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tokens: is comparable to our results. There are many possible drivers for high excitation conditions. Daddi et al. ( 2015 ) found that the ratio of CO(5–4) to lower J CO emission is well correlated with the average intensity of the radiation field and with Σ SFR among local spirals, local ULIRGs, and z ∼ 1.5 MS galaxies. This is consistent with the model of Narayanan Krumholz ( 2014 ). They combined hydrodynamic simulations of disk galaxies and galaxy mergers with molecular line radiative transfer calculations, motivated to examine the origin of a broad range of observed CO SLEDs in local and high-redshift galaxies. They found that CO SLEDs are well correlated with the galaxy-wide SFR surface density, which is attributed to the increase of gas density and temperature. As shown in Figure 8 , we find that their model for galaxies with Σ SFR ∼ 5 M ⊙ yr −1 kpc −2 agrees with our results. Figure 8. CO SLEDs normalized to the CO(1–0) line for the GRB 080207 host galaxy and other types of galaxies. The values for the Milky Way, M82, SMGs, and QSOs/radio galaxies (RGs) are taken from the compiled data in Carilli Walter ( 2013 ), (U)LIRGs are from Papadopoulos et al. ( 2012 ), and z ∼ 1.5 MS galaxies are from Daddi et al. ( 2015 ) and Bolatto et al. ( 2015 ). We plot the (U)LIRGs, SMGs, and QSOs/RGs with CO detection up to J ≥ 4. The average values are shown in bold dashed lines. The gray solid line represents the constant brightness temperature ( J 2 ). The gray dotted–dashed line represents the model of Narayanan Krumholz ( 2014 ) with Σ SFR = 5 M ⊙ yr −1 kpc −2 . Table 4 CO Brightness Temperature Line Ratio Note. The error is 1 σ . It might also be possible that the high CO excitation condition is attributed to a strong radiation from an active galactic nucleus (AGN). While no strong evidence for an AGN has been reported, the optical emission-line diagnostic with the Baldwin–Phillips–Terlevich (BPT) diagram (Baldwin et al. 1981 ) studied in Krühler et al. ( 2015 ) shows that the host galaxy is located in a starburst–AGN composite region. The significantly high radio-derived SFR (850 M ⊙ yr −1 ) derived in Perley Perley ( 2013 ) compared to other methods could be explained by the contribution of an AGN to the observed radio flux. Since peaks in CO SLED for AGNs are J ≳ 8–10 (e.g., Meijerink et al. 2006 ), where high interstellar medium (ISM) excitation occurs in X-ray dissociation regions, observations of higher J CO lines are important in diagnosing the presence of an AGN. 4.6. Implication for GRBs as a Tracer of SFR History There is ongoing debate as to whether GRBs occur in normal star-forming galaxies or occur under particular circumstances, and whether they can be used as an unbiased tool to trace star formation in the universe. It is suggested that GRBs prefer low-metallicity environments with a threshold of 0.3–0.7 Z ⊙ (e.g., Woosley Heger 2006 ; Graham Fruchter 2013 ; Vergani et al. 2015 , 2017 ; Perley et al. 2016b ). The metallicity effect on the