Early-type galaxy formation and evolution scenarios

1. Spectroscopic metallicity indicators in rest frame ultraviolet of z~2 galaxies C. Halliday, A. Cimatti, J. Kurk, M. Bolzonella, E. Daddi, M. Mignoli, P. Cassata, M. Dickinson, A. Franceschini, B. Lanzoni, C. Mancini, L. Pozzetti, A. Renzini, G. Rodighiero, P. Rosati & G. Zamorani the GMASS collaboration Early-type galaxy formation and evolution scenarios Massive early-type galaxies (ETGs) follow tight relations of their chemical, kinematical and photometric properties consistent with the homogenous formation of their stellar populations in a single burst of star formation at redshift z = 3-5.(e.g. Bender et al. 1993; Renzini 2006). Galaxies are however also believed to form in a Universe in which mass is assembled hierarchically. These two scenarios can be reconciled if the timescale of the assembly of a galaxy mass is independent of the formation of its stars. Observations find that hierarchical models do not predict the numbers of ETGs detected at redshift z>1 (K20 survey, Cimatti et al. 2002). Galaxy star formation histories are mass dependent (e.g. for nearby Universe, Kauffmann et al. 2003; ETGs: Faber et al. 1995, Kuntschner & Davies 1998, Thomas et al. 2005; Halliday 1999): less massive galaxies form their stars until more recent times (e.g. De Lucia et al. 2004), a scenario popularly known as “downsizing” (Cowie et al. 1996). The progenitor galaxies of the most massive ETGs should lie at increasingly high redshift (z>1.5). Different methods have been developed to isolate ETG progenitor candidates (e.g. BzK method, Daddi et al. 2004) which detect a mixture of very red passive and obscured starburst galaxies. GMASS (Galaxy Mass Assembly ultradeep Spectroscopic Survey) GMASS is based on an ESO Large Program (PI: A. Cimatti) of 145 hours using the FORS2 instrument at the VLT (Kurk et al.; Cimatti et al., Daddi et al. in preparation). The GMASS survey targets a 51 square arcminute area within the GOODS South field overlapping the K20 survey area and Hubble Ultra Deep Field (see Figure 1 below). GMASS aims to study the formation of massive ETGs by targetting galaxies at redshift z>1.4. Target selection was completed using Spitzer IRAC 4.5µm data such that mAB (4.5µm) < 23, and to have photometric redshift z PHOT > 1.4. Use of IRAC data ensures a target selection more sensitive to galaxy mass than K-band data, and less sensitive to dust extinction. No additional colour selection was applied. Spectroscopic redshifts have been determined for ~90% of targets in our blue grism, and ~70% in our red grism observations. Stellar masses have been measured using UBVRIzJHKs + 4 IRAC bands (Bolzonella et al. in preparation). Figure 1: diagram showing position of GMASS field in relation to K20, Hubble Ultra Deep Field and GOODS South field. Spitzer IRAC data for the GOODS South field was used in GMASS target selection. Figure 2: coadded GMASS spectra for galaxies in two bins divided by a stellar mass value of 1010 M○.The 1978 Ǻ index of R04 has been measured for both spectra. The continuua fitted in the measurement of equivalent width are shown Mass-metallicity relation: probe of galaxy chemical enrichment The mass-metallicity relation indicates a strong connection between the chemical enrichment history of a galaxy & its potential well. Studies by Erb, Shapley, Steidel & collaborators and Savaglio et al. have provided the first studies of the mass-metallicity relation to redshift z~2. Rix et al. (2004) (hereafter R04) modelled the rest frame UV spectra of star-forming regions for metallicities between 0.05 and 2 times solar. The galaxy evolutionary population synthesis code Starburst99 program of Leitherer et al. (2001) was used in conjunction with the WM-basic code (Pauldrach et al. 2001). A spectroscopic index centred on 1978Ǻ was identified by R04 to be a promising indicator of galaxy metallicity. We present GMASS spectra coadded by average combining spectra in two bins of galaxy divided at a stellar mass of 1010 M○. Galaxies were taken to have redshift (z ≥ 1.9) such that their rest frame UV spectra was observable. The spectroscopic index 1978 Ǻ as defined by R04 was measured for both coadded spectra. Spectra were normalised and an equivalent width was measured for the wavelength range 1935-2020 Ǻ. Equivalent width measurements of 3.10 Ǻ and 5.76 Ǻ were measured for the low and high stellar mass coadded spectrum respectively. Using the empirical relation equation 8 of R04. These values correspond to metallicities of 0.121 solar and 0.914 solar. Lower mass galaxies may have indeed experienced less chemical enrichment than more massive galaxies out to redshift 2.