Stellar-gas kinematic misalignments in EAGLE: enhanced SMBH growth in misaligned galaxies

Abstract

Stellar-gas kinematic misalignments are a transient phenomenon observed in $\sim11\%$ of the local galaxy population. According to current models, misaligned gas is expected to lose angular momentum and relax into the galactic plane on timescales of $\sim0.1$ Gyr, driving gas toward the central regions of the galaxy. Recent observational studies have found a higher incidence of active galactic nuclei in misaligned galaxies. We use the EAGLE simulation to explore the connection between stellar-gas misalignments and enhanced central black hole (BH) activity between $0<z<1$. We use a sample of $\sim5600$ galaxies with a stellar mass of $M_{*}\geqslant \mathrm{10^{9.5}}$ M$_\odot$ that feature long-lived stellar-gas alignment, counter-rotation, and unstable misalignments (non-coplanarity). Over time windows of $0.5$ Gyr, we find that galaxies experiencing an unstable misalignment have systematically enhanced BH growth during relaxation. Galaxies with long-term counter-rotation show little difference in BH growth compared to aligned galaxies. We suggest that this enhanced BH growth is driven by loss of angular momentum in unstable misaligned gas discs which is able to drive gas inward toward the vicinity of the BH. At $z\approx0.1$, we find a greater incidence of overmassive BHs in galaxies that have spent a greater fraction of time with unstable stellar-gas kinematic misalignments over the preceding $\approx2$ Gyr compared to control samples of aligned galaxies. In agreement with observations, we conclude that BH activity is enhanced in misaligned systems in EAGLE and suggest that the presence of overmassive BHs may be indicative of a past stellar-gas kinematic misalignment.

Figures (9)

• Figure 1: Median change in BH mass traced over a $0.50\pm0.05$ Gyr window as a function of the initial BH mass for samples of galaxies which are aligned (blue solid), counter-rotating (green dotted), and galaxies experiencing an unstable misalignment (red dash-dotted) for bins with a minimum bin count of 10. Shaded regions show the $25$ and $75$ percentiles. Grey scatter points show the total sample. Diagonal lines (dashed grey) show the relative increase in BH mass, with $\Delta M_{\mathrm{BH}}/ M_{\mathrm{BH,initial}}=1$ indicating a doubling of BH mass over this window. We find that counter-rotating galaxies exhibit similar BH growth to aligned galaxies, while galaxies experiencing an unstable misalignment show significantly higher ($\sim0.6$ dex) BH growth up to BH masses of $M_{\mathrm{BH, initial}} \lesssim 10^{7.5}$ M$_{\odot}$.
• Figure 2: Fractional BH growth distributions over a $0.50\pm0.05$ Gyr window for sub-samples of aligned (blue), counter-rotating (green), and galaxies experiencing an unstable misalignment (red). Errors are given as Poisson uncertainties. We find that unstable misaligned galaxies have higher fractional BH growth with a median of $0.129\Delta M_{\mathrm{BH}}/ M_{\mathrm{BH, initial}}$, while aligned and counter-rotating galaxies have similar growth fractions of $0.039\Delta M_{\mathrm{BH}}/ M_{\mathrm{BH, initial}}$ and $0.034\Delta M_{\mathrm{BH}}/ M_{\mathrm{BH, initial}}$, respectively.
• Figure 3: Median change in BH mass traced over a $0.50\pm0.05$ Gyr window as a function of the initial BH mass for samples of ETGs (top) and LTGs (bottom) which are aligned (blue solid), counter-rotating (green dotted), and galaxies experiencing an unstable misalignment (red dash-dotted) for bins with at least 10 galaxies. Shaded regions show the $25$ and $75$ percentiles. Grey scatter points show the total sample. Diagonal lines (dashed grey) show the relative increase in BH mass, with $\Delta M_{\mathrm{BH}}/ M_{\mathrm{BH,initial}}=1$ indicating a doubling of BH mass over this window. We find that BH growth is strongly enhanced ($\sim1.0$ dex) in unstable misaligned LTGs compared to aligned and counter-rotating morphological counterparts up to BH masses of $M_{\mathrm{BH, initial}} \approx 10^{6.9}$ M$_{\odot}$, beyond which we are limited by low number statistics. Compared to LTGs, the degree of enhanced BH growth among unstable misaligned ETGs is more modest at ($\sim0.4$ dex) up to BH masses of $M_{\mathrm{BH, initial}} \lesssim 10^{7.3}$ M$_{\odot}$.
• Figure 4: Median star-forming gas fraction within $r_{50}$ as a function of fractional BH growth averaged over a $0.50\pm0.05$ Gyr window for sub-samples of aligned (blue solid), counter-rotating (green dotted), and galaxies relaxing from an unstable misalignment (red dash-dotted). Shaded regions show the $25$ and $75$ percentiles. Grey scatter points show the total sample. Galaxies experiencing an unstable misalignment tend to be more gas rich overall, but have similar $f_{\rm{gas,SF}}$ compared to aligned galaxies among the fastest growing BHs.
• Figure 5: Median co-rotational energy fraction for star-forming gas within $r_{50}$ as a function of fractional BH growth averaged over a $0.50\pm0.05$ Gyr window for sub-samples of aligned (blue solid), counter-rotating (green dotted), and galaxies relaxing from an unstable misalignment (red dash-dotted). Shaded regions show the $25$ and $75$ percentiles. Grey scatter points show the total sample. Galaxies experiencing an unstable misalignment tend to have gas$_{\rm{SF}}$ discs with less rotational support compared to aligned and counter-rotating galaxies.