Venice06 poster

1
Spectroscopic metallicity indicators in rest
frame ultraviolet of z2 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 zgt1 (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 (zgt1.5). 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 zgt1.4. Target
selection was completed using Spitzer IRAC 4.5µm
data such that mAB (4.5µm) lt 23, and to have
photometric redshift z PHOT gt 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 ? 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 z2.
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 1978? 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 ? 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 ?.
Equivalent width measurements of 3.10 ? and 5.76
? 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.