Composite Bulges -- V. Detecting signatures of gas inflows in IFU data: The MUSE view of ionised gas kinematics in nearby galaxies

TL;DR

The paper addresses how gas inflows to galactic nuclei are driven by extended shocks associated with bars. It employs a multi-faceted approach combining residual-velocity maps, velocity-difference maps between emission lines, BPT ionisation diagnostics, and dust morphology using MUSE data for 21 nearby galaxies, with careful handling of multi-component gas and artefacts. The main finding is that extended shocks are present in ~52% of the sample (likely a lower limit due to AGN obscuration in some systems) and often reach well into the inner kpc, implying shocks as a major mechanism for nuclear inflows; higher-mass and barred galaxies show a stronger incidence of coherent shocks. The results also reveal that inner bars correlate with AGN-like emission and nuclear rings with higher shock prevalence, while AGN outflows can mask centrally traced shocks, highlighting the complex interplay between bars, rings, star formation, and AGN activity in shaping central gas dynamics.

Abstract

Using VLT/MUSE data, we study the ionised-gas kinematics in a mass- and volume-limited ($M_* \geq 10^{10} M_\odot$, $D \leq 20$\,Mpc) sample of 21 nearby galaxies to identify signatures of extended shocks within their inner kiloparsec, which appear as coherent velocity jumps in kinematic maps. By removing angular momentum, shocks in gas cause inflows, which can trigger nuclear star formation and fuel AGN activity. To identify the signatures of extended shocks, we examine residual velocity fields after subtracting a modelled rotating disc, and we study velocity difference between various gas tracers. Combining our kinematic analysis with BPT ionisation diagnostic maps and dust morphology, we find that 11 of 21 galaxies ($\sim$52%) show extended shock signatures with velocity jumps consistent with models of bar-driven shocks. This is likely a lower limit, as three additional galaxies ($\sim$15%) exhibit shocks along bars, potentially reaching the nucleus but obscured by AGN outflows. We trace shock signatures inwards close to the resolution limit, which suggests that shocks may be the prevailing mechanism of inflow in the central kpc of galaxies. The only two unbarred galaxies in our sample are also the only systems with unperturbed kinematics and no shocks, strongly linking the perturbed gas dynamics in centres of galaxies to the presence of bars. All galaxies with inner bars show LINER- or Seyfert-like nuclear emission, whereas galaxies without inner bars exhibit all emission types, indicating that regardless of gas supply, inner bars suppress star formation in galactic nuclei.

Figures (35)

• Figure 1: Stellar kinematic maps of NGC 4303. Panels show stellar velocity (top left), stellar velocity dispersion (top right), $h3$ (bottom left) and $h4$ (bottom right) moments of LOSVD. The black contours represent the continuum flux. Galaxy centre is at $\Delta\alpha$,$\Delta\delta$=[0,0].
• Figure 2: Images and diagnostic maps of NGC 4303. Unless stated otherwise, the x and y axes give positional offsets from the galaxy centre ($\Delta\alpha$, $\Delta\delta$) in arcseconds. 1 arcsecond corresponds to 82.4 parcsecs. The first three panels in the left-hand column show a large-scale image (taken either from CGS or SDSS) with the MUSE field of view marked as red box (top), a MUSE-based colour image with $40\arcsec \times 40\arcsec$ region marked with a cyan box (middle), and an unsharp mask image enclosed in cyan box region, highlighting dust features (bottom). All other panels show the region marked with a cyan box: top row -- H$\alpha$, [Nii]$\lambda$6583, [Oiii]$\lambda$5007 flux maps; second row -- velocity maps; third row--velocity dispersion maps. The bottom row shows the BPT map with its diagnostic diagram, the residual velocity map (H$\alpha$--disk model), and the velocity difference map ([Nii]$\lambda$6583-H$\alpha$). For the first three rows, the colour scale is consistent across all maps in a row, with the colour bar given at the right end.
• Figure 3: Residual velocity map of NGC 5248 (left) and NGC 1433 (right). The centre of the galaxies are marked with an "x". The black dashed lines mark the approximate locations of signatures of extended shocks. The signatures seen in NGC 5248 are classified as "coherent shocks" and in NGC 1433 as "broken shocks" (see Sect. \ref{['sec:grouping-shock-class']}).
• Figure 4: The BPT diagram of the galaxy sample analysed in this study, computed using integrated emission-line flux ratios within 150pc radius. The navy (solid) , black (dashed), and red (dotted dashed) lines show Kauffmann_03, Kewley_01a, and Cid_Fernandes_10 fits respectively. These lines determine the regions associated with different ionisation mechanisms: SF, Composite, LINER, and Seyfert. Each point in the diagram represents an individual galaxy, with the accompanying numbers listed in the index column of Table \ref{['tab:galaxies_shock_classes']} for identification. The points are colour-coded by the categories assigned based on the extended shock signatures observed: "coherent shocks" in brown, "broken shocks" in yellow, "other perturbations" in pink, "AGN dominated" in black, and "unperturbed" in cyan.
• Figure 5: The distribution of galaxies stacked in each kinematic category, colour-coded based on their nuclear emission types, both classifications are defined in Sects. \ref{['sec:grouping-shock-class']} and \ref{['sec:grouping-nuclear-emission']}. Different colours represent SF in blue, composite emission in green, LINER in orange, and Seyfert in red. NGC 1566, which has an uncertain nuclear emission type, is represented in grey.