Galaxy archaeology over the last 7 billion years

The stellar populations in galaxies can gain us insight into galaxy evolution. Their age and composition in elements heavier than hydrogen and helium (metallicity) are the result of the history of star formation in the galaxy. It is thus possible to infer the past history of galaxies through an 'archaeological' approach, i.e. by studying the fossil record imprinted in their current stellar populations. This 'archaeological' approach can be extended to earlier and earlier times in the Universe's history by looking at galaxies at larger and larger distances from us. This is the study conducted by a team of astronomers led by Anna Gallazzi (AstroFIt fellow at the Observatory of Arcetri) in collaboration with Eric Bell (University of Michigan), Stefano Zibetti (Observatory of Arcetri), Jarle Brinchmann (Leiden Observatory) and Daniel Kelson (Carnegie Observatory).

The properties of the stars in galaxies are encoded in the galaxy spectrum. In particular, several absorption features are present in the optical wavelength range that show distinct sensitivity to the age and the metallicity of the stellar populations (see Fig.1). By comparing these features with theoretical models (so-called "stellar population synthesis models") quantitative estimates of the physical parameters of the galaxy stellar populations can be derived.

Fig.1: Images and spectra of two galaxies at redshift 0.7, a quiescent galaxy (upper panel) and a star-forming  galaxy (lower panel). The black curve shows the data, the red curve is the bestfit stellar population model, while  the cyan curve shows any residual emission line from the star-forming gas (indicated by the dotted green lines). The most important absorption features to study the galaxy stellar populations are indicated in orange. The spectra have been obtained with the spectrograph IMACS on the Magellan telescope of the Las Campanas Observatory. The images have been obtained with the Hubble Space Telescope as part of the GEMS survey.

Large datasets of good-quality spectra for galaxies in the local Universe, such as those provided by the SDSS survey, have allowed to establish the relationships between the age and metallicity of the galaxy stellar populations with the galaxy mass. These indicate that the stars in more massive galaxies must have formed at earlier epochs and on shorter timescales than those in smaller galaxies. At the present time, the large majority of massive galaxies are elliptical galaxies with no active star formation, which contain more than half of the total mass and heavy elements locked into stars of the present-day Universe.

This was not the case at earlier times (corresponding to high cosmological redshift), when a larger fraction of massive galaxies was star-forming. The increase of passive galaxies and the decrease of star-forming galaxies from early times to the present-day indicates that the overall star formation activity has declined and galaxies have been transitioning from the active to the passive phase at a fast rate since about 8 billion years ago. By comparing the properties of high-redshift galaxies with those of their local counterparts it is possible to infer the mechanisms responsible for their transformation.

This kind of studies have been long hampered by the intrinsic faintness of distant galaxies and by the redshift, which moves the interesting part of the spectrum to the red, where observations are difficult due to the natural glow of our atmosphere and the usual low sensitivity of detectors. Anna Gallazzi and her collaborators have exploited the advanced capabilities of the IMACS multi-object spectrograph on the 6.5m-diameter Magellan telescope of the Las Campanas Observatories to obtain high-quality spectra for a sample of 80 galaxies at redshift of 0.7 (corresponding to 7 billion years ago), for the first time on individual objects spanning the full range of star formation activity.

Fig.2: The mean stellar age (a) and metallicity (b) of galaxies at redshift 0.7. Circles indicate quiescent galaxies with no ongoing star formation, while stars indicate star-forming galaxies with the color representing the efficiency of star formation. These distributions are directly compared with their analogs in the local Universe as studied in the Sloan Digital Sky Survey (SDSS, solid and dashed red curves - see Gallazzi et al 2005).

Their analysis provides the first characterization at these redshifts of how stellar age and metallicity of massive galaxies scale with their mass (see Fig.2). These relations could be directly compared with their analogs at the present epoch. Galaxies that were not forming stars already 7 billion years ago have properties consistent only with the oldest among the present-day population of quiescent galaxies. The study found that part of the high-redshift star-forming galaxies are already as metal-rich as local galaxies. Quenching of star formation in these systems explains the origin of the youngest among the present-day quiescent galaxies.

The results of this study are presented in the paper recently published on the Astrophysical Journal "Charting the evolution of the ages and metallicities of massive galaxies since z=0.7", Gallazzi et al, 2014, ApJ, 788, 72

Edited by Anna Gallazzi and Stefano Zibetti, 1/9/2014