Supplementary Material to:

An Introduction to Radio Astronomy

4^th edition Cambridge University Press 2019

Last updated 04/12/2021

Chapter 16 Active Galaxies

Starburst Galaxies

M82 and its supernovae

Radio and infra-red spectrum of the nearby (distance 3.5 Mpc) starburst galaxy M82 (this is a replacement for Fig 16.2 - see Errata on opening page):

The dotted lines represent emission from different radiation mechanisms:

· cyan: synchtrotron

· red: free-free

· blue: thermal

Figure from Peel et al 2011 MNRAS 416, L99 with permission.

Images of the starburst galaxy M82: top) an optical image, taken with the Subaru telescope, of the nearby starburst galaxy M82; bottom) a MERLIN/VLA radio image of the very central regions (~500 pc) of the galaxy. The optical image shows hydrogen gas (red) being flung out in directions above and below the main disc of the galaxy. The radio waves see through to the dust enshrouded core and reveal a population of very young supernova remnants (the bright points in this image); the inset shows how one of them has expanded over a decade or so. It is this high rate of supernova explosions which drives the galactic wind seen in the optical image. (image and text from http://www.jb.man.ac.uk/distance/radio/course/sourcesII/sourcesII5.html).

M82 at higher angular resolution. This false colour radio image has a higher resolution (100 milliarsec) was made from a combination of data from eMERLIN and the JVLA at C-Band (~5 GHz). These images complement Fig 13.8 and Fig 16.1 in the main text. (Image credit: Tom Muxlow)

Arp 220 and its supernovae

Another much studied starburst galaxy is Arp 220 at a distance 76 Mpc. Its supernova rate is ten times higher than M82 (see e.g. Lonsdale et al 2006 ApJ, 647, No 1; see also https://arxiv.org/abs/astro-ph/0604570). A further representation of the Arp220 as a “supernova factory” can be seen at https://www.flickr.com/photos/onsala/6194194999

Active Galactic Nuclei (AGN):

Review papers on central engines and relativistic jets

We recommend the conference introduction paper “Black Holes as Cosmic Dynamos” by Roger Blandford https://arxiv.org/pdf/1901.05164.pdf which provides accessible additions to the discussion of “central engines” in Chapter 16. For a fuller research-level discussion we recommend the extensive illustrated review “Relativistic Jets in Active Galactic Nuclei” by R.D. Blandford, D. M. Meier and A.C.S Readhead in Ann Rev. Astro. Astrophys 2018 – see also https://arxiv.org/pdf/1812.06025.pdf.)

Extended double radio sources

Thousands of images of extended radio sources associated with AGN have been made – in particular with the NRAO VLA/JVLA. As noted in the text a good place to start is “An Atlas of DRAGNs” compiled and maintained by P. Leahy, A. Bridle and R. Strom http://www.jb.man.ac.uk/atlas/. Another rich source of material is the NRAO Image Gallery http://images.nrao.edu/ and the NRAO Public Gallery https://public.nrao.edu/gallery/. Here we show just a few illustrative examples.

Left) an FRII source: The classic example of FRII morphology is the radio galaxy Cygnus A. This is a false colour representation of the gray-scale image (shown in Fig 16.4) by Perley et al., 1984 (Image credit NRAO/AUI)

Right) An FRI source: This is a false-colour VLA image of the source 3C449 shown in Fig 16.5; here the source is shown in its correct orientation on the sky (image courtesy of Robert Laing).

Imaging the AGN Torus in Cygnus A

Carilli et al (2019) https://arxiv.org/pdf/1904.01365.pdf have produced direct multi-frequency imaging with the VLA which strongly supports the picture of the central regions of an FRII radio source (section 16.5). In particular the images reveal a well-resolved elongated structure centred on the core and perpendicular to the radio jets whose spectrum is consistent with thermal free-free emission. Their interpretation is that the images show the outer parts of the inner circumnuclear torus.

An extended double radio galaxy: (see https://public.nrao.edu/gallery/hercules-a/). This superb image of the radio galaxy Hercules A was obtained with a combination of JVLA data from different arrays – see the entry under Chapter 10 of this Supplementary Material. (Credit: NASA, ESA, S. Baum and C. O'Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF) and the Hubble Heritage Team (STScI/AURA). For a multiwavelength view of Hercules A see http://hubblesite.org/news_release/news/2012-47.

Polarisation structure of an extended jet:

The jet in the FRI radio galaxy NGC315 mapped with the VLA; the linear polarization “vectors” indicate the direction of the magnetic field. The innermost portions of the jets are asymmetrical but on larger (kpc) scales the jets become symmetrical as the outward flow decelerates. For the astrophysical interpretation of the polarisation pattern see Worrall et al. 2007, MNRAS, 380, 2 (image courtesy of Robert Laing).

The inner jet in M87:

[Colour versions of Fig 16.9 in the main text – see section 16.3.3.]

The first 25 mas of the M87 jet imagesd with the VLBA at 43 GHz https://www.aoc.nrao.edu/~cwalker/M87/ many other images and several movies of the variations in the jet over many years can be seen at this site (courtesy R.C. Walker)

The inner 3 mas of the M87 jet imaged with the GMVA at 86 GHz. (courtesy J-Y Kim and T. Krichbaum)

The supermassive black hole in the nucleus of M87

VLBI observations with the Event Horizon Telescope (see Section 11.4.11 and Supp Mat Chapter 11) have unequivocally revealed the “shadow” of a 6 x 10⁹ solar mass black hole in the nucleus of the giant elliptical galaxy M87. The full scientific story (in 6 papers) can be followed starting from S. Doelman’s introduction at https://iopscience.iop.org/journal/2041-8205/page/Focus_on_EHT. The multi-faceted and exhaustive approach to creating the final image is explained in paper IV of the series. The reader of the text (Chapters 9 and 10) and this Supplementary Material (Chapters 9 and 10) will be able to follow much of the discussion in respect of the u,v coverage, the calibration of the visibility data; the general discussion of and modelling of the data; the use of closure phase and CLEAN in the imaging process; the importance of blind tests in establishing image credibility. The team also used a non-Fourier based approach to image formation which we have not discussed. The similarity of the results from independent methods and imaging teams gives great credence to the overall final composite image. (Image credit Event Horizon Consortium)

Interaction of radio lobes with hot intra-cluster gas

Fig 16.22 (main text) shows that the extended radio source Hercules A expands outwards through the hot X-ray emitting gas confined within a cluster of galaxies. Fig 16.22 establishes the principle but does not show details of the effect which the passage of the radio jets+lobes have on the hot cluster gas. The Chandra X-ray images of the Perseus cluster http://www-xray.ast.cam.ac.uk/papers/per_200ks.pdf (discussed in A.C. Fabian et al MNRAS 344, L43 (2003)) show shocks and ripples driven by the expanding radio lobes of Perseus A (3C84). The article “Observational Evidence for AGN Feedback” by A.C. Fabian https://arxiv.org/pdf/1204.4114.pdf has an up to date discussion of the relevant physics and a compilation of images showing radio/X-ray interactions in the Perseus cluster and around M87.

Faint Radio Sources

Radio Quiet AGN

The origin of the radio emission from radio quiet AGN (Section 16.7) remains, to an extent, controversial. A recent review of the issues and potential mechanisms has been given by Panessa et al (2019) https://arxiv.org/abs/1902.05917

Faint sources the GOODS-North field

Mosaicing (see section 11.5) extends the field-of-view of the primary antenna beam by combining a number of individual pointing centres - this is important for interferometric surveys of faint sources. The above figure shows a mosaiced image made with the VLA at 5.5 GHz using the pointing strategy described with the diagram in Supp. Mat. Chapter 11; the outer perimeter (diameter ~16.6 arcmin) traces the edges of the hexagonal pattern of beams clustered around the central one. The overall field is corrected for the primary beam response at each pointing in order correctly to recover the source flux densities across the mosaic - this process has the effect of raising the noise levels towards the edge of the mosaic whilst maintaining a flat sensitivity response across the central region and most of the field. The inset shows some faint (sub-milliJy) sources within the square outlined near the image centre. These data form part of the e-MERGE project (Muxlow et al., in preparation) which focusses on faint radio sources in the well-studied GOODS-N field (courtesy T. Muxlow).

The left hand contour plot above shows a JVLA image at L-band (centre frequency 1.5 GHz) of the strongest source in the mosaiced 5.5 GHz JVLA image above; the beam is 1.68x1.40 arcsec and the rms noise is 2.04 microJy/beam. The right hand contour plot shows the same source made with a combination of JVLA+eMERLIN data in which the resolution is 0.28 x 0.26 arcsec; the bottom contour is plotted at 3 microJy/beam and the total flux density is 1.29 milliJy. The JVLA image reveals a low surface brightness double-lobed structure characteristic of an FRI source; the addition of e-MERLIN data increases the angular resolution and sensitivity and reveals fine detail in the region of the inner south-pointing jet and core. The core has a peak brightness of 501 microJy/beam and is coincident with an I=21.6mag elliptical galaxy at a redshift of 1.0128. In the high resolution image the outer lobes are well resolved and do not contain any significant compact radio structure such as outer “hot-spots”.

The deepest e-MERGE images are obtained by combining L-band JVLA and e-MERLIN data at 1.5 GHz. In the first data release (DR1) images the rms noise is ~1.2 microJy/beam and hence 5s detections of unresolved sources can be made at a flux density of ~6 microJy. (In the second data release (DR2) the rms noise level will decrease to ~0.6 microJy/beam). With the DR1 data approximately 850 sources have been detected within a 15 arcmin diameter field in GOODS-N. Previous studies (see section 16.9.4) have shown that the percentage of starburst systems in faint source samples increases with decreasing flux density; below 100 μJy at 1.4 GHz >70% of the sources are starburst-type associated with major disc galaxies with redshifts 0.3−1.3. The e-MERGE observations confirm this trend down to ∼ 10 microJy and show that many luminous star-forming systems exhibit bright nuclear starbursts embedded within regions of extended star-formation. In addition the radio emission from a significant number of radio-quiet AGN (identified by AGN activity in other wavebands) is dominated at radio wavelengths by star-formation (Muxlow et al., in preparation) (images and text courtesy T. Muxlow).

Source counts at faint flux levels:

Euclidean-normalized source counts scaled to 1.4 GHz from Vernstrom et al (2016). The brown solid line is their P(D) count with the shaded area being the 68 per cent confidence region. For further discussion of this diagram see Vernstrom et al 2016 MNRAS, 462, 2934 (Fig 8; panel b) (also available https://arxiv.org/abs/1603.03085).

The MeerKAT 1.28 GHz DEEP-2 image by T. Mauch et al Astrophysical Journal vol 888, 61 (2020) currently provides the deepest source counts translated to 1.4 GHz. These results are discussed in a wider context by Matthews et al https://arxiv.org/pdf/2101.07827.pdf.

Prandoni (2018) https://arxiv.org/pdf/1806.10886.pdf also provides a concise overview of the faint source counts and their implications.

The MWA GLEAM survey counts at 88-200 MHz and the challenges posed by sidelobes when using phased arrays are discussed by Franzen et al (2018) https://arxiv.org/abs/1812.00666

Surveys for faint radio sources

There are several wide-area deep-field surveys for discrete radio sources currently (2018) underway. Examples are:

At low frequencies:

· The GLEAM survey with the MWA: see the following paper and references therein

https://arxiv.org/abs/1812.00666

· The LOFAR Two-metre Sky Survey (LoTSS; T.W. Shimwell et al., A&A, 598, A104, (2017)) is imaging the Northern Sky in the band 120-168 MHz: for an illustrated overview of LoTSS see http://www.astron.nl/LifeCycle2018/Documents/Talks_Session1/Williams_LifeCycle18.pdf also at https://www.lofar-surveys.org/. At the time of writing (April 2019) the survey is 35% complete; it will be completed in ~2024.

At mid-radio frequencies

· The ASKAP 1.3 GHz Evolutionary Map of the Universe (EMU) survey (see http://emu-survey.org/ and https://arxiv.org/pdf/1106.3219.pdf) will cover declination from -90 to +30 degrees at a frequency of 995 MHz and will reach an order of magnitude deeper than the VLA NVSS survey. The enormous number of sources expected to be detected (>5 x 10⁷) means that machine learning techniques will be vital for digesting the data.

· The 3-GHz VLA Sky Survey (VLASS https://public.nrao.edu/vlass/ and https://science.nrao.edu/science/surveys/vlass is well underway and the first science papers have been published.

In the (redshifted) 21cm line

· The ASKAP WALLABY survey http://www.atnf.csiro.au/research/WALLABY/ is aimed at detecting the HI emission from galaxies out to z~0.25.

· The WSRT APERTIF surveys https://www.astron.nl/radio-observatory/apertif-surveys

· The MeerKAT LADUMA survey: http://www.laduma.uct.ac.za/

Sky simulations for the SKA era

The Tiered Radio Extragalactic Continuum Simulation (T-RECS) https://arxiv.org/abs/1805.05222 (Bonaldi et al 2018) is a new simulation of the continuum radio sky in the frequency range 150 MHz to 20 GHz which models both radio loud and radio quiet AGNs and SFGs. It essentially supersedes the SKADS simulations of Wilman, et al., MNRAS, 388, 1335 (2008) referred to in Section 16.9.3. T-RECS forms the basis of several of the SKA simulated data challenges.