of the Planetary Camera detector (PC1), which has a plate scale of 0046 pixel$^{-1}$. We remove cosmic rays using LA Cosmic[^3] (van Dokkum 2001), which can detect cosmic ray hits of arbitrary shape and size. Two of the pixels in the center of the galaxy are saturated; we masked them out in the analysis below.

Precise sky subtraction is of great importance to obtain accurate photometric measurements and structural decomposition. From the surface brightness profile of the SDSS $r$-band image, \[f606w-sbp\] 0.3cm we know that UM 625 extends close to a radius of $20\arcsec$, which means that it fills nearly the entire field of PC1 (radius $\sim 18\arcsec$). We estimate the background level and its uncertainty from the outermost edges of the PC1 chip and from the flanking WF2 chips; the limiting surface brightness of the F606W image is 25.31 $\rm mag\,arcsec^{-2}$. To examine the effect of the star-forming ring on the underlying galaxy structure, we also plot the surface brightness profile with the ring region (the region enclosed by the green polygon as indicated in Figure 2) masked out. Figure 4 shows that the circumnuclear star-forming region mainly affects the profile on scales of 02$\lesssim r \lesssim$08; the outer 