Joanna Ramasawmy
Gather.town id
CSF19
Poster Title
Low-frequency radio spectra of star forming galaxies in the Lockman Hole
Institution
UK Astronomy Technology Centre (UKATC)
Abstract (short summary)
We investigate the radio properties of a sample of 850 μm-selected sources from the SCUBA-2 Cosmology Legacy Survey (S2CLS) using new deep, low-frequency radio imaging of the Lockman Hole field from the Low Frequency Array (LOFAR). Combining these data with additional observations at 324 MHz, 610 MHz, and 1.4 GHz from the Giant Metrewave Radio Telescope and the Jansky Very Large Array, we find a variety of radio spectral shapes and luminosities within our sample despite their similarly bright submillimetre flux densities. We characterise their spectral shapes in terms of multi-band radio spectral indices. Finding strong spectral flattening at low frequencies in ~20% of sources, we investigate the differences between sources with extremely flat low-frequency spectra and those with `normal' radio spectral indices (α > ‒0.25). As there are no other statistically significant differences between the two subgroups of our sample as split by the radio spectral index, we suggest that any differences are undetectable in galaxy-averaged properties that we can observe with our unresolved images, and likely relate to galaxy properties that we cannot resolve, on scales ≲1 kpc. We attribute the observed spectral flattening in the radio to free-free absorption, proposing that those sources with significant low-frequency spectral flattening have a clumpy distribution of star-forming gas. We estimate an average spatial extent of absorbing material of at most several hundred parsecs to produce the levels of absorption observed in the radio spectra. This estimate is consistent with the highest-resolution observations of submillimetre galaxies in the literature, which find examples of non-uniform dust distributions on scales of ~100 pc, with evidence for clumps and knots in the interstellar medium.
Plain text (extended) Summary
Taking advantage of the overlapping coverage of the Lockman Hole in both the Low Frequency Array (LOFAR) deep fields and the SCUBA-2 Cosmology Legacy survey (S2CLS) – both the deepest surveys in their respective wavelength regimes – we investigate the low-frequency radio properties of bright submillimetre galaxies (SMGs). These sources are dusty, star-forming galaxies with a median redshi of z = 2.61.
Combining these data with additional observations at 324, 610 and 1400 MHz, we find surprising diversity in the shapes of the radio spectra of this remarkably submillimetre-homogeneous sample.
To quantify the shape of the radio spectrum, we define two “colours” or spectral slopes α: a low-frequency slope between 150 and 324 MHz, and a high-frequency slope between 324 and 1400 MHz. Plotting these into a “colour-colour” type figure (see Fig. 1), we find that while the majority of sources lie in the α_low ≈ α_high ≈ -0.7 locus, the typically assumed power-law index of a star-forming galaxy's radio spectrum, ~20% of our sources have significantly flatter low-frequency spectra. We define a cut-off at α_low = -0.25, splitting our sources into two populations. Investigating characteristics of these two populations, such as distribution in luminosity and redshift, and mid-IR colour properties, we find no statistically significant differences between any of their observable properties.
We therefore infer that any difference must be on scales smaller than we can resolve. We attribute this spectral flattening to free-free absorption caused by a dense, clumpy ISM, and model the effect this absorption has on the spectrum (Fig. 2).
Figure 1 shows a “colour-colour” plot of the radio spectral indices of our sample. The dashed line marks our cutoff between extremely flat and normal spectral index sources, with the majority of sources located around the expected
alpha ≈ -0.7 locus but a non-neglible fraction (~20%)
with significantly flatter low-frequency spectral indices.
Figure 2 shows a simple model of the effects of free-free absorption on a typical star-forming galaxy's radio spectrum, taking typical values of electron density and temperature for increasing column densities of gas. The result of this in colour-colour space is shown on Figure 1 with the orange line.
Conclusions: Assuming that the spectral flattening observed is due to free-free absorption and that our sample is representative of the larger poulation of sources, our results are in agreement with a scenario in which ~20% of SMGs have dense due to dense, clumpy gas distributions, with column densities of several hundred parsecs. Very high resolution interferometric studies of large numbers of SMGs would be required to investigate this further. Our results present resolution-independent evidence from radio observations in support of this scenario of some fraction of SMGs having clumpy gas morphologies.
Combining these data with additional observations at 324, 610 and 1400 MHz, we find surprising diversity in the shapes of the radio spectra of this remarkably submillimetre-homogeneous sample.
To quantify the shape of the radio spectrum, we define two “colours” or spectral slopes α: a low-frequency slope between 150 and 324 MHz, and a high-frequency slope between 324 and 1400 MHz. Plotting these into a “colour-colour” type figure (see Fig. 1), we find that while the majority of sources lie in the α_low ≈ α_high ≈ -0.7 locus, the typically assumed power-law index of a star-forming galaxy's radio spectrum, ~20% of our sources have significantly flatter low-frequency spectra. We define a cut-off at α_low = -0.25, splitting our sources into two populations. Investigating characteristics of these two populations, such as distribution in luminosity and redshift, and mid-IR colour properties, we find no statistically significant differences between any of their observable properties.
We therefore infer that any difference must be on scales smaller than we can resolve. We attribute this spectral flattening to free-free absorption caused by a dense, clumpy ISM, and model the effect this absorption has on the spectrum (Fig. 2).
Figure 1 shows a “colour-colour” plot of the radio spectral indices of our sample. The dashed line marks our cutoff between extremely flat and normal spectral index sources, with the majority of sources located around the expected
alpha ≈ -0.7 locus but a non-neglible fraction (~20%)
with significantly flatter low-frequency spectral indices.
Figure 2 shows a simple model of the effects of free-free absorption on a typical star-forming galaxy's radio spectrum, taking typical values of electron density and temperature for increasing column densities of gas. The result of this in colour-colour space is shown on Figure 1 with the orange line.
Conclusions: Assuming that the spectral flattening observed is due to free-free absorption and that our sample is representative of the larger poulation of sources, our results are in agreement with a scenario in which ~20% of SMGs have dense due to dense, clumpy gas distributions, with column densities of several hundred parsecs. Very high resolution interferometric studies of large numbers of SMGs would be required to investigate this further. Our results present resolution-independent evidence from radio observations in support of this scenario of some fraction of SMGs having clumpy gas morphologies.
URL
joanna.ramasawmy@stfc.ac.uk
Poster file