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tokens: on the individual galaxies. We derived the source counts using a circular aperture with 13′′ radius, which, at a distance of 65 Mpc, corresponds to 4 kpc. To account for the background emission, we used annuli with 60′′–120′′ radii. We did not obtain statistically significant detections for any of the individual galaxies. To increase the signal-to-noise ratio, we co-added the X-ray emission associated with the individual galaxies. For the full sample of 51 LSBGs, this resulted in a total exposure time of 381.1 ks. We did not detect statistically significant emission from the stacked data, implying that the X-ray brightness of LSBGs remains below the detection threshold. We also stacked the subset of 17 UDG candidates and the 17 LSBGs as well as the subset of blue and red galaxies in our sample. We did not detect X-ray emission in any of the subsets. In the absence of detections, we set upper limits on the average luminosity of galaxies by calculating the confidence bounds of the stacked signal. We defined the 95% confidence region of the distribution, and used the upper confidence bound of the count rate as an upper limit. To convert the count rate to flux, we assumed that the emission from the hot gas can be described with an optically thin thermal plasma model. Specifically, we assumed a gas temperature of kT = 0.2 keV and a metallicity of 0.2 solar, which are typical in low-mass elliptical galaxies (e.g., Boroson et al. 2011 ; Goulding et al. 2016 ). Based on the resulting count rate upper limit of , we estimate the upper limit on the flux of for the full sample of 51 galaxies, which corresponds to a luminosity of , where d represents the average distance of the sample in Mpc. We obtained similar upper limits for the subset of UDG candidates and LSBGs and for the blue and red subsamples, which are listed in Table 1 . Table 1 X-Ray Upper Limits on the Stacked LSBGs 5. Discussion 5.1. Constraining the Halo Masses The prototypical UDG, Dragonfly 44, has a virial mass of (van Dokkum et al. 2016 , 2017 ). Using its effective radius of 4.6 kpc and assuming a Navarro–Frenk–White (NFW) dark-matter halo profile, we estimate that it has a total mass of . Based on the L X – M (5 r eff ) scaling relation, this total mass is linked with an X-ray luminosity of L 0.3–8 keV ≈ 2.4 × 10 39 erg s −1 in the 0.3–8 keV band. However, the luminosity upper limit measured for the sample of 51 field LSBGs, with an assumed distance of 65 Mpc, is in the 0.3–1.2 keV band. After converting the X-ray upper limit luminosity to the 0.3–8 keV band, we obtain . This value is ∼40 times lower than that expected for a galaxy with a massive dark-matter halo (see Figure 2 ). A similar conclusion is obtained for the subset of 17 UDG candidates and the subset of 17 LSBGs with 0.3–8 keV, as well as for the blue and red galaxy subsets. Figure 2. Empirical relation between the 0.3−8 keV band X-ray luminosity of hot diffuse gas and the total gravitating mass within 5 r eff (red