MaNGA AGN dwarf galaxies (MAD) -- IV. Revealing hidden AGN in dwarf galaxies with radio observations

TL;DR

The paper addresses the challenge of identifying AGN in low-mass, dwarf galaxies by combining MaNGA IFU optical diagnostics with high-resolution VLA radio imaging. It shows that one of four MaNGA-selected dwarf candidates (8442-1901) hosts AGN-driven radio emission with a jet power of about $Q_{\rm jet} \sim 10^{42}$ erg s$^{-1}$ and an outflow capable of escaping the dark matter halo, accompanied by a central decline in star formation indicative of negative feedback. The other three targets exhibit radio emission consistent with stellar processes or SNRs/SNe, aligning with their optical classifications and lack of a robust AGN radio signature. Overall, the results demonstrate that radio observations, when combined with IFU spectroscopy, can reveal hidden or switched-off AGN in dwarfs and quantify their potential impact on host galaxy evolution.

Abstract

Low-mass black holes hosted by dwarf galaxies offer valuable insights into galaxy formation and the growth of the massive black holes found in massive galaxies. Their detection as AGN is challenging due to their low luminosity and compact size. This can be circumvented employing multi-wavelength observational strategies, such as combining optical and radio observations, which enables the detection of AGN features that may be hidden in single-wavelength analyses We aim to detect any jet-like emission indicative of the presence of an AGN in a sample of four dwarf galaxies with AGN signatures based on spatially resolved emission line diagnostic diagrams with SDSS MaNGA. Confirming the presence of an AGN will prove IFU spectroscopy to be a resourceful tool for identifying hidden or switched-off AGN. Using VLA radio observations, we image the radio emission of the four dwarf galaxies and derive their integrated radio flux and luminosity. We compare these to that expected from star formation processes to determine the origin of the radio emission and probe if it is consistent with the results of the emission line diagnostic diagrams. We find that one out of the four galaxies shows AGN radio emission consistent with the analysis of the MaNGA IFU data. The kinetic jet power of this source is Qjet ~ 1e42 erg / s, indicating that dwarf galaxies can host radio jets as powerful as those of massive radio galaxies. This galaxy exhibits an AGN outflow able to escape the gravitational bound produced by the dark matter halo, along with a decrease in the star formation rate of the central region. This suggests the presence of negative feedback from the AGN, which could be suppressing star formation. The other three galaxies exhibit regions of radio emission consistent with a stellar origin and overlapping with the star-forming regions found in the IFU spectroscopy.

Figures (6)

• Figure 1: Left panel: BPT diagram used to distinguish between ionisation by AGN (red spaxels), star formation (blue spaxels), composite (green spaxels) and LINER (yellow spaxels). Figure adapted from from Mezcua2020. Middle panel: The left figure shows the spatial distribution of the BPT-classified spaxels (colour-coded as in the top left panel). Empty squares mark the IFU coverage, grey squares those spaxels with S/N > 1. The radio contours at (10, 20, 40, 70) times the off-source RMS noise have been added on the image. The right figure is the SDSS composite image. The pink hexagon shows the IFU coverage. Figure taken from Mezcua2020. Right panel : VLA radio image at 6 GHz. The colour bar indicates the flux of each pixel in Jy beam$^{-1}$. Regions selected for Gaussian fitting are indicated by a letter. In the lower right corner is the clean beam (solid white or solid black line), which indicates the minimum beam size resolved in the observation and whose size is shown in Table \ref{['tab:gausfit']}. The radio contours have been added on the image.
• Figure 2: Left: Spatial W$_{80}$ outflow velocity distribution of the galaxy 8442-1901. Middle: Star formation rate over the las 20 Myr. Right: SFR over the last 10 Myr. The spaxels with outflow signatures have red edges in the two SFR maps. The VLA radio contours were added in the three maps.
• Figure 3: Possible origin of the radio emission detected in our sample of MaNGA AGN candidates. The grey solid line represents the one-to-one relation of the expected luminosity of each possible origin. The grey dotted line represents the luminosity 3$\sigma$ above the expected luminosity for each possible origin. Unseen error bars are smaller than the size of the plotted symbols. Top: Under the assumption that the radio emission is thermal bremsstrahlung (i.e., the origin of the emission is star formation), the 6 GHz luminosity density of our sources versus the expected luminosity of the star formation has been plotted. Middle: 6 GHz luminosity densities of our sources versus the expected luminosity of individual SNR/SN. Bottom: 6 GHz luminosity densities of our sources versus the expected cumulative luminosity of the SNRs/SNe population. Sources above the 3$\sigma$ expected luminosity cannot be explained by the specific mechanism since individual regions cannot have luminosities that exceed what is expected for each mechanism in the entire galaxy. Sources below this region can have their origin in each specific mechanism.
• Figure 4: Same caption as in figure \ref{['fig:bpt1']}. The VLA contours are 5 times the off-source RMS noise.
• Figure 5: Same caption as in figure \ref{['fig:bpt1']}. The VLA contours are (5, 8) times the off-source RMS noise.