We also note that we do not have reason to think that we may be underestimating the stellar mass of Was 49b due to dust obscuration. We examined this possibility by measuring the K-band light ratio of Was 49b to Was 49a using an image from the UKIRT Infrared Deep Sky Survey (UKIDSS; Lawrence et al. 2007), taken on 2010 February 27. At 2.2 μm the AGN is manifest as a dominant, unresolved source in Was 49b (Figure 6, left). Because of the dominance of the AGN and the shallowness of the UKIDSS image, it was difficult to model any extended emission in Was 49b using galfit, as was done for the DCT images. We instead subtracted the AGN and measured the remaining emission by determining the expected K-band AGN luminosity given its intrinsic luminosity found in Section 3.1. Using five bright stars within a few arcminutes of Was 49, we made an empirical template of the PSF. We found that we could accurately model the stars in the image with a PSF with three Gaussian components. With our PSF template, we fit the unresolved source in Was 49b, allowing only the amplitude and position to vary (Figure 6, middle). We found a PSF K-band magnitude of 13.52, which corresponds to 





 erg s−1. Before subtracting this unresolved source from the K-band image, we performed a check to ensure that it is consistent with the expected AGN emission. Using the value of AV ∼ 12 for the direct AGN continuum calculated above and the extinction curve of Cardelli et al. (1989), we found AK ∼ 1.3, which implies an intrinsic AGN luminosity of LK ∼ 1044 erg s−1. If we assume that the intrinsic spectral energy distribution is approximately flat in the near-IR to the mid-IR for luminous AGNs, then we can use LK ∼ L6 μm ∼ 





 (e.g., Mateos et al. 2015) to calculate 





 erg s−1, consistent with the intrinsic value, given the scatter on EB–V/





 and L6 μm/





. As the observed unresolved source in Was 49b is consistent with expectations for the AGN, we subtracted it from the background-subtracted K-band image and photometered the remaining emission in Was 49b for comparison with Was 49a. We did not attempt to model any contribution to the K-band emission from the AGN in Was 49a, as the faint, soft X-ray source in Was 49a (Section 3.1) suggests that the 2–10 keV luminosity and therefore the expected K-band luminosity are negligible. We used the position and inclination angle for the disk of Was 49a (Table 2) to build an elliptical isophotal profile of Was 49a, which we interpolated across the position of Was 49b using a low-order polynomial and by taking the median isophotal value. Subtracting this profile and estimating the variance from a source-free region in the UKIDSS image, we found that the K-band light ratio is consistent with the Was 49 system being a 





 merger, where the confidence intervals are ∼90%. We emphasize that these confidence intervals assume that the AGN has been accurately subtracted from Was 49b. This may not be the case, as there is scatter in the relationship between the intrinsic X-ray luminosity and intrinsic IR luminosity of AGNs. If our model of the AGN overestimated the observed K-band luminosity of the AGN, then the K-band light ratio would be somewhat lower. It is less likely, however, that our model underestimated the observed K-band luminosity of the AGN, as increasing the modeled luminosity would quickly lead to negative flux values when subtracting it from the K-band data. With these considerations, we consider Was 49 to be a minor merger, with a mass ratio between ∼1:7 and ∼1:15.
Citation: Mateos et al. 2015
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