Cosmological evolution of the ISO/IRAS sources in NEPR
Context Understanding how galaxies form and evolve is a key goal of modern physical cosmology. A fundamental observable of galaxies is their luminosity function (LF), which has long been used to constrain galaxy formation models and to quantify star formation and evolution both in luminosity and in density.
The IR and far-IR spectral ranges are the best to deepen our knowledge of this subject since they trace the star formation that is responsible for galaxy formation. We exploited a multi-wavelength approach to study of the evolution of a far-IR selected sample of galaxies on which numerous studies of the far-IR evolution of galaxies still rely. It comprises a complete, 60 micron selected sample of galaxies (56) in the North Ecliptic Polar Region (NEPR), a sub-sample of the original 98 IRAS Deep Survey (IDS) fields with S(60μm)>50 mJy over an area of 6.25 square degrees (Mazzei et al. 2001).
Results
Thanks to our ISOCAM, optical/near-IR observations and spectroscopy (Aussel, et al. 2000), our sample, which provides a direct link between the IRAS and ISO surveys, and the forthcoming deeper Spitzer Space Telescope and AKARI cosmological surveys, is one of the far-IR selected complete samples with the larger spectral coverage.
The redshift distribution of our sample shows a tail extending up to 0.37, in particular 26% of the sources have redshifts z>0.1 (Della Valle et al. 2006).
To fully exploit the potential of this sample, ten times deeper than the IRAS Point Source Catalogue thus less liable to the effect of local density inhomogeneity, for investigating galaxy evolution, we calculated the 60 micron LF using the 1/V(max) method (Mazzei et al. 2007). Despite the fact that our redshift range exceeds 0.3, our 60micron LF, which extends up to log(L60)=12, does not show any evidence of evolution (red points in the figure 1). The earliest IR estimate of the LFs, derived from the IRAS data, indicated strong evolution so that LF increases with redshift as (1+z)3±1. Moreover, deep surveys at 15 micron carried out with ISO seem to require strong evolution of 15 micron sources. Mazzei et al. 2007 presented the bi-variate 15 micron LF, one of the few determinations based on ISO data, by convolving the 60 micron LF with the luminosity ratio distribution, L(15)/L(60) of our sample. This extends up to luminosity 100 times higher than before. Our results agree with the recent determinations in the common range of luminosity, however, above log(L15/L(solar))=10.6, results differ so that at log(L15/L_(solar))=11 we expect 10 times more sources than before (red points and red curve in figure 2).
Future work:
We aim at:
- i) investigating the evolution of the spectral energy distribution (SED) of such dusty galaxies over several Gyr in look-back time, i.e. over an interval in which the mean star formation rate in the Universe is known to evolve strongly; as already mentioned, the IDS sample is very suitable for this purpose; of course, a global view of the evolution of the SED, from optical to far-IR, overcomes the difficulties associated with uncertain dust extinction or bolometric corrections
- ii) exploring possible relationships between optical/near-IR morphology, dust temperature, dust extinction, luminosity, etc.
People:P. Mazzei, D. Bettoni, A. Della Valle, G. De Zotti
Collaboration: H. Aussel (FR), A. Franceschini (Padova Univ.)
Publications: Mazzei et al. (2007), A&338;A 462,21