LEGA-C at a Glance

For more information, see the LEGA-C Survey Paper

The Large Early Galaxy Astrophysics Census, LEGA-C, is an ESO Public Spectroscopic Survey (PSS) and obtains deep continuum spectroscopy of several thousand galaxies at redshifts 0.6 to 1. Contrary to redshift surveys that have covered larger galaxy samples at shallower depths, LEGA-C obtains much higher quality spectra for a smaller galaxy sample - spending 20 hours per pointing, compared to typically 1 hour for redshift surveys. These deep, high resolution spectra provide very detailed data of stellar continua for galaxies at half the Universe's age: stellar ages, masses and metallicities, as well as dynamical masses from stellar velocity dispersions. These are key pieces of information for the study of galaxy evolution, but have so far only been available for representative galaxy samples in the nearby universe, at lookback times up to approx. 1 Gyr. LEGA-C therefore enables for the first time several studies of key questions in galaxy evolution on long cosmological time scales (up to 8 Gyr), especially for early-type galaxies: dyamical evolution, mass assembly through merging and star formation, and the mechanisms that quench star formation.

Survey Strategy: LEGA-C utilizes the VIMOS multi-object spectrograph at the ESO Very Large Telescope, and covers 1.3 square degrees within the UltraVISTA - covered region in the COSMOS deep sky survey field, about 79% of ULTRAVista; see the section on the layout. Each of the 3900 target galaxies receives 20 hours of integration time. This will yield spectra with high quality continuum signal-to-noise (better than 10 per Angstroem) continuum spectra for approximately 2700 galaxies.

Targets are selected from the UltraVISTA K-band catalog (Muzzin et al. 2013, ApJS 206, 8), requiring a photometric or pre-existing spectoscopic redshift in the range 0.6 to 1, and a K-band total magnitude limit running from K(AB) = 21.1 at z = 0.6 to K(AB) = 20.4 at z = 1. The sliding limit is chosen to accommodate the increasingly red I - K colors with redshift, thus ensuring sufficient signal in the optical spectra. As shown in Figure 1 (left), our final sample (red dots) reaches stellar masses down to 1010 solar masses over the entire redshift range, and covers the entire color-color space of star-forming, dusty and quiescent galaxies (Figure 1, right).

U-V color - stellar mass diagram UVJ 2-color diagram
Figure 1: LEGA-C sample selection shown in the color-stellar mass and 2-color diagrams. LEGA-C targets are shown as red dots. See text for further explanations.

Figure 2 (left) shows that over the entire redshift range z= 0.6 to 1, LEGA-C covers the key spectral features required to accomplish its science goals. The variation in wavelength coverage at a given z is due to variations in slit positions on the mask. Similarly, the required continuum signal-to-noise (better than 10 per Angstrom) is reached over the entire LEGA-C redshift range, as was verified from the data taken over the first year of the survey (Figure 2, right).

wavelength range vs redshift signal to noise range vs redshift
Figure 2: Rest-frame wavelength coverage and quality of spectra as a function of redshift; year 1 data are shown as yellow dots in the right panel, confirming that LEGA-C achieves its predicted S/N.

Schedule: This project spans a total of 5 years, covering 128 nights. Observations began in December 2014; data will be released incrementally, starting mid-2016, as detailed in the sections on News and Data Releases.

A first look at the data: Figures 3,4 and 5 illustrate the high quality of the individual spectra and the multitude of features present at large S/N and spectral resolution, allowing to measure strengths and velocity widths of both emission and absorption lines and larger scale continuum features such as slopes and the Balmer break. These figures also demonstrate that LEGA-C provides these key features with high S/N consistently for the whole range of morphological and spectral types of galaxies at z = 0.6 to 1 .

Spectra of
      LEGA-C targets ordered by Sersic Index
Spectra of
      LEGA-C targets ordered by Sersic Index
Click images to enlarge
Figure 3: Spectra of 10 typical LEGA-C targets ordered by morphology (Sersic index). Flux density uncertainties are indicated in grey. Hubble Space Telescope image cutouts are shown on the right. Labels show object IDs, redshifts, stellar masses and star-formation rates. LEGA-C will produce ~3000 such spectra.

Spectra of
      LEGA-C targets ordered by Sersic Index
Click image to enlarge
Figure 4: Spectra of 5 disk galaxies in LEGA-C, ordered by inclination. See the caption of Figure 4 for explanations.

Compilation of 1-d spectra
Click to enlarge

Figure 5: Compilation of the extracted 1-D spectra for the galaxies observed during year 1 (654 objects). Each row represents one spectrum; galaxies are sorted from high (top) to low (bottom) specific star formation rate. This visualization of the data reveals 10 emission lines and as many as 50 unique absorption line features.

website contact: K. Noeske
Last modified: Thu Apr 28 14:20:22 CEST 2016