Revisiting the Gas Dynamics of Henize 2-10: Possible Drivers of the Starburst

TL;DR

Henize 2-10 is a nearby isolated dwarf experiencing a vigorous starburst, and this study tests external merger versus internal inflow scenarios as its trigger. By combining new VLA HI data with archival CARMA/SMA/ALMA CO observations, the authors map the distributions and kinematics of atomic and molecular gas, revealing an extended HI envelope without a clear HI tail and a prominent southeastern CO tail that is kinematically distinct from the main body. The analysis favors an infalling/accreting CO cloud as a plausible driver of the central starburst, though outflow or IGM accretion scenarios face challenges; higher-resolution HI data are needed to fully resolve the merger history and gas dynamics. The work highlights how cold gas inflow in dwarfs can fuel intense star formation and informs our understanding of starburst triggering in low-mass galaxies, with potential implications for the formation of super star clusters in the early universe. Specifically, the estimated ram pressure from infalling gas, $P/k \sim 3-4 \times 10^{8}\ \mathrm{K\ cm^{-3}}$, could be sufficient to trigger central star formation, reinforcing the inflow scenario as a viable mechanism.

Abstract

The triggers of starburst episodes are a key component to our understanding of the baryon cycle in galaxies. Galaxy mergers are a commonly suggested catalyst for starbursts, but once the galaxies coalesce into a single kinematically disturbed system, their merger history can be difficult to assess. This is particularly true for dwarf galaxies, which are expected to dominate the merger rate at all redshifts due to their large numbers. One such dwarf galaxy undergoing an enigmatic starburst episode is Henize 2-10, which appears to be isolated. Possible scenarios that might have caused the starburst episode include a previous merger or stochastic processes within the galaxy itself, such as self-regulation via feedback processes. We present new VLA 21-cm observations and unpublished archival CARMA CO data to investigate the dynamical state and star formation activity in the galaxy. We do not detect an HI tail consistent with the structure reported by Kobulnicky et al. (1995), which was suggested as evidence for a merger or interaction, but rather these new observations indicate an extended HI distribution. We also find that the HI appears dynamically decoupled from an extended CO feature (inferred to be a tidal tail in previous work), suggesting large-scale dynamical processes of some type are affecting the gas in this system. We provide a meta-analysis of available results to enhance our understanding of what might be triggering the starburst episode in Henize 2-10, and speculate that the large CO feature could be falling into the galaxy and potentially trigger starburst activity.

Figures (13)

• Figure 1: Finder charts for Hen 2-10. The raster is a Hubble Legacy Archive HST/WFPC2 H$\alpha$ (F656N) map originally published in Johnson:2000. In both panels, the purple contour shows the VLA 33 GHz continuum, which traces free-free emission from the embedded SSCs, from Costa:2021 at 5$\times$ their rms of 3.12 $\mu$Jy beam$^{-1}$. The gray contour shows this work's $^{12}$CO(1-0) moment 0 map at 4$\sigma_{CO}$; the CO beam is shown with the white ellipse in the lower left. Top. The NE and SW outflows and the locations of the two star-forming knots in the SE are annotated Mendez:1999. The black 'x' on centered in the 33 GHz contours represents the position of the central AGN. Bottom. The nominal positions of Regions A and B are annotated, and components C and D are shown with circles the size of the Kobulnicky:1995 OVRO synthesized beam in green and red, respectively. The cyan contour shows a representative "footprint" of the continuum subtracted F656N map. This contour was chosen to highlight NE and SW bubbles and to distinguish, for illustrative purposes, the CO "tail" from the CO main body, the latter of which is enclosed by the cyan contour. The black dashed line is drawn to guide the eye; it passes through the centers of the NE and SW bubbles at a PA = 49$^{\circ}$ from the +y axis.
• Figure 2: Line profiles in Hen 2-10. Left, top. The black symbols represent the surface brightness per velocity channel with 1$\sigma$ uncertainties, and the black dashed curve is the best fit 1D Gaussian. The inset box is the H i integrated intensity map; this FOV is $\sim$45 $\times$ 45. Left, bottom. The black dashed curve shows the residuals ( = measurement - model). Right, top. The $^{12}$CO(2-1) line profiles for three extracted regions are shown, and the inset box is the $^{12}$CO(2-1) integrated intensity map with the extracted regions overlaid. The region of main body of CO emission is marked with a green rectangle; its line profile is the green dash-dotted curve. The CO "tail" is the cyan rectangle; its line profile is the blue dotted curve. The black dashed curve is the emission within the black dashed rectangle, representing the total CO mass. The $^{12}$CO(1-0) profiles are qualitatively consistent with the profiles shown here. Right, bottom. The black dashed, green dash-dotted, and dotted blue curves show the residuals for the total, the main CO body, and the CO "tail", respectively. See Table \ref{['tab:regions']} for qualitative details of the regions.
• Figure 3: H i maps of (a) the integrated intensity (b) intensity weighted velocity field, (c) the peak intensity, and (d) the intensity weighted velocity dispersion, integrated over 730 -- 990 km s$^{-1}$. In each panel, the black contour shows 5$\sigma_{CO(2-1)}$. The synthesized H i beam is the red ellipse in the lower left, and the combined $^{12}$CO(2-1) beam is overlaid as the black hatched ellipse at the center of the H i beam. In all panels, the raster is masked below 3$\sigma_{HI}$. In (a), the gray contour is 6$\sigma_{HI}$, and the orange star represents approximately the tip of H i tail seen in Kobulnicky:1995. In (b), isovelocity contours are shown in increments of 20 km s$^{-1}$. In (c) the cyan contour is the H$\alpha$ "footprint". In (d), the red dashed line intersects both the NE and SW bubbles from Figure \ref{['fig:f555FinderA']}, and the dashed fuchsia contour is at the H i surface density of 1 $\textrm{M}_{\odot}$ pc$^{-2}$. The two regions of high velocity dispersion are denoted with the orange arrows (see text for discussion). Note, the $\sigma_v$$\sim$ 60 km s$^{-1}$ clump near (RA, Dec.) = (08:36:14.5, --26:23:28) is likely an artifact in the image.
• Figure 4: Position-velocity diagrams for the H i data. The inset raster map is the H i moment 1 data, marking the paths of the slices. All slices are anchored on Region A, which is marked with two dashed orange lines in the raster map and the vertical dashed orange line on the p-v slice. Emission in the position–velocity diagram is displayed with a two-tone colormap: signal above $\sigma_{HI}$ appears in color, and fainter emission is shown in grayscale.
• Figure 5: Left: Raster map of $\Sigma_{HI}$, masked below 3$\sigma_{HI}$. The diameter of the yellow circle is D$_{HI}$; the dashed cyan contour traces $\Sigma_{HI}$ = 1 $\textrm{M}_{\odot}$ pc$^{-2}$. The diameter of the orange circle is 4$^{\prime}$, which is D$_{HI}$ estimated from single dish H i mass estimates. Right: Reproduction of Figure 1 of Wang:2016, where the gray circles are galaxies in their sample, the red dashed line is the relationship from Broeils:1997, and the solid black line is Equation \ref{['eq:wang2016']}, with the shaded gray region marking the $\pm$3$\sigma$. The fuchsia star marks the mass enclosed by a circle with a diameter of 7 kpc to represent Hen 2-10. We do not correct D$_{HI}$ for inclination; the uncertainties represent the range in D$_{HI}$ for inclination angles spanning 15--80$^{\circ}$. The orange, vertical shaded region marks the range of H i masses derived from single dish observations.