Investigating Galactic Fountains in M101: Insights from Ionized, UV emissions and Neutral Gas

TL;DR

This work investigates galactic fountains in M101 by combining HI 21 cm maps, GALEX UV imaging, and SITELLE optical IFU data to identify fountain-driven cavities in a nearly face-on disk. The authors revisit the Kamphuis hole catalog and uncover 20 additional HI holes; nine high-confidence fountain sites are characterized, with diameters of $0.5$–$2.4$ kpc, expansion velocities of $7$–$54$ km s$^{-1}$, and kinetic ages of $18$–$93$ Myr, revealing how stellar feedback deposits energy into the ISM and ejects gas above the disk. Derived energetics using $E_{Ch}$ and $E_{Mc}$ imply $N_*$ between $10^{2}$ and $10^{4}$ SNe powering the holes, while many sites show UV/H$\alpha$ signatures consistent with triggered star formation along shell rims or interiors lacking ionizing sources. The face-on perspective, combined with high-resolution multiwavelength data, provides crucial constraints on feedback efficiency, gas recycling timescales up to $\sim 100$ Myr, and the spatial distribution of fountain activity, highlighting the value of such observations for understanding disk evolution and the baryon cycle.

Abstract

Spiral galaxy disks are thought to exist in a quasi-stationary state, between fresh gas accretion from cosmic filaments and disk star formation, self-regulated through supernovae feedback. Our goal here is to quantify these processes and probe their efficiency. While star formation can be traced at 10 Myr time-scales through H$α$ emission, the signature of OB stars, and at 100 Myr scale with UV emission, the gas surface density is traced by HI emission for the atomic phase. We choose to investigate feedback processes using fountain effects in M101, a nearby well-observed face-on galaxy. Face-on studies are very complementary to the more frequent edge-on observations of these fountains in the literature. We use high-resolution data from THINGS for the HI emission GALEX for UV, and SITELLE/SIGNALS IFU for the H$α$ tracer. We have identified 20 new HI holes, in addition to the 52 holes found by Kamphuis in 1993. We study in more detail the nine holes satisfying strong criteria to be true fountain effects, compute their physical properties, and derive their energy balance. Only one small HI hole still contains H$α$ and young stars inside, while the largest hole of 2.4 kpc and oldest age (94 Myr) is deprived of H$α$ and UV. For face-on disks, the possibility to study simultaneously the HI shell morphology, the stellar association, and kinematic evidence is of primordial importance. In M101, we have quantified how stellar feedback is responsible for carving the observed cavities in the atomic gas disk, and how it can expel above the disk the neutral gas, which is then unavailable for star formation during up to 100 Myr.

Figures (11)

• Figure 1: Multi-wavelength views of NGC 5457 or M101.
• Figure 2: Pictorial depiction of the scoring criteria. The red ellipse in all the panels corresponds to Hole 1. The dashed blue line on the top left panel is the path across which the intensity profile and the rightmost position–velocity diagrams have been retrieved.
• Figure 3: Hi moment-0 map of NGC 5457 (M101), overlaid with the locations of highly likely sites of galactic fountains. Regions are categorized based on quality flags ($Q$): red dashed ellipses mark holes from Kamphuis et al. with $Q \ge 15$, red solid ellipses show newly identified holes with $Q \ge 15$, purple dashed ellipses correspond to Kamphuis et al. holes with $10<Q<15$, and purple solid ellipses mark new detections with $10<Q<15$. The background blue-scale image shows the integrated Hi emission, with brightness scaled between the 10th and 98th percentiles. The nine regions in red are the identified sites of galactic fountains.
• Figure 4: Multiwavelength views of the four biggest sites of galactic fountains in M101 marked by red ellipses. From top: Hi velocity map overlayed with its 5$^\mathrm{th}$ and 95$^\mathrm{th}$ flux contours in black; Hi velocity dispersion; FUV flux map; H$\alpha$ flux map; H$\alpha$ velocity dispersion map; the ratio of [Sii] to H$\alpha$ flux overlayed with the positions of optically identified SNRs by 1997Matonick in black points.
• Figure 5: Comparison of properties between the 20 new Hi holes in blue and the 52 Kamphuis holes in grey. The medians of each sample are plotted as dashed lines. As expected, the new holes span smaller diameters and are younger than those of the Kamphuis sample. However, these younger holes do not seem to have a more circular shape than the older ones.