UGC 2369S: a Kpc Scale Triple Merger Candidate Identified in a Nearby Luminous Infrared Galaxy

Abstract

We present high spatial resolution ($\lesssim$1.0''), multi-wavelength observations of UGC 2369S, a nearby luminous infrared galaxy showing three distinct cores separated on kpc scales in near-infrared (NIR) imaging with significant X-ray emission. Utilizing optical/NIR adaptive optics (AO), radio, \chandra X-ray, as well as archival HST imaging, we perform a comprehensive study of AGN activity, obscuration, and host properties. As one of the clearest cases of a triple-nucleus merger at $\simeq$3 kpc separations, UGC 2369S is the first to be studied with high-resolution observations at multiple wavelength. We find that the northern core, having possibly the most massive black hole in the system ($\rm M_{BH}\simeq10^{8}\,M_{\odot}$) is consistent with a heavily obscured AGN. However, its high dust extinction ($\rm A_v>5$), hydrogen column density ($N_\mathrm{H}\gtrsim 10^{25}\,\rm cm^{-2}$) and non-detection of optical coronal lines and coronal X-ray emission leave the identification inconclusive. The other two cores show no evidence for black-hole activity and instead exhibit signatures of tidal disruption. From stellar mass surface density and stellar velocity dispersion maps, we infer that the strongly varying gravitational potential in this three-body system may have cannibalized the stellar bulge of the southwestern core, leaving a metal enriched remnant. An ongoing survey focusing on similar triple systems could help us understand how they evolve and help benchmark numerical simulations, providing insight into gravitational wave predictions and the formation of the most massive black holes.

Figures (21)

• Figure 1: HST/ACS F814W imaging of UGC2369. The target highlighted in this work is the southern luminous IR galaxy UGC 2369S. The FOVs of our KCWI and MUSE IFU observations are highlighted with boxes.
• Figure 2: Multi-wavelength observations of UGC 2369S. Top: HST ACS and NICMOS images. The contours overplotted on the ACS image indicate the Chandra $0.5-7\,$keV X-ray emission. The Chandra counts have been convolved with a Gaussian of $\sigma=0.25^{"}$; Bottom: MUSE and VLA 33 GHz observations. Three cores are detected above 5-sigma. Circular apertures with 1.5 arcsec diameter are indicated in MUSE pseudo-narrow band images, from which we extract spectra and estimate the stellar kinematics.
• Figure 3: Chandra broad band (ObsID: 28148, $0.5-7\,$keV) image of UGC 2369S. The cores are fitted with three PSFs generated by ChaRT. We fix the relative displacement between these three cores to the value found in the MUSE observation. Only the northern core has an unresolved point-like structure. We note that even for object 3, a significant amount of flux is generated in a diffuse star-forming region.
• Figure 4: Spectra extracted from KCWI data with circular apertures as presented in Figure \ref{['fig:optical image']}. The spectra are fitted by pPXF to extract stellar kinematics. The grey shaded areas are excluded from the fit. The velocity shift (V) is measured relative to the best redshift determined from prior run with stellar absorption, so it is expected to have very small value. The rest frame positions of the Ca II absorption are marked with orange dotted lines. The stellar absorptions are all redshifted relative to the nebular lines and [Fe II] emission, implying that the bulk of the line-emitting gas is outflowing.
• Figure 5: Light-weighted age distribution (with $7800-8800$Å flux level contour) derived from the pPXF fit suggests that the central starburst region is surrounded by a relatively old stellar population with age $\gtrsim$ a few Gyrs. The fitting is based on KCWI blue channel data covering restframe $3650-5400$Å. Object 3 and 1 have similarly old stellar populations, while object 2 assembled most of its mass in a recent starburst.