Resolved ISM properties and scaling relations in the barred galaxy NGC 3627: constraints from NIKA2 observations

TL;DR

This study presents resolved and global analyses of the barred galaxy NGC 3627 using new NIKA2 1.15 and 2 mm data, combined with a broad suite of ancillary maps. By fitting the SED with the THEMIS dust model within the HerBIE framework, the authors separate dust, free-free, and synchrotron components, deriving ISM maps and scaling relations at ~kpc scales. They find that 2 mm emission is predominantly dust-driven (≈87%), with notable synchrotron and free-free contributions in the bar-ends and nucleus, and that the 160 μm band traces molecular gas best while 1.15 mm traces dust mass. The analysis reveals a ring-like distribution of small grains depleted in high-radiation and central regions, a strong link between dynamical structures (bars, tides) and SF/dust, and an evolutionary picture in which regional ISM properties vary between active star-forming zones and more quiescent zones, consistent with a simple closed-box dust evolution framework. Overall, the work demonstrates the power of mm-wavelength insights for understanding ISM composition and evolution in barred galaxies and their role in shaping star formation and dust distributions.

Abstract

We investigate the interplay between star formation, interstellar medium (ISM) components, and dust properties in NGC 3627 using new NIKA2 1.15 and 2 mm observations from the IMEGIN Large Program. Our goal is to analyze dust and radio emission, decompose contributions in the millimeter-centimeter regime, and explore ISM properties within the galaxy. We perform spectral energy distribution fitting, at both global and spatial scales, using the THEMIS dust model within the HerBIE code, applied to data from 3.4 $μ$m to 6 cm. We decompose emission into dust, free-free, and synchrotron components, and examine correlations with gas surface density and star formation activity. Additionally, we analyze the small dust grain fraction and its variation across the galaxy. We find $\sim$10% radio emission at 2 mm, peaking at 18% in the southern bar-end, which hosts the highest star formation activity. However, an isolated star-forming region beyond this bar-end is the most efficient, as indicated by its elevated dust production efficiency and effective yield, predicted by our simplistic dust evolution model. The 160 $μ$m emission shows the strongest correlation with molecular gas, while 1.15 mm better traces the dust mass surface density. Small grains, which make up $\sim$13% of dust mass (2 $\times$ 10$^{7}$ M$_{\odot}$), are depleted in intense radiation fields, with a notable deficit in the southern tidal tail. ISM properties and chemical evolution indicate that dynamical processes, such as bar-driven gas flows and tidal interactions, are crucial in shaping the galactic structure, influencing star formation efficiency, and dust distribution.

Figures (14)

• Figure 1: NGC 3627 mapped with the NIKA2 camera at 1.15 mm (left panel) and 2 mm (right panel) and with native resolutions of 11.1$^{\prime\prime}$ and 17.6$^{\prime\prime}$ ($\sim0.6$ kpc and $\sim1$ kpc, respectively; see the white circles in the bottom-left corner in each panel). The maps are centered at RA$_\mathrm{J2000}$$=11^\mathrm{h}20^\mathrm{m}15^\mathrm{s}$, DEC$_\mathrm{J2000}$$=+12^{\circ}59^{\prime}30^{\prime\prime}$. The surface brightness contours correspond to 3, 5, 7 and 12 $\times$ RMS. The RMS values are 0.9 mJy beam$^{-1}$ and 0.3 mJy beam$^{-1}$ at 1.15 mm and 2 mm respectively. The maps are presented in projected galactic distances $\Delta$X, $\Delta$Y from the center (marked by crosses), in kpc.
• Figure 2: The global SED of NGC 3627 followed by the SEDs of typical pixels within selected regions which correspond to an isolated HII region, the southern bar-end, the nuclear region, the northern bar-end, the galactic bar, and the western and the eastern spiral arms of the galaxy (regions A to G, respectively). The SEDs are fitted with the HerBIE code (see Sect. \ref{['subsec:herbie']} for a detailed description). Observed spectral luminosities are represented by different blue symbols in each SED (with the exception of the NIKA2 measurements highlighted in red), while the corresponding, fitted models (and their uncertainties) are shown with continuous brown curves. The gray filled symbols in each SED denote the model values at the observational bands. The spectral luminosities for the global SED correspond to the values in the y-axis, while the rest of the SEDs are scaled by the factors indicated next to the model curves. The vertical dashed line at 74.5 $\mu$m, marking the wavelength that the IR SED peaks for the galactic center (C), provides a reference of how the peaks of the other SEDs are positioned relative to each other. The respective residuals between observations and model are provided in the panels below.
• Figure 3: Total modeled emission at 2 mm (left panel) and its decomposition into dust, synchrotron, and free-free emission, shown in the second, third, and last panels, respectively. The color bars are in mJy per 25$^{\prime\prime}$ beam, and the beam size is indicated in the bottom-left corner in each frame. The crosses indicate the regions A to G (see Fig. \ref{['fig:seds']}). Contours of X-ray emission (observed by Chandra) have been superimposed on the synchrotron and free-free emission maps, while the white contours in the first two panels are employed to aid in visualizing the galaxy's morphology. As proposed by 2012AA...544A.113W, the asymmetric X-ray extension in the eastern part of the galactic disc may be linked to a recent interaction with a dwarf galaxy approaching from this direction.
• Figure 4: Emission components contributing to the total flux at 1, 2 and 5 mm, 6 and 20 cm (top to bottom panels) in the regions A, B, C, D, E, F and G (left to right). The emission percentages for the dust, the free-free, and the synchrotron emissions are shown in orange, red, and blue bars, respectively. The actual percentage values for each emission mechanism, in each region, are indicated with numbers in the plots.
• Figure 5: The dust mass and stellar mass surface densities (top panels; left and right, respectively), the atomic mass and molecular mass surface densities (middle panels; left and right, respectively), and the star formation rate (SFR) and specific star formation rate densities (bottom panels; left and right, respectively). All maps share the same grid and resolution of 25$^{\prime\prime}$ (indicated by the white circle in the lower left corner). The methods used in deriving these maps are detailed in Sect. \ref{['subsec:ISM_morph']}.