Starforming galaxies

Among the various classes of objects considered in this review, star-forming galaxies at z = 2-3 are those for which in the last few years there has been most progress towards determining their physical properties, including metallicity. Here I concentrate in particular on recent results on galaxies in what used to be referred to as the 'redshift desert' (z — 2), which somewhat ironically has turned out to be one of the most intensely studied cosmic epochs, partly because of its accessibility to ground-based observations and partly because we now understand it to be the epoch when mass assembly in galaxies was at its peak.

Observations of galaxies in the z = 2-3 redshift range now span almost the entire electromagnetic spectrum, from X-rays to radio, and spectroscopic surveys include thousands of such objects with measured redshifts. Among this rich complement of data, the near-infrared bands have played a pivotal role. Although it is still somewhat difficult to access efficiently from the ground, given the high and variable terrestrial background and the lack until now of multi-object spectrographs, the observed near-IR regime has the advantage of sampling rest-frame optical wavelengths at which metallicity and other physical diagnostics are better developed than in many other wavelength intervals.

In particular, the rest-frame optical colours measured with J, H and K photometry are sufficient, when combined with the more easily obtained rest-frame UV colours, to give an adequate description of the galaxies' spectral energy distributions (SEDs). As shown by Shapley et al. (2005), the SEDs can be interpreted with state-of-the-art stellar-population models in terms of the past history of star formation experienced by the galaxies and one can deduce, among other quantities, an estimate of the assembled stellar mass, as illustrated in Figure 20.3.

For the UV-selected (and hence actively star-forming) galaxies at z ~ 2 considered by Erb et al. (2006b), assembled stellar masses are in the range - (109-10114)M©, with a mean <Mstars) = (3-4) x 1010M©. At one end of this range we have galaxies where star formation has apparently just turned on (in the

BX536 z = 1.977 t = 50 Myr Age = 202.60 Myr Mass = 5.5 x 1O1OM0

Observed Wavelength (|m)

10 11


10 11


Figure 20.3. Left panel: an example of SED fitting, reproduced from Shapley et al. (2005). A wide grid of parameter space in the spectral synthesis models of Bruzual & Charlot (2003) is explored to determine the age, assembled stellar mass, past history of star formation (continuous or exponentially declining) and reddening that best fit the observed optical and IR photometry. The two fits shown here are respectively with and without the Spitzer IRAC data. Shapley et al. found that the addition of the 3.6- 8-^m IRAC data, while reducing the uncertainties in the physical parameters deduced, does not lead to significantly different star-formation histories for most of the galaxies considered. Right panel: histograms of assembled stellar masses deduced from SED fitting of BX galaxies, reproduced from Erb et al. (2006b). The larger histogram is for a sample of 461 BX galaxies with near-IR photometry, while the smaller histogram pertains to a sub-set of these for which Erb et al. detected nebular Ha emission with near-IR spectroscopy.

last few 107 years) and most of the baryons are still in gaseous form (gas fractions f > 0.8). At the other end of the scale, some 15% of the BX galaxies in the Erb et al. (2006b) sample have ages comparable to the age of the Universe at the time when we observe them and have by then already turned most of their gas into stars (f < 0.2). When the gas fractions are taken into account, the total baryonic masses range from Mstars+gas — 1010 3MQ to 10115M0, in reasonable agreement with the dynamical masses indicated by the widths of the Ha emission lines and consistent with the masses Mdm — (10118-1012 2)MQ of the dark-matter halos in which these galaxies reside, as deduced from their clustering properties (Adelberger et al. 2005). As argued by Adelberger et al., such masses indicate that today's ellipticals and massive bulges are the descendants of the galaxies we see undergoing vigorous star formation at z = 2.

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