The complex relationships between AGN, bars and bulges

Abstract

Context. Via scaling relations, it is well-known that active galactic nuclei (AGN) and bulges are linked. This link was thought to be driven by mergers, but recent studies show that secular processes are the dominant mechanism of supermassive black hole growth. One such secular mechanism is gas inflow driven by large-scale bars. Since bulges can also grow via these bars, there is likely some common process between these three features. Aims. We investigate whether the observed correlation between AGN and bars is real or arises as a result of correlations between bars and bulges. Methods. Using a catalogue of AGN identifications and galaxy morphologies in the DESI Legacy Survey at $z\leq0.1$, we control for mass and colour and investigate the AGN fraction variation with bulge prominence and bar strength. Results. We first show that the variation in AGN fraction between strongly barred, weakly barred and unbarred galaxies does not qualitatively change if we additionally control for bulge prominence. Second, we find that in fixed bins of bulge prominence, the AGN fraction increases with increasing bar strength. In subsamples split by bar strength, the AGN fraction increases with bulge prominence, indicating that AGN presence correlates with both bar strength and bulge prominence simultaneously.

Figures (7)

• Figure 1: The distributions of stellar mass (left column), $(g-r)_{0}$ colour (middle column) and bulge prominence (right column) for AGN (top row), star-forming galaxies (middle row) and undetermined galaxies (bottom row). We show strongly barred galaxies in solid red lines, weakly barred in dashed navy blue, and unbarred in dotted teal. The AGN tend to have a higher bulge prominence, redder colour and higher stellar mass than their star-forming counterparts, although the ranges of these parameters do not vary significantly. The differences between the bar strengths are more apparent in star-forming galaxies than in AGN, with bulge prominence being particularly divided in undetermined galaxies.
• Figure 2: The fraction of galaxies in each bar strength that are AGN (red, positive diagonal), star-forming (SF; teal, square hatching) or undetermined (grey, negative diagonal). Error bars are shown in white. The AGN fraction increases as bar strength increases, although in each case the star-forming fraction is greater than the AGN fraction.
• Figure 3: The effect of bulge prominence on AGN fraction ($f_{\mathrm{AGN}}$) for each of strongly barred (red solid line), weakly barred (navy dashed line) and unbarred (teal dotted line) disk galaxies. Overall, $f_{\mathrm{AGN}}$ increases in each bar strength category with bulge prominence. At lower bulge prominences, $f_{\mathrm{AGN}}$ increases in each bulge bin with bar strength, however the difference between $f_{\mathrm{AGN}}$ in strongly and weakly barred galaxies disappears by $B\approx 0.48$, and the difference between all three bar categories disappears around $B\approx0.6$. The shaded regions show the $1\sigma$ uncertainties.
• Figure 4: The relationship between stellar mass and bulge prominence for our sample. Lines of best fit are shown in black. We split the sample by unbarred (teal), weakly barred (navy blue) and strongly barred (red) galaxies. There is a slight increase in the bulge prominence with stellar mass.
• Figure 5: Variation of $f_{\mathrm{AGN}}$ with the bulge surplus, as calculated in Equation \ref{['eq:bulge_surplus']}. The relationship for strongly barred galaxies is shown in solid red, for weakly barred in dashed navy blue, and for unbarred in dotted teal. The horizontal lines show the mean $f_{\mathrm{AGN}}$ for each bar category. Shaded regions show the $1\sigma$ uncertainties. Galaxies at a given stellar mass are more likely to be hosting an AGN if they also have a greater bulge surplus, and this relationship is steeper for unbarred galaxies than for strongly or weakly barred.