La mujer y su participación:

In the last twenty years or so, it has become more and more clear that in order to understand deeply galaxy evolution one has to take into account the energy that is absorbed by dust and re-emitted at mid- and far-infrared wavelengths. In fact, the derivation of the cosmic star formation history from ultraviolet measurements suffers from uncertainties in the obscuration corrections which must be applied to the rest frame emitted flux (e.g. Lilly et al. 1996; Madau et al. 1996). Thus, a more precise estimate of SFR is needed to investigate the SF activity at different cosmic times and environments. This possibility is offered by a calorimetric estimate coming from the mid- and far-IR emission. In the past, the two satellites ISO and Spitzer were already equipped with sensitive far-infrared detectors, although they focused on the study of local objects and luminous galaxies due to the high level of source confusion. These limitations have changed dramatically with the advent of

the Herschel satellite.

In the next sections we describe the satellite properties, focusing in particular on

the P hotoconducting Array Camera and Spectrometer (PACS) instrument on board of

Herschel. We will then present the main results existent in the literature obtained using

Herschel data.

1.3.1

The Herschel satellite

The key ingredient of our analysis is a robust estimate of the galaxy star formation activity. The rest-frame mid-infrared emission from dust in galaxies, in particular the emission detected in the 8 µm and 24 µm Spitzer bands, has been analyzed by a number of authors (Alonso-Herrero et al. 2006; Boselli, Lequeux, & Gavazzi 2004; Calzetti et al. 2005, 2007;

F¨orster Schreiber et al. 2004; P´erez-Gonz´alez et al. 2006; Rela˜no et al. 2008; Rieke et al. 2009; Roussel et al. 2001; Salim et al. 2009; Wu et al. 2005; Zhu et al. 2008), and a general correlation (but also a number of caveats) between mid-IR emission and SFR has been found. In the last two years, the Herschel Space Telescopeopened new windows of sensitivity at even longer wavelengths than those explored by Spitzer, and in turn, provided even more powerful tools for probing the evolution of the rate at which galaxies have assembled their gas and dust components.

The Herschel satellite (Pilbratt et al. 2010) is designed to explore the “cool universe” during its expected 3.5 year mission lifetime. To achieve its scientific goals, Herschel is equipped with a 3.5 m main mirror and marks the beginning of a new generation of “space giants”. Bigger than any of its predecessors at approximately 7.5 m height and 4 m width, its science payload consists of three instruments: PACS and SPIRE, both cameras and spectrometers that allow Herschel to take pictures in six different “colors” in the far-infrared, and HIFI, a spectrometer with extremely high spectral resolution. The main instrument used in this work is PACS (Poglitsch et al. 2010), which providesHerschel with far-infrared imaging and spectroscopic capabilities from 60 to 210µm.

1.3.2

Photoconducting Array Camera and Spectrometer

The requested observing time of Herschel is largely spent on deep and/or large scale pho- tometric surveys performed in scan map mode with the PACS photometer. Indeed, the opening of the 60-210µm window by PACS photometer addresses a wide range of key ques- tions of current astrophysics concerning the origins of stars, planetary systems, galaxies, and the evolution of the universe.

The PACS photometer detectors are bolometer arrays. Each pixel of the array can be considered as a little cavity in which sits an absorbing grid. The incident infrared radiation is registered by each bolometer pixel by causing a tiny temperature difference, which is measured by a thermometer implanted on the grid. The blue channel offers two filters, 60-85 µm and 85-130 µm and has a 32×64 pixel array. The red channel has a 130-210 µm filter and has a 16×32 pixel array. Both channels cover a field-of-view of

∼1.750×3.50, with full beam sampling in each band. The two short wavelength bands are selected by two filters via a filter wheel. The field of view is nearly filled by the square pixels, however the arrays are made of subarrays which have a gap of ∼1 pixel in between. For the long wavelength end 2 matrices of 16×16 pixels are tiled together. During a PACS scanmap observation, the telescope moves back and forth in a pattern of parallel scan lines that are connected by short turnaround loops. For science observations the multiplexing readout samples each pixel at a rate of 40 Hz. Because of the large number of pixels, data compression is required and hence raw data are binned to an effective 10 Hz sampling rate. The scan technique is the most frequently used Herschel observing mode. Scan maps are the default to map large areas of the sky, for galactic as well as extragalactic surveys, but at the same time they are also recommended for small fields and even for point sources. Scan maps are performed by slewing the spacecraft at a constant speed along parallel lines. Available satellite speeds are 10, 20, 60 arcsec/s in PACS prime mode and 20, 60 arcsec/s

(medium, fast) in PACS/SPIRE parallel mode. The number of satellite scans, the scan leg length, the scan leg separation, and the orientation angles (in array and sky reference) are freely selectable by the observer. Via a repetition parameter the specified map can be repeated n times. During the full scan map duration the bolometers are constantly readout with 40 Hz. However, due to satellite data rate limitations, the data are compressed to a final sampling rate of 10 Hz.

1.3.3

The PACS Evolutionary Probe survey

Members of the PACS instrument consortium, the Herschel Science Centre, and mission scientist M. Harwit have joined forces in the PACS Evolutionary Probe (PEP) deep extra- galactic survey (Lutz et al. 2011).

PEP encompasses deep observations of blank fields and lensing clusters, close to the Herschel confusion limit, in order to probe down to representative high-redshift galaxies, rather than being restricted to individually interesting extremely luminous cases. PEP is focused on PACS 70, 100, and 160 µm observations. SPIRE observations of the PEP fields are obtained in coordination with PEP by the HerMES survey (Oliver et al. 2010). Larger and shallower fields are observed by HerMES (70 deg2), as well as by the H-ATLAS survey (570 deg2, Eales et al. 2010), while the GOODS-Herschel program (Elbaz et al. 2011) provides deeper observation in (part of) the GOODS fields that are also covered by PEP. Finally, the Herschel lensing survey (Egami et al. 2010) substantially increases the number of lensing clusters observed with Herschel, adding about 40 clusters to the 10 objects covered by PEP.

The PACS Evolutionary Probe is one of the major Herschel Guaranteed Time (GT) extragalactic projects. It is structured as a “wedding cake” survey, based on four different layers in order to cover wide shallow areas and deep pencil-beam fields. PEP includes the most popular and widely studied extragalactic blank fields: COSMOS (2 deg2 ), Lockman Hole, EGS and ECDFS (450-700 arcmin2 _{), GOODS-N and GOODS-S ( ∼200 arcmin}2 _).

In addition, the observations of ten nearby lensing clusters offer the chance to break the PACS confusion limit thanks to the gravitational lensing (Altieri et al. 2010). PEP aims to resolve the cosmic infrared background (e.g. Berta et al. 2010 and Berta et al. 2011) and determine the nature of its constituents, determine the cosmic evolution of dusty star formation and of the infrared luminosity function, explore the relation between far-infrared emission and environment and determine clustering properties. Other main goals include study of AGN/host co-evolution, and determination of the infrared emission and energetics of known high redshift galaxy populations.