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Using extremely deep (rms 3.3 microJy/bm) 1.4GHz sub-arcsecond resolution MERLIN + VLA radio observations of a 8.5 by 8.5 field centred upon the Hubble Deep Field North, in conjunction with Spitzer 24 micron data we present an investigation of the radio-MIR correlation at very low flux densities. By stacking individual sources within these data we are able to extend the MIR-radio correlation to the extremely faint (~microJy and even sub-microJy) radio source population. Tentatively we demonstrate a small deviation from the correlation for the faintest MIR sources. We suggest that this small observed change in the gradient of the correlation is the result of a suppression of the MIR emission in faint star-forming galaxies. This deviation potentially has significant implications for using either the MIR or non-thermal radio emission as a star-formation tracer at low luminosities.
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In this paper we investigate the radio-MIR correlation at very low flux densities using extremely deep 1.4 GHz sub-arcsecond angular resolution MERLIN+VLA observations of a 8.5 by 8.5 field centred upon the Hubble Deep Field North, in conjunction with Spitzer 24micron data. From these results the MIR-radio correlation is extended to the very faint (~microJy) radio source population. Tentatively we detect a small deviation from the correlation at the faintest IR flux densities. We suggest that this small observed change in the gradient of the correlation is the result of a suppression of the MIR emission in faint star-forming galaxies. This deviation potentially has significant implications for using either the MIR or non-thermal radio emission as a star-formation tracer of very low luminosity galaxies.
We present the correlation between the far-infrared (FIR) and radio emissions from a composite sample of 72 nearby normal galaxies observed with the ISOPHOT instrument on board the Infrared Space Observatory. The galaxies in the sample have measurements at three FIR wavelengths (60, 100 and 170 micron), which allowed a direct determination of the warm and cold FIR emission components. This is the first time that the correlation has been established for the total FIR luminosity, of which most is carried by the cold dust component predominantly emitting longwards of the spectral coverage of IRAS. The slope of this correlation is slightly non-linear (1.10+/-0.03). Separate correlations between the warm and cold FIR emission components and the radio emission have also been derived. The slope of the warm FIR/radio correlation was found to be linear (1.03+/-0.03). For the cold FIR/radio correlation we found a slightly non-linear (1.13+/-0.04) slope. We qualitatively interpret the correlations in terms of star formation rate and find that both the FIR and radio emissions may be consistent with a non-linear dependence on star formation rate for galaxies not undergoing starburst activity.
We present a study of the low-frequency radio properties of star forming (SF) galaxies and active galactic nuclei (AGN) up to redshift $z=2.5$. The new spectral window probed by the Low Frequency Array (LOFAR) allows us to reconstruct the radio continuum emission from 150 MHz to 1.4 GHz to an unprecedented depth for a radio-selected sample of $1542$ galaxies in $sim 7~ rm{deg}^2$ of the LOFAR Bootes field. Using the extensive multi-wavelength dataset available in Bootes and detailed modelling of the FIR to UV spectral energy distribution (SED), we are able to separate the star-formation (N=758) and the AGN (N=784) dominated populations. We study the shape of the radio SEDs and their evolution across cosmic time and find significant differences in the spectral curvature between the SF galaxy and AGN populations. While the radio spectra of SF galaxies exhibit a weak but statistically significant flattening, AGN SEDs show a clear trend to become steeper towards lower frequencies. No evolution of the spectral curvature as a function of redshift is found for SF galaxies or AGN. We investigate the redshift evolution of the infrared-radio correlation (IRC) for SF galaxies and find that the ratio of total infrared to 1.4 GHz radio luminosities decreases with increasing redshift: $ q_{rm 1.4GHz} = (2.45 pm 0.04) times (1+z)^{-0.15 pm 0.03} $. Similarly, $q_{rm 150MHz}$ shows a redshift evolution following $ q_{rm 150GHz} = (1.72 pm 0.04) times (1+z)^{-0.22 pm 0.05}$. Calibration of the 150 MHz radio luminosity as a star formation rate tracer suggests that a single power-law extrapolation from $q_{rm 1.4GHz}$ is not an accurate approximation at all redshifts.
Wright et al. 2014 have embarked on a search for advanced Karadashev Type III civilisations via the compilation of a sample of sources with extreme mid-IR emission and colours. In this scenario, the mid-IR emission is then primarily associated with waste heat energy by-products. I apply the Mid-IR radio correlation to this $hat{G}$ sample (Griffith et al. 2015). I demonstrate that the mid-IR and radio luminosities are correlated for the sample with $q_{22}=1.35pm0.42 $. By comparison, the First Look Survey (FLS) has $q_{22}=0.87pm0.27$. The fact that the G-HAT sample largely follows the Mid-IR radio correlation, strongly suggests the vast majority of these sources are associated with galaxies in which natural astrophysical processes are dominant. This simple application of the mid-IR radio correlation can substantially reduce the number of false positives in the $hat{G}$ catalogue, since galaxies occupied by advanced Kardashev Type III civilisations would be expected to exhibit very high values of $q$. Indeed I identify 9 outliers in the sample with $q_{22} > 2$ of which at least 3 have properties that are relatively well explained via standard astrophysical interpretations e.g. dust emission associated with nascent star formation and/or nuclear activity from a heavily obscured AGN. I also note that the comparison of resolved Mid-IR and radio images of galaxies on sub-galactic (kpc) scales can also be useful in identifying and recognising artificial mid-IR emission from less advanced intermediate Type II/III civilisations. Nevertheless, from the bulk properties of the $hat{G}$ sample, I conclude that Kardashev Type-III civilisations are either very rare or do not exist in the local Universe.
We present radio and optical analysis of a sample of Low Luminosity Compact (LLC) objects, selected from FIRST survey and observed with MERLIN at L-band and C-band. The main criterion used for selection was luminosity of the objects and approximately one third of the CSS sources from the new sample have a value of radio luminosity comparable to FR,Is.The analysis of a radio properties of LLC sources show they occupy the space in radio power versus linear size diagram below the main evolutionary path of radio objects. We suggest that many of them might be short-lived objects, and their radio emission may be disrupted several times before becoming FR,IIs. The optical analysis of the LLC sources were made based on the available SDSS images and spectra. We have classified the sources as high and low excitation galaxies (HEG and LEG, respectively). The optical and radio properties of the LLC sample are in general consistent with brighter CSSs and large-scale radio sources. However, when LLC are added to the other samples, HEG and LEG seem to follow independent, parallel evolutionary tracks. LLC and luminous CSS behave like FR,II sources, while FR,I seem to belong to a different group of objects, concerning ionization mechanisms. Based on our results, we propose the independent, parallel evolutionary tracks for HEG and LEG sources, evolving from GPS - CSS - FR.
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