Cosmic-ray electron propagation in NGC 3044 from radio continuum observations

TL;DR

The study tackles how cosmic-ray electrons propagate in the edge-on spiral NGC 3044 and whether they drive a galactic wind. It combines new 943 MHz ASKAP data with higher-frequency VLA observations to constrain CRE transport using 1D advection–diffusion models (via SPINNAKER), revealing an advection-dominated wind with a velocity profile that accelerates with height and reaches the escape threshold of $v_{ m esc}\approx 400$ km s$^{-1}$ at $z\approx 3$ kpc. The results include two-component synchrotron scale heights ($h_{\rm thin}\approx 0.43$ kpc and $h_{\rm thick}\approx 1.91$ kpc), a magnetic-field gradient ($B\sim16\mu$G midplane to $\sim6\mu$G halo), and a diminishing CRE energy density with height, implying CR-driven wind dynamics. A potential $R\approx3$ kpc superbubble is identified in the north halo, consistent with a multiphase feedback structure, while the integrated spectrum shows a non-thermal slope of $\alpha\approx-0.61$ with low-frequency turnover likely due to free-free absorption.

Abstract

Star-forming edge-on galaxies often exhibit extended halo radiation in multiple bands, providing ideal laboratories for studying the transfer of matter from the disk to the halo. We investigate the transport of cosmic-ray electrons (CREs) and the associated galactic wind, and assess their impact on the surrounding medium in NGC 3044. We obtained the NGC 3044 total intensity image at 943 MHz from the Australian SKA Pathfinder (ASKAP) observations with a resolution of 16 arcsec and an rms noise of 20 $μ$Jy beam$^{-1}$. The sensitivity is higher than the previous observations at similar frequencies. We find that the ASKAP intensity profiles perpendicular to the disk can be fit with two exponential components. The scale heights of the thin and thick disks are $0.43 \pm 0.13$ kpc and $1.91 \pm 0.26$ kpc, respectively. By jointly fitting total intensity and spectral index profiles with one-dimensional advection and diffusion models, we find that CREs are advected outward from the disk with the velocity increasing with height in a power law. Beyond $\sim3$ kpc, the velocity exceeds the escape speed of $\sim400$ km s$^{-1}$, indicating a strong wind. We further identify a possible superbubble of radius $\sim3$ kpc filled with soft X-ray emitting hot gas and surrounded by an HI shell and a bright H$α$ rim. These results demonstrate that radio continuum observations provide a powerful probe of cosmic-ray-driven winds in normal star-forming spiral galaxies.

Figures (14)

• Figure 1: Layout of the 36 ASKAP beams with circles in the "closepack36" footprint configuration, overlaid on the $I$ image from CASDA. The radius of the circles is $0.9\degr$, which is approximately the primary beam width. The blue circles indicate the beams that cover NGC 3044.
• Figure 2: Total intensity images from ASKAP at 943 MHz with an rms noise of $\rm 20\, \mu Jy\,beam^{-1}$ (top left panel), from VLA at 1.57 GHz with an rms noise of $\rm 30\, \mu Jy\,beam^{-1}$ (top right panel) and at 6 GHz with an rms noise of $\rm 10\,\mu Jy\,beam^{-1}$ (bottom left panel). The VLA images have been convolved to the ASKAP resolution of $16\arcsec$. The ASKAP total intensity contours overlaid on an SDSS u, g, and r three-color image is shown in the bottom right panel. The Contours levels are at $3\sigma\times2^n$ ($n=0,\,1,\,2,\,\ldots$). The region of the superbubble is outlined by the yellow dashed semicircle.
• Figure 3: Rotated ASKAP synchrotron emission intensity image of NGC 3044. The definition of $x$ and $z$ axes is outlined. The vertical strips split into rectangles are also overlaid. Each rectangular box has a width of $43\arcsec$ (4.2 kpc) and a height of $5\farcs3$ (0.5 kpc).
• Figure 4: Intensity profiles of the strips outlined in Fig. \ref{['fig:cover']}. The red curves show the two-component exponential fits, performed only on data points above $3\sigma$ ($\sigma=20\,\mathrm{\mu Jy\,beam^{-1}}$).
• Figure 5: Top: Scale heights of the thin disk (blue circles) and thick disk (orange triangles) of NGC 3044 at 943 MHz. Bottom: Thin-to-thick disk intensity ratios as a function of position along the major axis, showing the mid-plane (peak) ratio, $I_{\rm thin}(0)/I_{\rm thick}(0)$, and the vertically integrated (area) ratio. The dashed line marks equal contributions from the two components.