Arp 299A: The Movie

Watch the summary of the main results of my team in the Luminous Infrared Galaxy Arp 299-A, where we have found an extremely prolific factory of massive (more than eight solar masses) stars. The movie spans about two and a half years of observations with the European VLBI Network (EVN), at a wavelength of 6 cm, reaching milliarcsecond angular resolution (corresponding to about 1.5 pc linear resolution at the distance of Arp 299-A (D=44.8 Mpc). We are witnessing their death right now, at a rate of about 1 supernova year, which puts into trouble standard wisdom about converting far-infrared luminosity into supernova rates. Further, we also discovered the exact location of the supermassive black hole (A1 in the movie) in the nucleus of Arp 299-A. Very interestingly, a supernova that popped-off in 2003 (A0 in the movie) is just 2 pc away from the galaxy monster, showing the existence of a rather peaceful relationship between the AGN and the powerful starburst, which is concetrated in the innermost ~100 pc.


An extremely prolific supernova factory in the buried nucleus of the starburst galaxy IC694

The central kiloparsec of many local luminous infrared galaxies are known to host intense bursts of massive star formation, leading to numerous explosions of core-collapse supernovae. However, the dust-enshrouded regions where those supernovae explode hamper their detection at optical and near-infrared wavelengths.

We investigated the nuclear region of the starburst galaxy IC 694 (=Arp 299-A) at radio wavelengths, aimed at discovering recently exploded core-collapse supernovae, as well as determining their rate of explosion, which carries crucial information about star formation rates, the initial mass function, and the starburst processes in action.

We used the electronic European VLBI Network (eEVN) to image with milliarcsecond resolution the 5.0 GHz compact radio emission of the innermost nuclear region of IC 694. Our observations detected a rich cluster of 26 compact radio emitting sources in the central 150 pc of the nuclear starburst in IC 694. The high brightness temperatures observed for the compact sources are indicative of a non-thermal origin for the observed radio emission, implying that most, if not all, of those sources are young radio supernovae and supernova remnants. We found evidence of at least three relatively young, slowly evolving, long-lasting radio supernovae (A0, A12, and A15 in the image) that appear to have unusual CCSN properties, suggesting that the conditions in the local circumstellar medium (CSM) play a significant role in determining the radio behaviour of expanding SNe. Their radio luminosities are typical of normal RSNe, which result from the explosion of type IIP/b and type IIL SNe. All of these results provide support for a recent (less than 10-15 Myr) instantaneous starburst in the innermost regions of IC 694, and confirm that the inner regions of Arp 299-A are an extremely prolific supernova factory.

The above results appeared published as a Letter to A&A (A&A, 507, L17) in November 2009 and are part of a three-year long monitoring of Arp 299-A, using both the full EVN, as well as the eEVN, aimed at directly determining the core-collapse supernova of the galaxy - model independently - and testing the standard far infra-red/core-collapse supernovae relationships.

Authors: Miguel A. Perez-Torres (1), Cristina Romero-Canizales (1), Antxon Alberdi (1), Antonis Polatidis(2,3) (2008, A&A, 507, L17)

1 Instituto de Astrofisica de Andalucia (IAA-CSIC), Granada, Spain
2 ASTRON, Dwingeloo, The Netherlands
3 JIVE, Dwingeloo, The Netherlands


Caption: Top: 5 GHz VLA archival observations of Arp 299 on 24 October 2000, displaying the five brightest knots of radio emission in this merging galaxy. Middle and bottom: contour maps drawn at five times the rms of our 5 GHz eEVN observations of the central 500 light years of the luminous infrared galaxy Arp 299-A on 8 April 2008 and 5 December 2008, revealing a large population of relatively bright, compact, non-thermal emitting sources. The size of the FWHM synthesized interferometric beam was (0.6 arcsec x 0.4 arcsec) for the VLA observations, and (7.3 milliarcsec x 6.3 milliarcsec) and (8.6 milliarcsec x 8.4 milliarcsec) for the EVN observations on 8 April 2008 and 5 December 2008, respectively. To guide the reader's eye, we show in cyan the components detected only at the 5 December 2008 epoch. The attained off-source root-mean-square (rms) noise level was 39 microJy/beam and 24 microJy/beam for the 8 April 2008 and 5 December 2008 observations, respectively, and enables 26 compact components to be detected above 5 rms. The peak of brightness in the images is of ~900 microJy/beam, corresponding to component A1 at both epochs.

3. EVN Scientific Highlights

Serendipitous discovery of the long-sought AGN in Arp 299-A

Within the constellation of Ursa Major, at a distance of 44 Mpc lies the luminous infrared galaxy Arp 299, which is the result of the merging of two galaxies, IC 694 (also known as Arp 299-A) and NGC 3690 (=Arp 299-B1). The large far-infrared luminosity of Arp 299-A, which accounts for about 50% of the total in Arp 299, should result in about 1 SN/yr if standard relations between far-infrared luminosity and core-collapse supernova (CCSN) rates hold.

We observed Arp 299-A with the eEVN in 2008, and found indeed a large number of compact sources in its inner 150 pc, most likely young CCSNe and supernova remnants (SNRs) (see EVN Newsletter No. 25 and Perez-Torres et al. 2009, A&A, 507, L17). We therefore requested further observations, this time using the full-EVN at 1.7 and 5.0 GHz, to characterize the nature of all these compact sources and with the final aim of determining the core-collapse supernova rate for this galaxy after a three-year long monitoring.

Fig. 1 shows an "spin-off" result from these observations. The top panel shows the inner 150 pc region of Arp 299-A as imaged with the full EVN at 5.0 GHz, with all the white blobs being the compact sources we wanted to characterize. What caught our interest was the line of objects toward the top right of this image (the A1-A5 region). This was not a chance alignment, but could either be a chain of supernovae and SNRs in a super star cluster approximately 500 years old, or a core and jet of a hidden AGN. The chances that the objects belong to such superstar cluster are less than 3 in a million, and therefore we ruled out this possibility. On the other hand, the morphology, radio luminosity, spectral index and ratio of radio-to-X-ray emission of the A1~VA5 region are consistent with a low-luminosity AGN (LLAGN). We therefore identified the A1~VA5 region with the long-sought AGN in Arp 299-A.

Interestingly, the object labelled A0 is not part of the AGN. Actually, it is a supernova discovered in 2004 by Neff and collaborators. At a mere projected distance of less than 2 pc from the AGN, it is one of the closest to a central supermassive black hole ever detected. And we hypothesize that this vicinity of massive stars could be the reason why Arp 299-A AGN is so weak ~V nearby massive stars heating their surroundings and dispelling the material that it would normally accrete.

Full details can be found in A&A ( Perez-Torres et al. 2010, A&A, 519, L5), where our Figure is one of the three highlights for the Vol. 519 of the September 2010 issue.

Authors: Miguel A. Perez-Torres, Antonio Alberdi, Cristina Romero-Canizales (IAA-CSIC,Granada, Spain), and Marco Bondi (IRA-INAF, Bologna, Italy)


3. EVN Scientific Highlights

Multi-epoch EVN observations of Arp299-A

Arp299-A is a luminous infrared galaxies at a distance of 45 Mpc hosting recent and intense star-forming activity as indicated by the relatively high frequency of supernovae (SNe) discovered at optical and near-infrared (NIR) wavelengths in its outer, less extinguished, regions. The innermost 150 pc nuclear region of Arp299-A is so dusty that even NIR observations will miss a significant fraction of SNe. Only VLBI observations couple the necessary angular resolution and high sensitivity to detect new radio SNe, i.e. core-collapse SNe, allowing to measure the SN rate directly and indipendently of models.


Figure. EVN image at 5 GHz of Arp299-A obtained by stacking all six epochs. The resolution is (10 x 8) mas. The peak is 907 microJy/beam and the 1-sigma sensitivity is 18.5 microJy/beam.

The EVN has been monitoring Arp299-A at 6 cm with one observation every 6 months since April 2008, and partial results have been published in Perez-Torres et al. (Letters to A&A, 2009; see also EVN Newsletter No. 25 - Jan 2010) and Perez-Torres et al. (Letters to A&A, 2010; see also EVN Newsletter No. 28 - Jan 2011). The outcome of the first 6 epochs of observations is illustrated in Figure 1, which shows the EVN image at 5 GHz of Arp299-A obtained stacking all six epochs, the deepest image ever of Arp 299-A at this frequency. The image has a resolution of (10 x 8 mas; i.e. 2.2 x 1.7 pc) and a 1-sigma sensitivity of 18.5 microJy/beam. Twenty-six compact sources are detected in a region of about (150 x 110) pc, eight of which are new objects. The radio luminosity, time variability behaviour and spectral index (derived using previous observations) of the compact objects are consistent with them being a mixed population of core-collapse SNe and supernova remnants (SNR). We find clear evidence for at least two new CCSNe in less than two years, implying a lower limit to the CCSNe rate in the nuclear region of Arp 299-A of 0.80 SN/yr. This value is essentially the CCSN rate expected for the whole Arp 299-A galaxy, indicating that most of the SN activity is taking place in a very compact, nuclear starburst, as previously suggested by us. The monitoring of Arp 299-A using the e-EVN and the full EVN is currently underway and more spectacular results from this supernova factory are expected.

Authors: Marco Bondi (INAF-IRA), Miguel Perez-Torres (IAA-CSIC), Ruben Herrero-Illana (IAA-CSIC), Antonio Alberdi (IAA-CSIC)

Reference: Bondi et al. 2012, A & A, 539, 134.