Measuring Star Formation Rate and Far-infrared Color in High-redshift Galaxies Using the CO(7-6) and [N II] 205 mum Lines Article - 2015

Nanyao Lu, Yinghe Zhao, C. Kevin Xu, Yu Gao, Tanio Díaz-Santos, Vassilis Charmandaris, Hanae Inami, Justin H. Howell, Lijie Liu, Lee Armus, Joseph M. Mazzarella, George C. Privon, Steven D. Lord, David B. Sanders, Bernhard Schulz, Paul P. van der Werf

Nanyao Lu, Yinghe Zhao, C. Kevin Xu, Yu Gao, Tanio Díaz-Santos, Vassilis Charmandaris, Hanae Inami, Justin H. Howell, Lijie Liu, Lee Armus, Joseph M. Mazzarella, George C. Privon, Steven D. Lord, David B. Sanders, Bernhard Schulz, Paul P. van der Werf, « Measuring Star Formation Rate and Far-infrared Color in High-redshift Galaxies Using the CO(7-6) and [N II] 205 mum Lines  », The Astrophysical journal letters, 2015. ISSN 2041-8205

Abstract

To better characterize the global star formation activity in a galaxy, one needs to know not only the star formation rate (SFR) but also the rest-frame, far-infrared color (e.g., the 60-100 mum color, C(60/100)) of the dust emission. The latter probes the average intensity of the dust heating radiation field and scales statistically with the effective SFR surface density in star-forming galaxies including (ultra-)luminous infrared galaxies ((U)LIRGs). To this end, here we exploit a new spectroscopic approach involving only two emission lines : CO(7-6) at 372 mum and [N ii] at 205 mum([N ii]<SUB>205mum</SUB>). For local (U)LIRGs, the ratios of the CO(7-6) luminosity (L<SUB>CO(7-6)</SUB>) to the total infrared luminosity (L<SUB>IR</SUB> ; 8-1000 mum) are fairly tightly distributed (to within &tilde ;0.12 dex) and show little dependence on C(60/100). This makes L<SUB>CO(7-6)</SUB> a good SFR tracer, which is less contaminated by active galactic nuclei than L<SUB>IR</SUB> and may also be much less sensitive to metallicity than L<SUB>CO(1-0)</SUB>. Furthermore, the logarithmic [N ii]<SUB>205mum</SUB>/CO(7-6) luminosity ratio depends fairly strongly (at a slope of &tilde ; -1.4) on C(60/100), with a modest scatter (&tilde ;0.23 dex). This makes it a useful estimator on C(60/100) with an implied uncertainty of &tilde ;0.15 (or &lsim ;4 K in the dust temperature (T<SUB>dust</SUB>) in the case of a graybody emission with T<SUB>dust</SUB> &gsim ; 30 K and a dust emissivity index beta >= 1). Our locally calibrated SFR and C(60/100) estimators are shown to be consistent with the published data of (U)LIRGs of z up to &tilde ;6.5.

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