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Curated by RSF Research Staff

Resolved structure of a galactic active nucleus

Galactic nuclei host supermassive black holes and some of them are so bright that the central region can be more luminous than the remaining galaxy light. These nuclei are called active galactic nuclei (AGN). Much of the energy output of AGNs is of non-thermal emissions which are synchrotron emissions, Compton scattering or stimulated emission. The light or energy emitting source gives clues as to the energy mechanism.

The immediate environments of these black holes typically include a tori of dust and gas and, as material falls toward the black hole, the gas radiates copiously at all wavelengths. Although the models for these active galactic nuclei work reasonably well, it is difficult to obtain direct evidence of the inner structures of AGN because they are so far away and their dimensions are thought to be only tens to hundreds of light-years.

Nonthermal Radiation (a) Charged particles, especially fast electrons (red), emit synchrotron radiation (blue) while spiraling along magnetic field lines (black lines). This process is predominant for active galaxies. (b) Variation in the intensity of thermal and synchrotron (nonthermal) radiation with frequency. Thermal radiation, described by a blackbody curve, peaks at some frequency that depends on the temperature of the source. Nonthermal synchrotron radiation, by contrast, is most intense at low frequencies. It is independent of the temperature of the emitting object

Astronomer David Wilner and his colleagues used the ALMA millimeter telescope facility to study the nearest AGN, Arp 220, which is thought to be particularly active after having recently undergone a merger with another galaxy. The nucleus has been actively forming stars in the nuclear disk, feeding either a luminous AGN or exceptionally intense starburst at the center, and blowing out dust and gas at the same time. The nucleus is certainly in a phase of rapid evolution.

The local galaxy Arp 220, captured by the Hubble Space Telescope, is a good candidate to study galactic nuclei. The bright core of the galaxy, paired with an overlaid artist's impression of jets emanating from it, indicate that the central black hole's activity is intensifying.

The two merging nuclei are about 1200 light-years apart, and each has a rotating disk of molecular gas a few hundred light-years in scale. Vigorous star formation is evident in the region as well as at least one molecular outflow inferred from the large velocities seen. But there are numerous unresolved structural issues about these inner regions, including how gas flows to, from, and between the two merging nuclei and precisely which subregions are responsible for the dominant luminosity sources.

The team analyzed ALMA high-resolution data of Arp 220 at ∼3 mm wavelengths. They spatially resolved the continuum structure of the individual nuclei and decomposed the nuclei into plasma and dust emission to characterize the luminosity source in the western nucleus. The astronomers used these high-resolution millimeter observations to tackle these questions because thick dust, which blocks much of the view at shorter wavelengths, is relatively transparent in these bands. They report that each nucleus has two concentric components, the larger ones probably associated with starburst disks somehow activated by the black holes; the smaller ones, roughly 60 light-years in size, contribute as much as 50% of the submillimeter luminosity, nearly double the previous estimates. In fact, one of the cores alone has a luminosity of about three trillion suns, larger than the entire emission of other AGN, not to mention the relatively small volume that is producing it.

Map of the galactic nucleus of ARP 220 showing a bipolar feature.

The cores in Arp220 also seems to have a third, extended linear feature that could represent the outflow seen before only in the spectroscopic (velocity) data. A luminous active galactic nucleus is a plausible energy source for these high values while other explanations remain to be explored. The continuum image also reveals a third structural component of the western nucleus—a pair of faint spurs perpendicular to the disk major axis. It could be attributed to a bipolar outflow from the highly inclined western nuclear disk.

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