22 research outputs found
LOFAR/H-ATLAS: The low-frequency radio luminosity - star-formation rate relation
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.Radio emission is a key indicator of star-formation activity in galaxies, but the radio luminosity-star formation relation has to date been studied almost exclusively at frequencies of 1.4 GHz or above. At lower radio frequencies the effects of thermal radio emission are greatly reduced, and so we would expect the radio emission observed to be completely dominated by synchrotron radiation from supernova-generated cosmic rays. As part of the LOFAR Surveys Key Science project, the Herschel-ATLAS NGP field has been surveyed with LOFAR at an effective frequency of 150 MHz. We select a sample from the MPA-JHU catalogue of SDSS galaxies in this area: the combination of Herschel, optical and mid-infrared data enable us to derive star-formation rates (SFRs) for our sources using spectral energy distribution fitting, allowing a detailed study of the low-frequency radio luminosity--star-formation relation in the nearby Universe. For those objects selected as star-forming galaxies (SFGs) using optical emission line diagnostics, we find a tight relationship between the 150 MHz radio luminosity () and SFR. Interestingly, we find that a single power-law relationship between and SFR is not a good description of all SFGs: a broken power law model provides a better fit. This may indicate an additional mechanism for the generation of radio-emitting cosmic rays. Also, at given SFR, the radio luminosity depends on the stellar mass of the galaxy. Objects which were not classified as SFGs have higher 150-MHz radio luminosity than would be expected given their SFR, implying an important role for low-level active galactic nucleus activity.Peer reviewedFinal Published versio
Magnetic field evolution in interacting galaxies
Violent gravitational interactions can change the morphologies of galaxies
and, by means of merging, transform them into elliptical galaxies. We aim to
investigate how they affect the evolution of galactic magnetic fields. We
selected 16 systems of interacting galaxies and compared their radio emission
and estimated magnetic field strengths with their star-forming activity,
far-infrared emission, and the stage of tidal interaction. We find a general
evolution of magnetic fields: for weak interactions the strength of magnetic
field is almost constant (10-15muG) as interaction advances, then it increases
up to 2x, peaks at the nuclear coalescence (25muG), and decreases again, down
to 5-6muG, for the post-merger remnants. The magnetic field strength for whole
galaxies is weakly affected by the star formation rate (SFR), while the
dependence is higher for galactic centres. We show that the morphological
distortions visible in the radio total and polarized emission do not depend
statistically on the global or local SFRs, while they do increase with the
advance of interaction. The constructed radio-far-infrared relations for
interacting and non-interacting galaxies display a similar balance between the
generation of cosmic rays, magnetic fields, and the production of the thermal
energy and dust radiation. The process of strong gravitational interactions can
efficiently magnetize the merger's surroundings, having a similar magnetizing
effect on intergalactic medium as supernova explosions or galactic winds. If
interacting galaxies generate some ultra-high energy cosmic rays (UHECRs), the
disk or magnetized outflows can deflect them (up to 23 degrees), and make an
association of the observed UHECRs with the sites of their origin very
uncertain.Comment: 17 pages, 16 figures, 5 tables. Published in Astronomy and
Astrophysics, minor changes to v