Astrophys. J., 802, L11 (2015/March-3)
Measuring star formation rate and far-infrared color in high-redshift galaxies using the CO(7-6) and [N II] 205 µm lines.
LU N., ZHAO Y., XU C.K., GAO Y., DIAZ-SANTOS T., CHARMANDARIS V., INAMI H., HOWELL J., LIU L., ARMUS L., MAZZARELLA J.M., PRIVON G.C., LORD S.D., SANDERS D.B., SCHULZ B. and VAN DER WERF P.P.
Abstract (from CDS):
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 µm 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 µm and [N ii] at 205 µm([N ii]205µm). For local (U)LIRGs, the ratios of the CO(7-6) luminosity (LCO(7–6)) to the total infrared luminosity (LIR; 8-1000 µm) are fairly tightly distributed (to within ∼0.12 dex) and show little dependence on C(60/100). This makes LCO(7–6) a good SFR tracer, which is less contaminated by active galactic nuclei than LIRand may also be much less sensitive to metallicity than LCO(1–0). Furthermore, the logarithmic [N ii]205µm/CO(7-6) luminosity ratio depends fairly strongly (at a slope of ∼ -1.4) on C(60/100), with a modest scatter (∼0.23 dex). This makes it a useful estimator on C(60/100) with an implied uncertainty of ∼0.15 (or ≲4 K in the dust temperature (Tdust) in the case of a graybody emission with Tdust≳ 30 K and a dust emissivity index β ≥ 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 ∼6.5.
galaxies: active - galaxies: ISM - galaxies: star formation - infrared: galaxies - ISM: molecules - submillimeter: galaxies
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