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Observations at optical and infrared (IR) wavelengths of distant red 2 galaxies (up to redshift of z ~ 2) probe the rest-frame UV range, in particular the mid-UV. The first high z red galaxies detected were two faint radio sources from the Lieden-Berkeley Deep Survey (LBDS): LBDS 53W091 (z = 1.55) and LBDS 53W069 (z = 1.43) (Dunlop et al. 1996, Spinrad et al. 1997, Dunlop et al. 1999). The analysis of these systems was soon a subject of much debate. For instance, Spinrad et al. (1997) determined an age of 3.5 Gyr for LBDS 53W091, which posed complications to explain galaxy formation under an Einstein-De Sitter universe. This age was soon contested by a series of authors (Bruzual & Magris 1997, Heap et al. 1998, Yi et al. 2000) that derived much younger ages (< 2 Gyr), which allowed for more comfortable estimates for the formation redshift (zF) of the galaxies. Subsequent analyses revived the polemic by confirming the first determinations, i.e. ascribing ages in excess of 3 Gyr (Nolan et al. 2003, Ferreras & Yi 2004). Aside from the different methodologies used for the age determinations, it was clear that our poor knowledge of the UV spectrum of the presumably well understood MS stars (e.g., Peterson, Dorman & Rood 2001 was (and still is to some extent) a major drawback that has prevented the unambiguous determination of the main properties (age and chemical composition) of these distant systems.

More recently, a series of deep surveys have been conducted (Cimatti et al. 2002, Abraham et al. 2004, McCarthy et al. 2004) and now include well over 300 systems with similar spectrophotometric properties as those of the prototypical LBDS 53W091. Cimatti et al. (2008) presented what perhaps is the best spectrum representative of distant red objects. Within the Galaxy Mass Assembly ultra-deep Sky Survey (GMASS) program, they selected 13 passive galaxies (with 1.3 < z < 2.0) on the basis of their red UV color, defined as the magnitude difference between two bands (each of 400 Å width) centered at 2900 and 3300 Å, and constructed a stacked spectrum that totalled nearly 500 hours of observing time at the Very Large Telescope. By comparing that spectrum with single stellar populations (SSPs) from several population synthesis codes (Bruzual & Charlot 2003, Maraston 2005), they determined, from the rest-frame UV alone, ages that ranged from 0.7 to 2.8 Gyr and metallicities in the range 0.2 to 1.5 Zodot. By adding to the comparison near and mid IR photometric data, they significantly constrained the ages to 1-1.6 Gyr and found that Z = Zodot provided the best results.

In Fig. 3, we show the GMASS stacked spectrum of the 13 red galaxies (black) together with three different SSPs of various ages and chemical compositions. As a qualitative demonstration of the AMD in the UV, we note that the observed spectrum is very similar to the middle two SSP fluxes constructed with quite different parameters.

Figure 3

Figure 3. GMASS composite spectrum (in black) compared to three theoretical SSP energy distributions calculated with UVBLUE database and the synthesis code of Bressan, Chiosi & Fagotto (1994) with the updates described in Chavez et al. (2009). Qualitatively, high z galaxies can be well represented by either a very young (0.6 Gyr) and solar metallicity or a rather old (8 Gyr) subsolar ([M/H] = -1.7) populations.

It is beyond the scope of this paper to discuss any detail on the procedures so far delevoped to establish the age and chemical composition of distant systems. We, nevertheless, believe that in general the spectrophotometric analysis of distant objects has been carried out with stellar libraries that might be inadequate, in particular concerning the spectral resolution and capabilities of representing real stars.

2 In the context of this paper, we ascribe red galaxies to the extremely red objects (EROs), that are intrinsically red, and not to the dust-enshrouded star-forming systems. Back.

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