppioni et al. 2016) to distant galaxies with radio and far-IR detection (Smolčić et al. 2017; Delvecchio et al. 2017). It is gaining increasing popularity eavery day, thanks to its versatility, its speed, and the uniqueness of the simultaneous 3-components fitting procedure.

Using Herschel far-IR data, combined with X-ray properties, and SED fitting, Delvecchio et al. (2015) studied the relation of AGN accretion, star formation rate (SFR) and stellar mass (M*) of star-forming galaxies up to z = 2.5. The average accretion rate of star forming galaxies correlates with SFR and with M∗. The dependence on SFR becomes progressively more significant at z > 0.8. This may suggest that SFR is the original driver of these correlations. Average AGN accretion and star formation increase in a similar fashion with offset from the star-forming "main-sequence". Accretion on to the central black hole and star formation broadly seem to trace each other, irrespective of whether the galaxy is evolving steadily on the main-sequence or bursting.

The Herschel PEP survey

PEP

I have been working for almost 10 years in the IR group of the Max Planck Institute für Extraterrestrische Physik (MPE, Munich), working at the far-IR PACS Evolutionary Probe (PEP, Lutz et al. 2011) survey, carried out by the Herschel Satellite.

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 targeted The PEP survey includes six among the most widely studied blank fields across the sky, two high-redshift galaxy clusters and ten lensing clusters, in a "wedding cake" strategy. It included wide and shallow fields (e.g. COSMOS), deep pencil-beam fields (GOODS-N/S), and even deep lensing clusters.

As of September 2017, the survey has produced 79 refereed publications, counting more than 5000 citations, and covering all main topics of IR extragalactic cosmology. We have resolved the CIB; studied the SEDs and properties of sub-mm galaxies; studies the far-IR/radio correlation and its evolution; classified and groupe the SEDs of star forming galaxies; studied the AGN content of IR galaxies; studied all possible galaxy properties as a function of position in the M*-SFR space; measured the dust and gas content of star forming galaxies at least up to redshift z=3. For a complete list of PEP publications see this link.

The dust and gas content of galaxies

M(mol) cosmic density (Berta et al. 2013b)

Tightly connected, dust and gas are the key ingredients of star formation, which is governed by their complex, mutual interplay as in a cyclic dance. Stars form in cold, dense molecular clouds and dust works as catalyst in transforming atomic hydrogen into molecular hydrogen. Gas is again expelled from stars during their lifetime in the form of winds and finally at the supernova (SN) stage. Dust is believed to be mainly produced in the envelopes of asymptotic giant branch (AGB) stars and at the end of the life of massive stars during the explosive SN phase. Supernovae shocks, on the other hand, destroy dust grains, which can form again in the interstellar medium (ISM) by an accretion process. Dust absorbs the ultraviolet (UV) emission of young stars, allowing gas to cool and condense to form new stars.

We used deep far-infrared data from the PEP/GOODS-Herschel surveys and restframe ultraviolet photometry to study the evolution of the molecular gas mass function of normal star-forming galaxies. This is the very first estimate of the gas mass function at redshift z>0! Computing the molecular gas mass, by scaling star formation rates through depletion timescales, or combining infrared (IR) luminosity and obscuration properties as described in the literature, we obtained M(mol) for roughly 700, z = 0.2−3.0 galaxies near the star-forming "main sequence". The number density of galaxies follows
a Schechter function of M(mol). The characteristic mass M* is found to strongly evolve up to z ∼ 1 and then flatten at earlier epochs, resembling the IR luminosity evolution of similar objects. Integrating the mass function, we studied the evolution of the M(mol) density (see Figure, Berta et al. 2013b).

Genzel et al. (2015) studied the dependence of M(mol) as a function of position in the M*,SFR,z space and provide useful relations mapping these three parameters into M(mol). Berta et al. (2016) analysed in greater detail all possible sources of uncertainty and systematics affecting SED-based dust and gas mass estimates.

The cosmic infrared background

CIB (Lutz et al. 2014)

The cosmic infrared background (CIB) includes roughly half of the energy radiated by all galaxies at all wavelengths across cosmic time, as observed at the present epoch. We exploited the PEP survey data to study the CIB and its redshift differential, at 70, 100 and 160 μm, where the background peaks (Berta et al. 2010, 2011; Magnelli et al. 2013; Lutz et al. 2014).

We defined number counts spanning over more than two orders of magnitude in flux: from a fraction of mJy to few hundreds mJy. Stacking of 24 μm sources and P(D) statistics extend the analysis down to even fainter fluxes. Thanks to the wealth of ancillary data in PEP fields, it was possible to study the differential number counts d2N/dS/dz and CIB up to z = 5.

We resolved roughly 75% of the CIB in individual sources. Employing the P(D) analysis, this fraction increase up ∼89%! More than half of the resolved CIB was emitted at redshift z ≤ 1. The 50%-light redshifts lie at z = 0.58, 0.67 and 0.73 at the three PACS wavelengths. The distribution moves towards earlier epochs at longer wavelengths: while the 70 μm CIB is mainly produced by z ≤ 1.0 objects, the contribution of z > 1.0 sources reaches 50% at 160 μm. Most of the CIB resolved in the
three PACS bands was emitted by galaxies with infrared luminosities in the range 1e11−1e12 Lsun (i.e. LIRGs).

The SWIRE survey and ground-based follow-up

SWIRE Tadople

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 have been a collaborator. SWIRE was the largest Spitzer Legacy Program. It included deep imaging of 6 high-latitude fields, totaling 49 square degrees in all the seven Spitzer bands. The main aim of the survey was 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.

A deep optical follow-up of SWIRE ELAIS-S1 field was 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) included B,V,R WFI@2.2m and I,z VIMOS@VLT imaging down to B=26 and V,R=25.5. I took 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 provided optical identifications, colors, rough morphologies, photometric redshifts, for the IR sources detected by Spitzer in this area (Berta et al., 2006).

This rich multi-wavelength dataset allowed us to: study the complete SEDs of Spitzer sources; derive the physical properties of galaxies up to z~3; search for distant (z>1) galaxy clusters, study the statistical properties (e.g. number counts) of starburst, evolved galaxies and AGNs; study the cosmic star formation density as a function of redshift; build the luminosity and mass functions of galaxies and study their dependence on redshift and environment.

The assembly of massive galaxies

SWIRE IR-peakers MF

The IRAC Camera onboard Spitzer observed 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 content. We took 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".

Thanks to the rich multiwavelength dataset in the ELAIS-S1 SWIRE field, I have derived the stellar mass function of these galaxies up to redshift z~3. The z=2−3 stellar mass function between 1e11 and ∼1e12 Msun was probed with unprecedented detail, at that time. A significant evolution was found not only for galaxies with M*∼1e11 Msun, but also at higher masses. The comoving number density of these galaxies was lower by more than a factor of 10 at z=2−3, with respect to the present epoch. SWIRE 5.8 μm peakers more massive than 1.6 × 1e11 M provide 30−50% of the total stellar mass density in galaxies at z=2−3 (Berta et al., 2007b).

Observing in the Infrared

Herschel telescope (1785-1789)

The development of efficient infrared detectors, operating at the focal plane of Space Observatories opened new frontiers to the study of galaxy formation and evolution. Each technological improvement and new IR astronomical Satellite triffered a small revolution in the way we know and understand the distant Universe.

The IRAS (1984), ISO (1995), Spitzer (2003), Herschel (2009) and - soon to come - JWST, SPICA,... satellites constitute a logical sequence in the study of galaxy infrared properties, with a well modulated improvement of observing capabilities.

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 timea 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; select dust-shrouded quasars independently of orientation; and discovery hyper-luminous galaxies out to redshifts z<5 (see Genzel & Cesarsky 2000, for a review).

The NASA Great Observatory Spitzer 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), lauched on May 14th, 2009, completed its mission in 2013. It performed 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). Designed to observe the `cool Universe', the extragalactic legacy of Herschel included: resolving the cosmic infrared background (CIB); building full spectral energy distributions (SEDs) of normal star forming galaxies and AGNs up to redshift z~3 and beyond; derive calorimetric star formation rates (SFR) at the peak of the cosmic star formation density; measure the dust and gas content of star forming galaxies; and much more.