Spectra Of Starburst Galaxies Research Articles

LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies

Aims. The shape of low-frequency radio continuum spectra of normal galaxies is not well understood, the key question being the role of physical processes such as thermal absorption in shaping them. In this work we take advantage of the LOFAR Multifrequency Snapshot Sky Survey (MSSS) to investigate such spectra for a large sample of nearby star-forming galaxies. Methods. Using the measured 150 MHz flux densities from the LOFAR MSSS survey and literature flux densities at various frequencies we have obtained integrated radio spectra for 106 galaxies characterised by different morphology and star formation rate. The spectra are explained through the use of a three-dimensional model of galaxy radio emission, and radiation transfer dependent on the galaxy viewing angle and absorption processes. Results. Our galaxies’ spectra are generally flatter at lower compared to higher frequencies: the median spectral index αlow measured between ≈50 MHz and 1.5 GHz is −0.57 ± 0.01 while the high-frequency one αhigh, calculated between 1.3 GHz and 5 GHz, is −0.77 ± 0.03. As there is no tendency for the highly inclined galaxies to have more flattened low-frequency spectra, we argue that the observed flattening is not due to thermal absorption, contradicting the suggestion of Israel & Mahoney (1990, ApJ, 352, 30). According to our modelled radio maps for M 51-like galaxies, the free-free absorption effects can be seen only below 30 MHz and in the global spectra just below 20 MHz, while in the spectra of starburst galaxies, like M 82, the flattening due to absorption is instead visible up to higher frequencies of about 150 MHz. Starbursts are however scarce in the local Universe, in accordance with the weak spectral curvature seen in the galaxies of our sample. Locally, within galactic disks, the absorption effects are distinctly visible in M 51-like galaxies as spectral flattening around 100–200 MHz in the face-on objects, and as turnovers in the edge-on ones, while in M 82-like galaxies there are strong turnovers at frequencies above 700 MHz, regardless of viewing angle. Conclusions. Our modelling of galaxy spectra suggests that the weak spectral flattening observed in the nearby galaxies studied here results principally from synchrotron spectral curvature due to cosmic ray energy losses and propagation effects. We predict much stronger effects of thermal absorption in more distant galaxies with high star formation rates. Some influence exerted by the Milky Way’s foreground on the spectra of all external galaxies is also expected at very low frequencies.

The Extraordinary Infrared Spectrum of NGC 1222 (Markarian 603)

The infrared spectra of starburst galaxies are dominated by low-excitation lines of [Ne II] and [S III], and the stellar populations deduced from these spectra appear to lack stars larger than about 35 M⊙. The only exceptions to this result until now were low-metallicity dwarf galaxies. We report our analysis of the mid-infrared spectra obtained with the Infrared Spectrograph on Spitzer of the starburst galaxy NGC 1222 (Mrk 603). NGC 1222 is a large spheroidal galaxy with a starburst nucleus that is a compact radio and infrared source, and its infrared emission is dominated by the [Ne III] line. This is the first starburst of solar or near-solar metallicity known to us that is dominated by high-excitation lines and is a likely host of high-mass stars. We model the emission with several different assumptions as to the spatial distribution of high- and low-excitation lines and find that the upper mass cutoff in this galaxy is 40-100 M⊙.

Ambiguities in Fits to the Complex X‐Ray Spectra of Starburst Galaxies

Spectral fits to X-ray data from both NGC 253 and M82 provide ambiguous results. The so-called best-fit results depend on the instrument with which the data were obtained and, obviously, on the choice of spectral model composition. We show that different spectral models can be fit equally well to BeppoSAX data of both galaxies. Metallicities are unreliable in general, with a strong dependence on the choice of model. Preference to one particular spectral model can be given only by combining spectroscopic and imaging X-ray data from all available satellites (ROSAT, ASCA, and BeppoSAX). Based on spectra of NGC 253, we demonstrate that a model consisting of two or more thermal plasma components plus a hard power-law continuum and Fe Kα line emission can explain all observations. These model components represent the integral spectrum of thermal gas and compact sources in starburst galaxies that are most likely supernova remnants and X-ray binaries. The same model can fit the X-ray data of M82, but there the evidence, from ROSAT imaging, of the existence of compact sources which might represent high-mass X-ray binaries is weaker. This implies that its hard X-ray emission, which is extended in ROSAT images, might—if truly diffuse—be dominated by a very hot (several keV energy) thermal gas component.