The impact of bars on the properties of HII regions in the TIMER survey

TL;DR

The paper investigates how galactic bars modify the physical properties of HII regions by analyzing 17 barred galaxies from the TIMER survey with VLT-MUSE IFU data. Using PYHIIEXTRACTOR to identify HII regions and decontaminate diffuse ionised gas, the authors classify regions into circumnuclear, bar, and disc (with inner/outer disc subdivisions) and derive O/H, N/O, A_V, L(Hα), Σ_*, Ne, and EW(Hα). They find that HII regions in the bar and especially in the circumnuclear disc are more metal-rich, dustier, denser, and more Hα-luminous than disc regions, with inner-disc regions slightly more enriched than outer disc ones; at fixed stellar density, bar and disc regions show similar chemical enrichment. The results support a scenario where bar-driven gas inflows concentrate molecular gas toward galaxy centers, driving enhanced star formation and ISM conditions, while the bar strength mainly modulates the quantity of gas rather than the enrichment level, and massive galaxies show systematically higher abundances and densities. The study highlights the importance of proper DIG subtraction and provides a large catalog of HII regions for future work on bar-driven secular evolution.

Abstract

In this study we perform a comparative analysis of the properties of the HII regions located in different areas of barred galaxies, with the aim of investigating the impact of bars on the physical properties of the ionised gas. Based on integral field spectroscopy data for 17 barred galaxies covering approximately the central 6x6 kpc, we detect a total of 2200 HII regions, of which 331 are located within the nuclear disc (also known as circumnuclear regions), 661 in the bar region, and 1208 in the disc. Among the physical properties of the HII regions, we explore the O/H and N/O abundances, H$α$ luminosity, dust extinction, electron density, and H$α$ equivalent width. We find clear differences in the properties of the HII regions between the nuclear disc, the bar and the disc, that could be explained by an enhancement in the molecular gas concentration in the central parts driven by bar-induced gas flows. As this gas is channelled towards the galaxy centre, the most extreme values in the analysed properties are found for the circumnuclear HII regions. Unlike the bar strength, galaxy mass does seem to affect the properties of the HII regions, with massive galaxies presenting higher values in most of the properties, possibly due to the increased amount of gas in these systems. This study provides evidence that the bar-driven redistribution of material within the galaxy inner parts causes significant differences in the HII region properties depending on their location within the galaxies.

Figures (8)

• Figure 1: Modelling of the Hii regions by PYHIIEXTRACTOR for the galaxy NGC 4981. We show the observed H$\alpha$ map (top panel), the derived model of the candidate Hii regions and the DIG together (middle-top panel) and separately (Hii regions, middle-bottom panel; DIG, bottom panel). Orange circle and red ellipse in top panel delimit the nuclear disc and bar regions, respectively.
• Figure 2: Sketch of a barred galaxy where the different populations of Hii regions are identified (circumnuclear, pink; bar, orange; inner disc, purple; outer disc, grey). The three contours correspond to the limits of the nuclear disc (dashed pink circle), bar region (dotted orange ellipse), and inner disc (dashed-dotted green ellipse), respectively.
• Figure 3: Comparison of the distributions of oxygen abundances ( top), N/O abundances ( middle), and L(H$\alpha$) ( bottom) for different populations of Hii regions: disc (green) versus bar (orange) regions ( left), disc (green) versus circumnuclear (pink) regions ( middle), and regions in the disc inside (purple) versus outside (grey) the bar radius ( right). The distribution for the bar regions is also represented in right panels with unfilled black histograms. In the top left corner of first panels we include the typical (median) errorbar of each parameter, derived by propagating the errors in the involved emission line intensities (the intrinsic error in the used calibrations is not included for O/H and N/O abundances). In the bottom right corner of the panels the Mann-Whitney U test p-value for each pair of compared distributions is shown, and for those which have values below 5% (i.e. presenting statistically significant differences), the Cliff's $\delta$ is also provided as a measure of the magnitude of the differences. These are categorised as negligible (N), small (S), medium (M) and large (L) differences (see Sec. \ref{['sec:results']} for details on these statistics).
• Figure 4: Same as Fig. \ref{['fig:hists1']} but for the dust extinction ( top), the $[\ion{S}{ii}]\lambda6717/[\ion{S}{ii}]\lambda6731$ line ratio as a proxy for the electron density ( middle), and the H$\alpha$ equivalent width ( bottom). See caption above for more details.
• Figure 5: Resolved $\Sigma_\star-{\rm O/H}$ ( top) and $\Sigma_\star-{\rm N/O}$ ( bottom) relations. Hii regions have been segregated as in Fig. \ref{['fig:hists1']}, following Sec. \ref{['sec:population']}. Solid and dashed lines mark the median trend and the median absolute deviation, respectively, in bins of 0.4 log $\Sigma_\star$. The average absolute difference of the median trend between the two represented distributions (|$\Delta$|) is given in the bottom right corner of the panels.