Mid-IR Spectroscopy of gas-rich disk galaxies at z=1.5

Details Mid-IR Spectroscopy of gas-rich disk galaxies at z=1.5 Mid-IR Spectroscopy of gas-rich disk galaxies at z=1.5

Mar 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008sptz.prop50512d&link_type=abstract

Spitzer Proposal ID #50512

Physics

Scientific paper

We have recently obtained the first detection of CO in two ordinary massive z=2 galaxies. These massive disk galaxies were observed with the IRAM Plateau de Bure Interferometer, selected to lie in the mass-star formation rate correlation at z~1.5, thus being representative massive high-z galaxies. The CO detection implies that giant molecular gas reservoirs are present in these sources, with gas fractions reaching 50-70% of the total galaxy masses. With an infrared luminosity of L(FIR)~10^12 L(sun), these BzK disk-like galaxies are borderline ULIRGs but with star formation efficiency similar to local spirals, and an order of magnitude lower than in submm galaxies, unveiling a new formation mode previously unknown in the distant universe. We propose here IRS spectroscopy of these two CO detected galaxies at z=1.5 in order to unveil their mid-IR rest frame spectral properties and measuring the equivalent widths of PAH features at 6.2, 7.7, 8.6 and 11.3 micron and of the 9.7 micron silicate absorption. The extreme richness in molecular gas in these sources should affect their mid-IR spectral properties, and we will investigate how the diversity in gas/star ratios in distant galaxies is important for the interpretation of mid-IR luminosities of the high-z populations. In particular, the fact that these ULIRGs behave like scaled up spiral galaxies in their star formation properties could be reflected in their mid-IR spectral properties. Obtaining the PAH ratios will be crucial to constrain better the CO to H2 mass conversion ratio Xco. These observations will represent a legacy from Spitzer for the years to come for the interpretations of mid-IR properties of distant star forming galaxies.

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