Spatially-resolved interstellar dust properties in the face-on spiral galaxy M 99 as observed by NIKA2

Abstract

Large dust grains in thermal equilibrium dominate the far-infrared and contribute to the millimetre continuum of star-forming galaxies, but constraining their properties is difficult due to free-free and synchrotron contamination. We study spatial variations in the dust spectral index, mass, and grain properties in the nearby face-on spiral galaxy M 99. We use new 1.15 and 2 mm continuum observations from NIKA2 on the IRAM 30 m telescope (IMEGIN Guaranteed Time Large Programme) combined with multiwavelength data from UV to radio. The infrared-to-radio SED is decomposed into dust, free-free, and synchrotron components using the hierarchical Bayesian code HerBIE. Dust is modelled via a modified blackbody (MBB) with variable millimetre spectral index beta and the THEMIS dust model with fixed beta. We perform spatially-resolved analysis at scales ~1.75 kpc (~25''), covering the centre, spiral arms, and inter-arm regions. MBB fits reveal beta variations from ~1.6-1.7 in diffuse regions to ~2.3-2.5 in dense star-forming areas, likely due to grain coagulation and changes in silicate-to-carbonaceous ratios. Dust masses from variable beta are up to ~4x higher than fixed-beta models, which systematically bias dust-to-stellar and dust-to-gas ratios. The small grain fraction rises from ~10% in the centre to ~15% in the diffuse disc, anti-correlated with the interstellar radiation field; gas-phase metallicity plays a minor role within 8 kpc. Synchrotron spectral index ranges from ~0.6-0.7 in star-forming regions to ~1.2 in the diffuse medium, consistent with cosmic-ray electron ageing.

Figures (29)

• Figure 1: NIKA2 maps of M 99 at 1.15 mm (left) and 2 mm (right) obtained through the Scanam_nika data reduction pipeline in mJy/px (cutouts of $8^\prime\times6^\prime$; pixel size of $3^{\prime\prime}$). The mean RMS is 0.034 mJy/px at 1.15 mm and 0.0055 mJy/px at 2 mm. White solid lines show the $[2.5,5,7.5]\times\sigma$ contours. The FWHM is $\sim12^{\prime\prime}$ at 1.15 mm and $\sim18^{\prime\prime}$ at 2 mm, corresponding to $\sim0.8$ and $\sim1.25$ kpc (cf. white circles, bottom-left).
• Figure 2: Integrated NIR-to-radio SED modelling of M 99 with HerBIE. Filled black circles show observed photometry used in the fit; grey diamonds indicate corresponding synthetic photometry. Left panel: full SED fit (black line), including stellar emission (starBB, blue), dust emission using powerU with THEMIS (green), and radio continuum (radio, red). Right panel: fit limited to $\lambda \ge 100$$\upmu$m using a single-T MBB (MBB1, green), excluding stellar emission. Empty circles denote data not included in the fit. Residuals are shown below each panel.
• Figure 3: PHANGS environmental masks by Querejeta2021AA...656A.133Q degraded to match SPIRE 250 $\upmu$m angular resolution (i.e., 18$^{\prime\prime}\sim1.25$ kpc) and pixel size (i.e., 6$^{\prime\prime}\sim0.4$ kpc). We show the disc in cyan, the spiral arms in blue and the centre in magenta. For reference, we overlay the IRAC 3.6 $\upmu$m contours (black solid lines).
• Figure 4: NIR-to-radio SED modelling of M 99's spiral arms (blue), disc (cyan), and centre (magenta) with HerBIE. Filled circles show observed photometry; grey diamonds indicate synthetic photometry. Left panel: full SED fits (solid lines) including stellar emission (starBB, yellow areas), dust emission using powerU with THEMIS (blue, cyan, magenta areas), and radio continuum (radio, red areas). Right panel: fits restricted to $\lambda \ge 100$$\upmu$m, modelling dust with single-T MBB (MBB1, coloured areas) and excluding stellar emission. Empty circles were not included in the fit. Monochromatic luminosities ($\nu\,$L$_\nu$) assume a distance of 14.4 Mpc (see Table \ref{['tab:NGC4254prop']}). Residuals are shown below each panel.
• Figure 5: Pixel-by-pixel $\chi^2_{\rm red}$ maps at $25^{\prime\prime}$ resolution; bottom left corner). Left panel: $\chi^2_{\rm red}$ from the NIR-to-radio SED fitting with THEMIS. Right panel: $\chi^2_{\rm red}$ from the FIR-to-radio ($\lambda \geq 100$$\upmu$m) SED fitting with a single-T MBB. Insets show the $\chi^2_{\rm red}$ distribution: the black dashed vertical lines indicate a $\chi^2_{\rm red}=1$. Cyan solid lines represent the SPIRE 350 $\upmu$m contours at $[5, 15, 35, 55]\times\sigma$.