Gas Accretion from a Neighbouring Galaxy Fuels the Low-luminosity AGN in NGC 4278

TL;DR

The paper addresses how a seemingly quiescent elliptical galaxy can fuel a low-luminosity AGN. It leverages high-sensitivity HI mapping with FAST to reveal external gas accretion from a neighboring galaxy (NGC 4286) that forms a rotating gas disk in NGC 4278 and a curved HI bridge between the pair. The accreted neighbor is gas-poor, with an HI-to-stellar mass ratio around 0.02, supporting a tidal accretion scenario that fuels the LLAGN and may relate to TeV gamma-ray emission. The findings highlight tidal, externally supplied gas as a viable fueling channel for LLAGNs in gas-poor ellipticals and connect HI gas dynamics to high-energy phenomena.

Abstract

How a seemingly `dead' host galaxy provides fuel for its active galactic nuclei (AGN) remains an unresolved problem. Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we present a new high-sensitivity atomic-hydrogen (HI) observation toward the nearby elliptical galaxy NGC 4278 and its adjacent region. From the observation, we found that external gas accretion from a neighbouring galaxy fuels the low-luminosity AGN in NGC 4278 through tidal interactions. The accreted gas entering NGC 4278 exhibits a rotating gas disk. And the accreted galaxy has been gas-poor and has an HI to stellar mass ratio of about 0.02. Due to the process of gas accretion, it is likely that relativistic jets are generated in the AGN of NGC 4278. The emission of TeV gamma rays in NGC 4278 is likely to be associated with the newly accreted HI gas.

Figures (5)

• Figure 1: Large-scale overview of NGC 4278. H i column-density map of NGC 4278 from the FAST observation shown in cyan contours overlaid on the DESI-RGB ($g, r, z$) image in color scale. The cyan contours begin at 5.4$\times$10$^{17}$ cm$^{-2}$ (3$\sigma$) in steps of 3.8$\times$10$^{18}$ cm$^{-2}$. The red circle represents TeV source 1LHAASO J1219+2915, while the purple contours indicate the X-ray emission. The radio jets of NGC 4278 are shown in the upper-right corner. The green arrow indicates a cuting direction and position of the position-velocity diagram shown in Fig. 4. The FAST beam in a white circle is shown in the bottom-left corner.
• Figure 2: Global H i profiles of NGC 4278 and NGC 4286 shown in a black lines. The green lines indicate the BusyFit fitting result. The values of the errorbars are 0.4 Jy for NGC 4286, while 1.0 Jy for NGC 4278.
• Figure 3: Position-velocity diagrams of NGC 4278 in color scale overlaid with the black contours. a, the direction of cutting for the slice from the FAST data is through the center of the galaxy along the major axis (PA=65$^{\circ}$). The black contours begin at 3$\sigma$ ( 1.4 mJy beam$^{-1}$) in steps of 3$\sigma$. b, the direction of cutting for the slice from the FAST data is through the center of the galaxy along the minor axis (PA=140$^{\circ}$). c, the slice from the WRST data along the major axis. The black contours begin at 3$\sigma$ (0.3 mJy beam$^{-1}$) in steps of 4$\sigma$. d, the slice from the WRST data along the major axis. The large white pluses in panels a and c mark the centre and systemic velocity of NGC 4278, while the red circles in panels b and d indicates a fitting shell.
• Figure 4: a, the velocity-position diagram in color scale overlaid with the black contours. The direction of cutting for the slice is along the direction of the accretion, as shown in Fig 1. The black contours begin at 3$\sigma$ (1.4 mJy beam$^{-1}$) in steps of 3$\sigma$. b, H i column-density map in cyan and blue contours overlaid on the DESI-RGB image in color scale. The cyan solid contours from the FAST data begin at 1.1$\times$10$^{18}$ cm$^{-2}$ (3$\sigma$) in steps of 1.1$\times$10$^{18}$ cm$^{-2}$, while the cyan dashed contours begin -3$\sigma$. The integrated velocity for the cyan contours ranges from 620.0 $\rm km \, s^{-1}{$ km s^-1$}$ to 695.0 $\rm km \, s^{-1}{$ km s^-1$}$. The red circle represent Tev source 1LHAASO J1219+2915, whose size represents the positional error of this source. The FAST beam in a white circle is shown in the bottom-left corner.
• Figure 5: The WRST H i column-density map in cyan contours overlaid on the DESI-RGB image in color scale. The WRST cube data is convolved to the same beam as FAST. The cyan contours begin at 3.0$\times$10$^{18}$ cm$^{-2}$ (3$\sigma$) in steps of 6.0$\times$10$^{18}$ cm$^{-2}$, while the cyan dashed contours begin -3$\sigma$. The integrated velocity for the cyan contours ranges from 620.0 $\rm km \, s^{-1}{$ km s^-1$}$ to 695.0 $\rm km \, s^{-1}{$ km s^-1$}$. The FAST beam in a white circle is shown in the bottom-left corner.