Towards a resolved Kennicutt-Schmidt law at high redshift

¹ LERMA, Observatoire de Paris, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
e-mail: jonathan.freundlich@obspm.fr
² Max–Planck–Institute für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany
³ Dept. of Physics & Astronomy, Frederick Reines Hall, University of California, Irvine, CA 92697, USA
⁴ Dept. of Physics, Le Conte Hall, University of California, CA 94720 Berkeley, USA
⁵ Dept. of Astronomy, Campbell Hall, University of California, CA 94720 Berkeley, USA
⁶ IRAM, 300 rue de la Piscine, 38406 St. Martin d’Hères, Grenoble, France
⁷ Dept. of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
⁸ CEA, IRFU, SAp, 91191 Gif-sur-Yvette, France
⁹ Universitätsternwarte der Ludwig–Maximiliansuniversität, Scheinerstrasse 1, 81679 München, Germany
¹⁰ Observatorio Astronómico Nacional – OAN, Apartado 1143, 28800 Alcalá de Henares, Madrid, Spain
¹¹ Max Planck Institut für Astrophysik, Karl Schwarzshildstrasse 1, 85748 Garching, Germany
¹² School of Physics and Astronomy, Tel Aviv University, 69978 Tel Aviv, Israel
¹³ Steward Observatory, 933 N. Cherry Ave., University of Arizona, Tucson, AZ 85721-0065, USA

Received: 21 December 2012
Accepted: 24 March 2013

Abstract

Massive galaxies in the distant Universe form stars at much higher rates than today. Although direct resolution of the star forming regions of these galaxies is still a challenge, recent molecular gas observations at the IRAM Plateau de Bure interferometer enable us to study the star formation efficiency on subgalactic scales around redshift z = 1.2. We present a method for obtaining the gas and star formation rate (SFR) surface densities of ensembles of clumps composing galaxies at this redshift, even though the corresponding scales are not resolved. This method is based on identifying these structures in position-velocity diagrams corresponding to slices within the galaxies. We use unique IRAM observations of the CO(3–2) rotational line and DEEP2 spectra of four massive star forming distant galaxies – EGS13003805, EGS13004291, EGS12007881, and EGS13019128 in the AEGIS terminology – to determine the gas and SFR surface densities of the identifiable ensembles of clumps that constitute them. The integrated CO line luminosity is assumed to be directly proportional to the total gas mass, and the SFR is deduced from the [OII] line. We identify the ensembles of clumps with the angular resolution available in both CO and [OII] spectroscopy; i.e., 1–1.5′′. SFR and gas surface densities are averaged in areas of this size, which is also the thickness of the DEEP2 slits and of the extracted IRAM slices, and we derive a spatially resolved Kennicutt-Schmidt (KS) relation on a scale of ~8 kpc. The data generally indicates an average depletion time of 1.9 Gyr, but with significant variations from point to point within the galaxies.