Low-mass Active Galaxies in the SAMI Galaxy Survey with Spatially-resolved Spectroscopy

Abstract

The smallest supermassive black holes (BHs), which provide constraints on BH seeds, reside in low-mass galaxies. Here, we present a systematic analysis of 990 low-mass galaxies in the SAMI Galaxy Survey to identify emission from accreting BHs using integral field spectroscopy (IFS). Employing a novel automated scoring algorithm based on spatially resolved narrow emission-line diagnostics, we find signatures of active galactic nuclei (AGNs) in 41 galaxies, as well as an additional 46 less secure candidates. The galaxies have stellar masses in the range $10^{9.4} \lesssim M_\star/M_\odot \lesssim 10^{10}$ (down to $10^{8.5}$ including less secure candidates), redshifts $z \lesssim 0.06$, and morphologies ranging from early-type ellipticals to late-type spirals. Our AGN fraction of 4% (9% including the less secure candidates) is significantly higher than those reported by studies using single-fiber spectroscopy ($\lesssim 1$--2%). Indeed, our additional analysis of single-fiber spectra of the objects in our sample demonstrates that many of our AGN candidates detected via IFS are missed. This work highlights the advantages of IFS, particularly its ability to capture extended or off-nuclear emission from accreting BHs.

Figures (20)

• Figure 1: An example of spaxel maps (top row) and emission line diagnostic diagrams (bottom row) used in this work, as defined in kewley_host_2006, for SAMI galaxy CATID 287827. The top row shows the spatial distribution of spaxels classified by each of the three diagnostic diagrams: [NII] diagram – star-forming (purple), Composite (green), and AGN (yellow); [SII] diagram – star-forming (blue), LINER (red), and Seyfert/AGN (orange); [OI] diagram – star-forming (light blue), LINER (light purple), and Seyfert/AGN (pink). Spurious AGN and Composite spaxels (see §\ref{['sec:selection_scheme']}) are colored gray. White indicates omitted spaxels due to either a lack of data or high uncertainty in the flux measurements for that spaxel (see §\ref{['sec:parent_sample']}). The coordinates in the diagnostic maps are relative to the galactic center as supplied by SAMI. The bottom row shows the same spaxels plotted on their respective diagnostic diagrams.
• Figure 2: Example of the AGN spaxel clustering analysis for galaxy CATID 382563. Left to right: (1) Initial [NII] diagnostic map showing AGN (yellow), Composite (green), and star-forming (purple) spaxels (see Figure \ref{['fig:287827_bpt']} for color scheme). (2) Boolean AGN mask in which AGN and Composite spaxels are assigned a value of one and all other spaxels zero. (3) PSF-convolved AGN mask using a 2D Gaussian kernel ($\sigma = 1.7$ spaxels, corresponding to $2"$ FWHM). (4) Spaxels with PSF-weighted values below the adopted threshold of 0.2, shown in gray and considered spurious (see §\ref{['sec:selection_scheme']}). (5) Final [NII] BPT map with spurious AGN classifications re-labeled in gray.
• Figure 3: Distribution of PSF-Weighted AGN Score for all [NII]-BPT AGN or Composite classified spaxels in the parent sample. Red dotted line indicates our adopted PSF value threshold of 0.2.
• Figure 4: Example galaxies in Sample A. We show a representative galaxy for each score value of 1 to 6, with a larger score indicating more evidence for an AGN. The score is shown in the top right corner of the first column for each row, with the accompanying letter indicating the galaxies are in Sample A. The first six columns show the [NII], [SII], and [OI] classification spatial maps interspersed with the corresponding diagnostic diagrams. The color code for all maps and diagrams follows Figure \ref{['fig:287827_bpt']}. The final column shows the $grz$-band $25" \times 25"$ optical cutout from the DESI Legacy Imaging Survey.
• Figure 5: Center panel: Galaxy stellar mass versus redshift. Sample A galaxies are shown in blue, Sample B galaxies are in red, and our parent sample of 990 low-mass galaxies is shown in gray. Top panel: Redshift (z) histograms of the three samples. Right panel: $\log M_*/M_{\odot}$ histograms of the three samples.