---
title: 'UGC 6728: Low-Mass Seyfert Galaxy'
url: https://www.emergentmind.com/topics/dwarf-seyfert-galaxy-ugc-6728
type: topic
---

# UGC 6728: Low-Mass Seyfert Galaxy

UGC 6728 is a nearby ($z=0.0065$) low-luminosity Seyfert 1 galaxy in the “dwarf AGN” regime, hosting a supermassive black hole (SMBH) of mass $M_{\rm BH}\approx7\times10^5\;M_\odot$—the lowest-mass AGN nucleus yet directly characterized via both reverberation and spin measurements. The galaxy exhibits a barred lenticular (SB0) morphology with prominent ansae at the ends of the bar. UGC 6728 serves as a critical anchor for black hole–host scaling relations and accretion physics at the faint end of the SMBH mass function.

## 1. Host Galaxy Morphology and Stellar Population

High-resolution HST imaging in F547M (UVIS), F814W (UVIS), and F160W (IR), analyzed via two-dimensional GALFIT decomposition, reveals UGC 6728 as a classic SB0 lenticular with the following structural components: 
- **Bulge**: Sérsic index $n=1.0\pm0.1$, effective radius $R_e=2.11''$, $f_{\rm bulge}/f_{\rm tot}\sim0.11$.
- **Bar**: nearly straight, $n=0.5\pm0.1$, $R_e=6.82''$, axis ratio $b/a\sim0.45$, contributes $\sim23\%$ of the light.
- **Ansae**: modeled with truncated exponential Sérsic profiles, accounting for $\sim9\%$ of the flux.
- **Disk**: exponential ($n=1$), $R_e=15''$, carrying $\sim57\%$ of the luminosity.

Composite color imaging and color gradients (e.g., $V-I=1.20$, $V-H=2.82$) indicate a red, evolved stellar population devoid of current star formation or dust features. The total $V$-band magnitude is $13.81\pm0.03$, yielding $M_V=-18.77\pm0.31$ at a distance $D=32.5\pm3.5$ Mpc determined via the globular cluster luminosity function ($m_{\rm TO}(V)=24.9\pm0.1$ mag, $\sigma=0.8\pm0.1$ mag, blue peak fraction $73\%$) [2007.05590].

Stellar mass is estimated as $\log(M_\star/M_\odot)=9.9\pm0.2$ based on empirical $M/L$–color relations [2007.05590].

## 2. AGN Physical Parameters: Mass, Spin, and Scaling Relations

A dedicated two-month reverberation mapping campaign combining nightly APO/DIS spectroscopy (H$\beta$ broad-line region) and simultaneous photometry yielded a transverse BLR radius $R_{\rm BLR}=3.6\times10^{13}$ cm, from a time delay $\tau=1.4\pm0.8$ days. Population-averaged virial factor $f=4.3\pm1.1$ and line width $\sigma_{\rm line}=783.7\pm92.3$ km/s result in a direct black hole mass $M_{\rm BH}=(7.1\pm4.0)\times10^5\;M_\odot$ [1608.03893, 2311.15104].

Penalized pixel-fitting of high-resolution (Ca II triplet) spectra yields a bulge stellar velocity dispersion $\sigma_\star=51.6\pm4.9$ km/s. UGC 6728 resides on both the $R_{\rm BLR}$–$L_{5100}$ ($\log\lambda L_{5100}=41.97\pm0.11$ erg/s) and $M_{\rm BH}$–$\sigma_\star$ relations, and thus anchors the low-mass end of AGN scaling laws [1608.03893].

Spin constraints from relativistic reflection (Suzaku/NuSTAR fits to the relxillCp model) give $a=0.97^{+0.20}_{-0.27}$, inclination $i=49.5^\circ\pm14.5^\circ$, and support a high-spin, low-mass SMBH [2311.15104].

## 3. Nuclear Accretion Flow and X-ray Spectral Properties

Long-term broad-band X-ray monitoring (XMM-Newton, Suzaku, Swift/XRT, NuSTAR) from 2006–2021 establishes UGC 6728 as a low-mass “bare” AGN nucleus, with negligible intrinsic $N_H$ (few$\times 10^{19}$–$10^{20}$ cm$^{-2}$) and no significant X-ray absorption.

Spectral evolution tracks a transition from a “hard” ($\Gamma\sim1.4$) to “soft” ($\Gamma\sim1.85$) power-law state over 15 years, interpreted through a two-component advective flow (TCAF) model:
- Disc accretion rate $\dot{m}_d$ rises from $\sim0.05$ to $\sim0.08\,\dot{m}_{\rm Edd}$.
- Sub-Keplerian halo rate increases in parallel.
- Hot Compton cloud (“CENBOL”) radius contracts from $\sim31.6\,r_g$ to $\sim11.0\,r_g$.
- Soft X-ray excess (0.3–2 keV) is strong initially ($\sim$factor 2.5 above hard) and weakens in later epochs.

The observed spectral steepening is explained by cooling/contraction of the corona under increased accretion, with high-energy photons produced via inverse Compton scattering in a compact, optically thin region (Thomson depth $\tau<1$) [2311.15104].

No prominent Compton hump is detected ($R_{\rm refl}\sim0.74$ during Suzaku epoch, $\sim0.1$ in NuSTAR), and relativistic reflection signatures (broad Fe K$\alpha$) are weak or absent.

## 4. Multiwavelength Variability: X-ray and Radio Coupling

Simultaneous five-year X-ray (Swift/XRT) and radio (VLA/X-band, K-band) monitoring reveals a coordinated event [2601.15383]:
- X-ray flare phase (MJD 59317–59512): count rate increases from $0.24\to0.82$ cts/s, peak flux $F_X=1.44\times10^{-11}$ erg/cm$^2$/s ($L_X=2.4\times10^{41}$ erg/s).
- Radio flux remains steady pre-flare ($S_\nu\sim0.12$ mJy), then drops by $\geq$factor 2 (to $<0.065$ mJy) during the late flare/post-flare period before returning to baseline.

X-ray hardness ratio ($HR\sim0.7$–$1.0$) and spectral index ($\Gamma=2.00\pm0.10$ at peak) do not vary measurably during the flare or radio dip. Multi-band cross-correlation returns zero lag, supporting a compact, co-spatial emission region.

## 5. Interpretation: Magnetic Reconnection and Plasmoid Occultation

A disk magnetic reconnection event is favored to explain the X-ray/radio coupling. The energetics are consistent: magnetic power from reconnection $\dot{W}_B\sim1.9\times10^{42}$ erg/s matches the observed flare power ($E_X/(20\,\rm d)\sim8.4\times10^{41}$ erg/s). The contemporaneous radio dimming is quantitatively modeled as free–free absorption by a magnetically ejected blob (“plasmoid”), requiring optical depth $\tau_\nu\sim3$ and electron columns consistent with BLR values ($N_e\lesssim10^{21}$–$10^{23}$ cm$^{-2}$), without causing X-ray obscuration.

Occultation timescales and inferred velocities ($v\sim300$ km/s) yield blob displacement of $144$–$9410\,R_S$ over 10–674 days, consistent with transient eclipse of the compact radio source (possibly a nascent jet) but not the X-ray corona [2601.15383]. Standard AGN coronal flaring is disfavored due to lack of spectral variation and the coordinated radio/X-ray behavior.

## 6. Placement in the Broader AGN Framework

UGC 6728 exemplifies the low-mass extension of AGN accretion and feedback physics:
- It demonstrates that disc–corona coupling, spectral state transitions, and magnetic reconnection phenomena persist to the regime of $M_{\rm BH}\sim10^5$–$10^6\;M_\odot$.
- The absence of strong X-ray reflection features suggests mass-dependent decreases in torus or disc covering factors, potentially affecting AGN unification at low masses.
- As a “bare nucleus” with low Eddington ratio ($\lambda_{\rm Edd}=0.002$–$0.006$) and weak radio output, UGC 6728 provides a bridge between more massive, canonical Seyfert galaxies and stellar-mass accretors in X-ray binaries, while its structure and SMBH properties anchor models of black hole growth and spin evolution [1608.03893, 2311.15104, 2601.15383, 2007.05590].

## 7. Summary of Key Parameters

| Quantity                              | Value                         | Uncertainty       |
|----------------------------------------|-------------------------------|-------------------|
| Redshift $z$                          | $0.0065$                      |                   |
| Distance $D$                          | $32.5$ (or $28.1$) Mpc        | $\pm 3.5$ Mpc     |
| $M_{\rm BH}$                          | $7.1\times10^5\,M_\odot$      | $\pm 4.0\times10^5\,M_\odot$ |
| Black hole spin $a$                    | $0.97$                        | $+0.20/-0.27$     |
| Stellar mass $\log(M_\star/M_\odot)$   | $9.9$                         | $\pm 0.2$         |
| $M_V$ (galaxy)                        | $-18.77$                      | $\pm 0.31$        |
| $\sigma_\star$                        | $51.6$ km/s                   | $\pm 4.9$ km/s    |
| $\log\lambda L_{5100}$ (AGN)           | $41.97$ [erg/s]               | $\pm 0.11$        |
| Eddington ratio $\lambda_{\rm Edd}$    | $0.002$–$0.006$               |                   |
| X-ray photon index $\Gamma$ (2021)     | $1.85$                        | $\pm 0.07$        |
| Typical radio flux (X-band)            | $0.12$ mJy                    |                   |

UGC 6728 represents a fully characterized case study for AGN accretion, variability, and host structure at the lowest reliably measured SMBH masses.

Source: https://www.emergentmind.com/topics/dwarf-seyfert-galaxy-ugc-6728