Star-forming dwarf galaxies: the correlation between far-infrared and radio fluxes

Abstract

The far-infrared -radio correlation connects star formation and magnetic fields in galaxies, and has been confirmed over a large range of far-infrared / radio luminosities, both in the local Universe and even at redshifts of z ∼ 2. Recent investigations indicate that it may even hold in the regime of local dwarf galaxies, and we therefore explore here the expected behavior in the regime of star formation surface densities below 0.1 M⊙ kpc −2 yr −1 . We derive two conditions that can be particularly relevant for inducing a change in the expected correlation: a critical star formation surface density to maintain the correlation between star formation rate and the magnetic field, and a critical star formation surface density below which cosmic ray diffusion losses dominate over their injection via supernova explosions. For rotation periods shorter than 1.5 × 10 7 (H/kpc) 2 yrs, with H the scale height of the disk, the first correlation will break down before diffusion losses are relevant, as higher star formation rates are required to maintain the correlation between star formation rate and magnetic field strength. For high star formation surface densities ΣSFR, we derive a characteristic scaling of the non-thermal radio to the far-infrared / infrared emission with Σ 1/3 SFR , corresponding to a scaling of the non-thermal radio luminosity Ls with the infrared luminosity L th as L 4/3 th . The latter is expected to change when the above processes are no longer steadily maintained. In the regime of long rotation periods, we expect a transition towards a steeper scaling with Σ 2/3 SFR , implying Ls ∝ L 5/3th , while the regime of fast rotation is expected to show a considerably enhanced scatter, as a well-defined relation between star formation and magnetic field strength is not maintained. The scaling relations above explain the increasing thermal fraction of the radio emission observed within local dwarfs, and can be tested with future observations by LOFAR as well as the SKA and its precursor radio telescopes.

Radio emission during the formation of stellar clusters in M 33

“…The extended sources show a remarkably constant ratio of F 5 GHz /F th with an equal share between the thermal and nonthermal flux density. The model of Schleicher & Beck (2016) on a larger scale and for a continuous star formation rate predicts an increase of the total-to-thermal radio flux ratio with Σ SFR faster than what we observe for compact star forming regions or GMCs. Schleicher & Beck (2016) explain the increasing trend as being due to turbulent magnetic field amplification by star formation.…”

“…The model of Schleicher & Beck (2016) on a larger scale and for a continuous star formation rate predicts an increase of the total-to-thermal radio flux ratio with Σ SFR faster than what we observe for compact star forming regions or GMCs. Schleicher & Beck (2016) explain the increasing trend as being due to turbulent magnetic field amplification by star formation. However, as the authors point out, discrete injection events cannot maintain a correlation between star formation rate and magnetic field strength.…”

Tracing star formation with non-thermal radio emission

“…This coupling is reflected in the FIR-radio correlation which has been observed in the local Universe (Niklas & Beck 1997;Yun et al 2001) and seems to hold up to z ≈ 3 (Seymour et al 2009;Jarvis et al 2010;Sargent et al 2010;Bourne et al 2011;Magnelli et al 2015;Pannella et al 2015). The physical interpretation of this correlation is based on star formation, which is related to cosmic rays, and thus synchrotron emission, as well as to FIR emission, which origins from dust heated by stellar UV radiation (Bell 2003;Groves et al 2003;Schleicher & Beck 2016). It has been suggested that the FIR-radio correlation can, however, break down at high redshifts due to increasing energy losses of cosmic rays in the stronger cosmic microwave background and at higher gas densities (Schleicher & Beck 2013;Schober et al 2016).…”

What drives galactic magnetism?

“…Hence, the strength estimated from the presumably reduced synchrotron emission can be undervalued. Additionally, at such low SSFR the turbulence injection timescale (or timescale of massive star formation) can become longer than the dissipation timescale of CR electrons and brake the equipartition between magnetic fields and CRs resulting in decrease in synchrotron emission and B (see Schleicher & Beck 2016).…”