Formation of A Nuclear Star Cluster Through A Merger Event In The Low Surface Brightness Galaxy AGC 223218

TL;DR

This study investigates the NSC in the low surface brightness galaxy AGC 223218 using multiwavelength imaging, spectroscopy, and X-ray data to test NSC formation pathways and search for IMBH evidence. Structural modeling with a double-Sérsic disk, combined with SED and MLCR mass estimates, reveals a relatively large NSC mass fraction and a young, metal-poor NSC compared to its host. Stellar population analysis via pPXF shows the NSC is dominated by a young component, while emission-line diagnostics indicate shocks and a possible AGN-like ionization environment; X-ray measurements greatly exceed the expectation from X-ray binaries, implying an IMBH with a small accretion rate. The results favor a merger-driven NSC formation in this LSBG, highlight a potential IMBH in the NSC, and offer a nearby laboratory for studying BH seeding and central star cluster assembly in low-density galaxies.

Abstract

We present the properties of the nuclear star cluster (NSC) in the low surface brightness galaxy AGC 223218. The disk of the galaxy can be modeled using two S$\acute{\rm e}$rsic components with distinct central positions: one representing the inner bright disk and the other corresponding to the extended outer disk. We estimate the stellar masses of the NSC and the host galaxy using two methods: spectral energy distribution (SED) fitting and mass-to-light versus color relations (MLCRs). The stellar mass ratio of the NSC to AGC 223218 is 0.094 based on the SED method and 0.072 using MLCRs. The NSC presents a younger stellar population and a lower [Fe/H] value than the host, as determined from SDSS and LAMOST spectra analysis using pPXF fitting. AGC 223218 is located at the boundary between the Seyfert and star-forming regions in the [SII]-BPT diagram, whereas in the [NII]-BPT diagram, it falls in the track of star-forming SDSS galaxies. This suggests the presence of strong shocks in AGC 223218. We propose that the NSC in AGC 223218 may have formed as a result of a merger event. Furthermore, the observed X-ray luminosity of AGC 223218 with eROSITA is two orders of magnitude higher than the expected X-ray luminosity from X-ray binaries, suggesting the presence of an intermediate-mass black hole (IMBH) in the NSC. To account for the observed X-ray luminosity, we estimate the IMBH accretion rate to be approximately 0.001.

Figures (8)

• Figure 1: DECaLS g band surface brightness of AGC 223218. The galaxy exhibits the asymmetric structure. Three main clumps (including the NSC, the brightest one) in the brighter disk, and other clumps are further out.
• Figure 2: Multi-band images of AGC 223218. Panel (a): g band image from DECaLS DR9. Panel (b): NUV band image. Panel (c): FUV band image. Panel (d): eROSITA 0.2-2.3 keV image. The white-filled circle in the left corner of each panels represents the resolution. The field of view of g band, NUV, and FUV is 90$"$ x 90 $"$. For X-ray data, the color code image is the smoothed image with a Gaussian kernel profile with $\sigma$ = 24$"$, and the X-ray counts are shown with black (one count) pixels. The white square in the X-ray image is the same region (90$"$ x 90 $"$) of the g band image. Two white stars represent the fitting centers of the double-S$\acute{\rm e}$rsic components of the g band image, which are the same as Figure \ref{['galfit']}.
• Figure 3: Panel (a): the final masked g band image from DECaLS. Panel (b): the double-S$\acute{\rm e}$rsic model image. Panel (c): the residual image. Two red stars represent the fitting centers of the double-S$\acute{\rm e}$rsic components.
• Figure 4: The HI spectrum from ALFALFA. The blue solid and red dotted lines represent the heliocentric velocity center of the HI gas and emissions of ionized gas. The red dashed line represents the NSC heliocentric velocity from NED. The black lines are velocity uncertainties of the velocities.
• Figure 5: Diagram of the stellar mass of host galaxy (M$_{\rm *gal}$) vs. the mass ratio of NSC to the host M$_{\rm *NSC}$/M$_{\rm *gal}$. The green and gray squares are nucleated galaxies and nucleated star-forming dwarf galaxies from 2021AA...650A.137F and 2022AA...667A.101F. The red and purple stars represent AGC 223218, with stellar masses derived using the SED fitting and MLCR methods, respectively. The blue star denotes NGC 4395, another LSBG.