Far-infrared to centimeter emission of very nearby galaxies with archival data

TL;DR

The paper addresses the challenge of characterizing galaxy emission from the far-infrared to centimeter regime by assembling a uniform, 18-band SED dataset for six nearby galaxies using all-sky surveys (IRAS, DIRBE, Planck, WMAP) convolved to 1°. It implements a multi-component emission model including thermal dust, free-free, synchrotron, AME, and CMB background fluctuations, and fits the data with an eight-parameter framework via Levenberg–Marquardt and MCMC to capture parameter degeneracies. The main findings are that AME is marginal or undetectable in integrated galaxy SEDs, dust properties (β and T_d) vary significantly across galaxies, and CMB background fluctuations contribute non-negligibly to the mm–cm flux at 1° resolution, sometimes causing degeneracies with dust emissivity. The study demonstrates the importance of consistent foreground/background treatment and suggests that higher-resolution observations and facilities like SRT, QUIJOTE, and SKA will be essential to disentangle these components and to probe AME and dust physics in galaxies more accurately.

Abstract

Compared to the well-studied infrared and radio domains, galaxy emission in the millimeter (mm) - centimeter (cm) range has been less observed. In this domain, galaxy emission consists of thermal dust, free-free and synchrotron emissions with a possible additional contribution from anomalous microwave emission (AME) peaking near 1 cm.The aim of this study is to accurately characterize the integrated spectral energy distribution (SED) of galaxies in the mm-cm range. We used COBE-DIRBE, IRAS, Planck, and WMAP all-sky surveys, brought to the same resolution of $1^\circ$, to cover 18 photometric bands from 97$μ$m to 1.3 cm. Given the low angular resolution and mixing with foreground and background emission that hampers the detection of the galaxy, our sample consists of 6 of the brightest, nearby galaxies: LMC, SMC, M31, M33, NGC 253 and NGC 4945. We subtract Milky Way dust emission, distant unresolved galaxies, and foreground point sources in the fields. We fit each integrated SED with a model of thermal dust, free-free, synchrotron, AME and Cosmic Microwave Background (CMB) temperature fluctuations. The integrated SEDs of our sample of galaxies are well fitted by the model within the uncertainties, although degeneracies between the different components contributing to the mm-cm emission complicate the estimation of their individual contributions. We do not clearly detect AME in any of our target galaxies, and AME emissivity upper limits are weak compared to Galactic standards, suggesting that the signal of AME might be diluted at the scale of a whole galaxy. We infer positive CMB fluctuations in the background of 5 out of our 6 galaxies. This effect might be related to the degeneracy between the dust emissivity index and CMB fluctuations in the background, or linked to the specific spatial distribution of CMB fluctuations coupled with the low resolution and small number statistics.

Figures (10)

• Figure 1: (a) Maps of M31 at $1^{\circ}$ of resolution, in 18 photometric bands of COBE-DIRBE, IRAS, Planck and WMAP from 97 $\mathrm{\upmu}$m to 13 cm, before any subtraction of foreground and background emission. (b) Maps around M31 after the subtraction of foreground and background sources. The solid and dashed white lines delineate the on-source and background regions used for the aperture photometry, respectively.
• Figure 2: Integrated SED of M31. The gray points represent the flux densities of the galaxy with the CMB fluctuations in the background. This is the SED that is fitted with our model, together with the radio data from the literature (blue points). The black points correspond to the flux densities of the galaxy where the CMB fluctuations are subtracted with the values obtained with the best model of $\mathrm{\delta_{CMB}}$. The different emission components of our model obtained for the best fit are also represented. The blue points correspond to radio data from the literature. The green points correspond to literature data from Fritz_2012Bennett_2013planck2015. On the bottom panel, residuals between the observed and modeled flux densities of M31 are shown.
• Figure 3: Posterior probability distributions for each free parameter of the model for the SED of M31. The values of the best-fit parameters obtained with the MCMC are written above each histogram.
• Figure 4: SEDs of LMC (top) and SMC (bottom) as observed with gray points and without CMB with black points (subtracted from the best model), and radio data in light blue (from Tables \ref{['table_radio_data_lmc']} for LMC and \ref{['table_radio_data_smc']} for SMC). Data points from the literature israel_2010planck_2011 are overlaid in green. The best fit model spectra are overlaid for the global model and individual emission components. On the right, the corner plot displays probability distributions of each free parameter of the models. The values of the best-fit parameters obtained with the MCMC are written above each histogram.
• Figure 5: Same as Figure \ref{['seds_lmc_smc']} for M33. IR literature data are overlaid in green tibbs, and radio data in light blue (from Table \ref{['table_radio_data_m33']}).