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Here be Spiral DRAGNs

Periodic Reporting for period 1 - Spiral DRAGNs (Here be Spiral DRAGNs)

Reporting period: 2016-05-01 to 2018-04-30

Until recently, it was thought that large double-lobed radio sources are always hosted by massive elliptical galaxies in the local Universe and that the merger phenomenon responsible for the elliptical morphology is also responsible for triggering the active galactic nuclei (AGN) at their centre. Consequently, the existence of spiral galaxies that host double-lobed radio sources, known as “spiral DRAGNs”, is in contradiction to leading galaxy formation models. However, these systems have been confirmed to exist, calling into question our current understanding of galaxy formation. In this project the role of these systems in galaxy formation is examined in detail: either reconciling their existence with current models or challenging the idea of a unified model. The project addresses both the environment in the immediate vicinity of the AGN and whether the properties of the central black hole in these spiral hosts are similar to those of the black holes hosted by elliptical galaxies, as well as the large scale environment of spiral DRAGNs to determine whether it is the environment that is responsible for their unique nature as there is already some evidence that suggests all detected spiral DRAGNs reside in high density environments. To do this the project uses radio measurements from Very Long Baseline Interferometry (VLBI) observations as well as a combination of archival and new data at multiple wavelengths.
1. The discovery of a new spiral DRAGN in data from the Giant Metrewave Radio Telescope (GMRT) at 322 MHz. This single galaxy represents an increase of 20% in the total sample size of all known spiral DRAGNs. The host galaxy, MCG+07-47-10, is a face-on late-type Sbc galaxy with distinctive spiral arms and prominent bulge suggesting a high black hole mass. Using WISE infra-red and GALEX UV data we show that this galaxy has a star formation rate of 0.16-0.75 M⊙ yr-1, and that the radio luminosity is dominated by star-formation. We demonstrate that this spiral DRAGN has similar environmental properties to others of this class, but has a comparatively low radio luminosity of L(1.4 GHz) = 1.12 × 10^22 W Hz-1, two orders of magnitude smaller than other known spiral DRAGNs. We suggest that this may indicate the existence of a previously unknown low-luminosity population of spiral DRAGNS.

2. The first observation of 0313-192, the archetypal spiral DRAGN, at very long baseline interferometry (VLBI) resolutions. 0313-192 is an edge-on spiral galaxy that appears to host a 360 kpc double-lobed radio source. The core of this galaxy is clearly detected at multiple frequencies using the Very Long Baseline Array, signifying an ongoing active nucleus in the galaxy. This rules out the possibility that the spiral DRAGN is merely a chance alignment. The radio core has L(1.4 GHz) ~ 3.0 × 10^23 W Hz-1. Radio components are detected to the south-west of the core, but there are no detections of a counter jet. Assuming a symmetric, relativistic jet, we estimate an upper limit to the inclination angle of θ ≲ 72 deg. The VLBI-detected radio jet components are extremely well aligned with the larger scale radio source suggesting little to no jet disruption or interaction with the interstellar medium of the host galaxy.

3. Investigations of the circumgalactic medium in the environment of proto-cluster galaxies from observations with the with the Atacama Large Millimeter Array and Australia Telescope Compact Array revealing a cold molecular halo. The molecular cold galactic medium was found to be metal-rich and not diffuse, confirming a link between the cold gas and in situ star formation. This demonstrated that galaxies are able to grow using recycled gas from the cold galactic medium and not necessarily only directly through accretion of gas from the cosmic web.
1. The project has increased the known sample of spiral DRAGNs by 20% with the discovery of MCG+07-47-10.

2. The project has produced the first VLBI scale image of a spiral DRAGN, revealing the central active galactic nucleus for the first time.
New Spiral DRAGN found with the GMRT (Mulcahy, Mao et al. A&A 595, L8 (2016))