A GNIRS Spectroscopic Survey of Massive Galaxies at z~2.3


Introduction

The past few years have seen a tremendous increase in our knowledge of the galaxy population at 2 < z < 3. Recently it was found that red galaxies make up more than 2/3 of the most massive galaxies in this redshift range (van Dokkum et al. 2006). As most of these galaxies are beyond the limits of optical spectroscopy they are underrepresented in available samples of high-redshift galaxies. To obtain a full spectroscopic census of the universe at 2 < z < 3 we have conducted a near-infrared spectroscopic survey of 36 massive galaxies with GNIRS on Gemini-South. This program was chosen as a GNIRS Key Science Program by the NOAO Director. On this page we give an overview of the survey and provide access to the fully reduced spectra and corresponding data products. A full description of the survey and data products can be found in Kriek et al. 2008 .

Sample Selection

Nearly all galaxies observed as part of our survey are selected from the MUlti-wavelength Survey by Yale-Chile (MUSYC). The galaxies were selected to be bright in K (K < 19.7), to have photometric redshifts roughly between 2 and 3, and to be representative for a mass-limited sample at 2 < z < 3 (see Kriek et al. 2008).

Observations

The galaxies were observed during six observing runs in September 2004, May 2005, January 2006, February 2006, December 2006, and March 2007.

All observation were taken with the Gemini Near-InfraRed Spectrograph (GNIRS) in cross-dispersed mode, in combination with the 32 lines/mm grating and the 0.675'' slit. This setting provides a wavelength coverage of 1-2.5 micron, corresponding to the full rest-frame optical wavelength regime for z~2.5 galaxies. We applied a blind offset from a nearby star (< 60''), to center on the science target. We observed the galaxies following an ABA'B' (+1.5,-1.2,+1.2,-1.5 arcsec) dither pattern, such that we can use the average of the previous and following exposure as sky frame. This has two advantages above the classic ABBA dither pattern. First, it reduces the noise in the sky image. Second, time variations of the sky emission are partly canceled out. We used individual integration of 10 minutes and integrated for ~2 hours on average per galaxy.

For telluric standards we choose nearby AV0 stars. For continuum spectroscopy it is crucial to match the airmass of the telluric and science target. We observed tellurics about every 2 hours and linearly combined the spectra of the stars -- observed both before and after a science sequence -- to perfectly match the airmass of the galaxy.

Reduction

The reduction is performed using custom IDL scripts. In summary, we subtract the sky, mask cosmic rays and bad pixels, straighten the spectra, combine the individual exposures, stitch the orders, and finally correct for the response spectrum. A detailed description of how to reduce GNIRS cross-dispersed spectra of faint galaxies can be found here.

Data

Please send me an email to obtain access to the data (home)

Publications