CHANG-ES XXXVII. Effects of spectral aging on radio scale heights

TL;DR

...We investigate spectral aging effects on radio halo scale heights in 16 edge-on galaxies by combining LOFAR 144 MHz and VLA 3 GHz data, fitting beam-convolved two-component models to vertical profiles to isolate thin and thick discs, and deriving thick-disc spectral indices and scale-height ratios. The mean thick-disc scale-height ratio is $1.26 \pm 0.16$, and the mean thick-disc spectral index is $\alpha_2 = -0.72 \pm 0.09$, with strong negative trends of $\alpha_2$ with total mass and moderate positive trends with the SFR-to-mass surface-density ratio, suggesting winds and advection influence CRE transport. A moderate positive correlation is found between scale-height ratio and $\alpha_2$, indicating that galaxies with flatter spectra tend to have larger low-frequency halo scale heights, consistent with wind-driven transport, while calorimetric halos show little frequency dependence; overall, scale-height ratios are well below expectations for pure advection or diffusion, implying significant adiabatic losses or energy-independent escape effects. The results support the use of scale-height ratios as proxies for cosmic-ray transport and galactic winds, but highlight the need for full radio SED modelling to disentangle loss mechanisms and quantify the relative roles of advection, diffusion, and energy losses in shaping halos.

Abstract

Context. Cosmic rays and magnetic fields play an important role for the formation and evolution of galaxies. Radio continuum observations allows us to study them in the haloes of edge-on galaxies. Aims. We explore the frequency dependence of the radio scale height which depends on cosmic ray transport and electron cooling. We test the influence of fundamental galaxy properties, such as star-formation rate (SFR), mass and size. Methods. We used radio continuum data of 16 edge-on galaxies from the Continuum HAloes in Nearby Galaxies -- an EVLA Survey (CHANG-ES). We included maps from the LOw Frequency ARray at 144 MHz and from the Jansky Very Large Array at 3 GHz with 7" angular resolution. We extracted vertical intensity profiles within the effective radio radius and fitted beam-convolved double-exponential models to separate thin and thick discs. For the thick radio discs, we computed mean spectral indices and scale-height ratios between 144 MHz and 3 GHz. Results. We find a mean scale-height ratio of 1.26 \pm 0.16. This is much lower than what we would expect for either cosmic ray diffusion or advection if synchrotron and inverse Compton losses dominate for the electrons. There is a moderate positive correlation between the ratio and spectral index of the thick disc: galaxies with high ratios have flat radio spectra. The ratio does not depend on any other galaxy parameter. The radio spectrum of the thick disc, as indicated by the radio spectral index, steepens with total mass (strong correlation) and flattens with SFR-to-mass surface density (moderate correlation). Conclusions. Galaxies with galactic winds have flat radio continuum spectra and large scale heights at low frequencies. This shows effective transport of cosmic rays in such systems.

Figures (7)

• Figure 1: Showing vertical radio continuum intensity profiles at $\unit[144]{MHz}$ of the galaxies in our sample. We show the intensity profiles of the central strip for each galaxy, with exception of NGC 2820 and 4388 where the profile in the eastern strip is shown. The left panel shows the profiles in units of []arcsec, the right panel shows them in units of []kpc in projection of the assumed distances. Solid lines represent two-component exponential model profiles after deconvolution with the effective beam. The intensities were rescaled arbitrarily to separate the profiles for clearer visualization.
• Figure 2: Location of the strips and boxes used for fitting vertical radio continuum intensity profiles overlaid on the LOFAR map of NGC 891. The red lines denote the measured scale heights of the thick disc for each strip, the blue lines denote the measured scale heights of the thin disc for each strip. We use three strips with a width in terms of the effective radius $r_e$ of $2/3\,r_{\rm e}$. For NGC 891, we calculate the average intensity in $60$ boxes per strip, each box with a height of $3\arcsec$, corresponding to approximately half of our angular resolution.
• Figure 3: Ratio of scale heights of LOFAR observations at $\unit[144]{MHz}$ and VLA $S$-band observations with errors for all galaxies of the sample. The dotted black line indicates the mean scale height ratio of the galaxies in the sample.
• Figure 4: Scale height ratio plotted against different galaxy properties. The panels show the scale height ratio versus total mass makarov_hyperleda_2014, star forming radiuswiegert_chang-es_2015, SFR surface density within $r_\star$heesen_chang-es_2025, total mass surface density scaled to our distances irwin_continuum_2012, ratio of SFR-to-mass surface density, SFR from $\mathrm{H\alpha}$ and mid-infrared vargas_chang-es_2019. Data points represent the galaxies in our sample with their associated uncertainties.
• Figure 5: Spectral index of the thick disc measured between LOFAR observations at $\unit[144]{MHz}$ and VLA $S$-band observations with errors for all galaxies of the sample. The red line represents the mean spectral index of the galaxy sample. The yellow points represent the measured spectral index in the different strips. The blue points (with error bars) denote the inverse-variance-weighted mean per strip and its standard error.