The Second Nucleus of NGC 7727: Direct Evidence for the Formation and Evolution of an Ultracompact Dwarf Galaxy

We present new observations of the late-stage merger galaxy NGC 7727, including HST/WFPC2 images and long-slit spectra obtained with the Clay telescope. NGC 7727 is relatively luminous ($M_V = -21.7$) and features two unequal tidal tails, various bluish arcs and star clusters, and two bright nuclei 480 pc apart in projection. These two nuclei have nearly identical redshifts, yet are strikingly different. The primary nucleus, hereafter Nucleus_1, fits smoothly into the central luminosity profile of the galaxy and appears--at various wavelengths--"red and dead." In contrast, Nucleus_2 is very compact, has a tidal radius of 103 pc, and exhibits three signs of recent activity: a post-starburst spectrum, an [O III]_5007 emission line, and a central X-ray point source. Its emission-line ratios place it among Seyfert nuclei. A comparison of Nucleus_2 ($M_V = -15.5$) with ultracompact dwarf galaxies (UCDs) suggests that it may be the best case yet for a massive UCD having formed through tidal stripping of a gas-rich disk galaxy. Evidence for this comes from its extended star-formation history, long blue tidal stream, and elevated dynamical-to-stellar-mass ratio. While the majority of its stars formed $\gtrsim 10$ Gyr ago, about 1/3 by mass formed during starbursts in the past 2 Gyr. Its weak AGN activity is likely driven by a black hole of mass $3\times 10^6 - 3\times 10^8 M_\odot$. We estimate that the former companion's initial mass was less than half that of then-NGC 7727, implying a minor, but significant, merger. By now this former companion has been largely shredded, leaving behind Nucleus_2 as a freshly minted UCD that probably moves on a highly eccentric orbit.
25 pages, 11 figures, 6 tables (pdflatex, emulateapj). Accepted for publication in The Astrophysical Journal. Figure 2 has been degraded to reduce file size