Radio spectra and sizes of ALMA-identified submillimetre galaxies: evidence of age-related spectral curvature and cosmic ray diffusion?. (arXiv:1904.08944v1 [astro-ph.GA])

Radio spectra and sizes of ALMA-identified submillimetre galaxies: evidence of age-related spectral curvature and cosmic ray diffusion?. (arXiv:1904.08944v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Thomson_A/0/1/0/all/0/1">A.P. Thomson</a> (JBCA &amp; Durham), <a href="http://arxiv.org/find/astro-ph/1/au:+Smail_I/0/1/0/all/0/1">Ian Smail</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Swinbank_A/0/1/0/all/0/1">A.M. Swinbank</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Simpson_J/0/1/0/all/0/1">J.M. Simpson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Arumugam_V/0/1/0/all/0/1">V. Arumugam</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stach_S/0/1/0/all/0/1">S. Stach</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Murphy_E/0/1/0/all/0/1">E.J. Murphy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rujopakarn_W/0/1/0/all/0/1">W. Rujopakarn</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Almaini_O/0/1/0/all/0/1">O. Almaini</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+An_F/0/1/0/all/0/1">F. An</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Blain_A/0/1/0/all/0/1">A.W. Blain</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chen_C/0/1/0/all/0/1">C.C. Chen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cooke_E/0/1/0/all/0/1">E.A. Cooke</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dudzeviciute_U/0/1/0/all/0/1">U. Dudzeviciute</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Edge_A/0/1/0/all/0/1">A.C. Edge</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Farrah_D/0/1/0/all/0/1">D. Farrah</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gullberg_B/0/1/0/all/0/1">B. Gullberg</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hartley_W/0/1/0/all/0/1">W. Hartley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ibar_E/0/1/0/all/0/1">E. Ibar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Maltby_D/0/1/0/all/0/1">D. Maltby</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Michalowski_M/0/1/0/all/0/1">M.J. Michalowski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Simpson_C/0/1/0/all/0/1">C. Simpson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Werf_P/0/1/0/all/0/1">P. van der Werf</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wardlow_J/0/1/0/all/0/1">J.L. Wardlow</a>

We analyse the multi-frequency radio spectral properties of $41$
6GHz-detected ALMA-identified, submillimetre galaxies (SMGs), observed at
610MHz, 1.4GHz, 6GHz with GMRT and the VLA. Combining high-resolution
($sim0.5”$) 6GHz radio and ALMA $870,mu$m imaging (tracing rest-frame
$sim20$GHz, and $sim250,mu$m dust continuum), we study the
far-infrared/radio correlation via the logarithmic flux ratio $q_{rm IR}$,
measuring $langle q_{rm IR}rangle=2.19pm 0.07$ for our sample. We show that
the high-frequency radio sizes of SMGs are $sim1.8pm 0.4times$
($sim2$–$3$kpc) larger than those of the cool dust emission, and find
evidence for a subset of our sources being extended on $sim 10$kpc scales at
1.4GHz. By combining radio flux densities measured at three frequencies, we can
move beyond simple linear fits to the radio spectra of high-redshift
star-forming galaxies, and search for spectral curvature, which has been
observed in local starburst galaxies. At least a quarter (10/41) of our sample
show evidence of a spectral break, with a median $langlealpha^{1.4,{rm
GHz}}_{610,{rm GHz}}rangle=-0.61pm 0.05$, but $langlealpha^{6,{rm
GHz}}_{1.4,{rm GHz}}rangle=-0.91pm 0.05$ — a high-frequency flux deficit
relative to simple extrapolations from the low-frequency data. We explore this
result within this subset of sources in the context of age-related synchrotron
losses, showing that a combination of weak magnetic fields ($Bsim35,mu$G)
and young ages ($t_{rm SB}sim40$–$80,$Myr) for the central starburst can
reproduce the observed spectral break. Assuming these represent evolved (but
ongoing) starbursts and we are observing these systems roughly half-way through
their current episode of star formation, this implies starburst durations of
$lesssim100$Myr, in reasonable agreement with estimates derived via gas
depletion timescales.

We analyse the multi-frequency radio spectral properties of $41$
6GHz-detected ALMA-identified, submillimetre galaxies (SMGs), observed at
610MHz, 1.4GHz, 6GHz with GMRT and the VLA. Combining high-resolution
($sim0.5”$) 6GHz radio and ALMA $870,mu$m imaging (tracing rest-frame
$sim20$GHz, and $sim250,mu$m dust continuum), we study the
far-infrared/radio correlation via the logarithmic flux ratio $q_{rm IR}$,
measuring $langle q_{rm IR}rangle=2.19pm 0.07$ for our sample. We show that
the high-frequency radio sizes of SMGs are $sim1.8pm 0.4times$
($sim2$–$3$kpc) larger than those of the cool dust emission, and find
evidence for a subset of our sources being extended on $sim 10$kpc scales at
1.4GHz. By combining radio flux densities measured at three frequencies, we can
move beyond simple linear fits to the radio spectra of high-redshift
star-forming galaxies, and search for spectral curvature, which has been
observed in local starburst galaxies. At least a quarter (10/41) of our sample
show evidence of a spectral break, with a median $langlealpha^{1.4,{rm
GHz}}_{610,{rm GHz}}rangle=-0.61pm 0.05$, but $langlealpha^{6,{rm
GHz}}_{1.4,{rm GHz}}rangle=-0.91pm 0.05$ — a high-frequency flux deficit
relative to simple extrapolations from the low-frequency data. We explore this
result within this subset of sources in the context of age-related synchrotron
losses, showing that a combination of weak magnetic fields ($Bsim35,mu$G)
and young ages ($t_{rm SB}sim40$–$80,$Myr) for the central starburst can
reproduce the observed spectral break. Assuming these represent evolved (but
ongoing) starbursts and we are observing these systems roughly half-way through
their current episode of star formation, this implies starburst durations of
$lesssim100$Myr, in reasonable agreement with estimates derived via gas
depletion timescales.

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