* Astronomy

Blobrana · L

NGC 3393

Blobrana · L

Title: Evidences of merging in the Seyfert galaxy NGC 3393 revealed by modelling the spectra
Authors: M. Contini

The discovery of two active black holes in the Seyfert galaxy NGC 3393, separated by about 490 light years, revealed a merging event. This led us to look for other evidences of galaxy collision and merging through the analysis of the observed spectra in different frequency ranges. We found preshock densities higher by a factor of about 10 in the NGC 3393 NLR than in other AGN and patches of ionised matter beyond the observed NLR bulk. They can be explained by compression and heating of the gas downstream of shock waves created by collision. Metallicity in terms of the O/H relative abundance, is about 0.78 solar. Mg/H depletion by a factor of about 3 compared with solar cannot be explained by Mg trapping into dust grains, due to rather high shock velocities. The low O/H and Mg/H abundances indicate mixing with external matter during collision. Twice solar N/H is predicted by modelling the spectra of high shock velocity clouds reached by a Ts =8.6 10^4 K black-body flux. This suggests that Wolf-Rayet stars could be created by galaxy collision in the central region.

Read more (92kb, PDF)

Blobrana · L

Title: A Close Nuclear Black Hole Pair in the Spiral Galaxy NGC 3393
Authors: G. Fabbiano, Wang Junfeng, M. Elvis, G. Risaliti

The current picture of galaxy evolution advocates co-evolution of galaxies and their nuclear massive black holes (MBHs), through accretion and merging. Quasar pairs (6,000-300,000 light-years separation) exemplify the first stages of this gravitational interaction. The final stages, through binary MBHs and final collapse with gravitational wave emission, are consistent with the sub-light-year separation MBHs inferred from optical spectra and light-variability of two quasars. The double active nuclei of few nearby galaxies with disrupted morphology and intense star formation (e.g., NGC 6240 and Mkn 463; ~2,400 and ~12,000 light-years separation respectively) demonstrate the importance of major mergers of equal mass spirals in this evolution, leading to an elliptical galaxy, as in the case of the double radio nucleus (~15 light-years separation) elliptical 0402+379. Minor mergers of galaxies with a smaller companion should be a more common occurrence, evolving into spiral galaxies with active MBH pairs, but have hitherto not been seen. Here we report the presence of two active MBHs, separated by ~430 light-years, in the Seyfert galaxy NGC 3393. The regular spiral morphology and predominantly old circum-nuclear stellar population of this galaxy, and the closeness of the MBHs embedded in the bulge, suggest the result of minor merger evolution.

Read more (316kb, PDF)

Blobrana · L

NASA'S Chandra Finds Nearest Pair Of Supermassive Black Holes

Astronomers using NASA's Chandra X-ray Observatory discovered the first pair of supermassive black holes in a spiral galaxy similar to the Milky Way. Approximately 160 million light years from Earth, the pair is the nearest known such phenomenon.
The black holes are located near the center of the spiral galaxy NGC 3393. Separated by only 490 light years, the black holes are likely the remnant of a merger of two galaxies of unequal mass a billion or more years ago.
Read more

Credit: M. Malkan (UCLA), HST, NASA

Blobrana · L

Title: The Parsec-scale Accretion Disk in NGC 3393
Authors: Paul T. Kondratko, Lincoln J. Greenhill, James M. Moran

We present a Very Long Baseline Interferometry image of the water maser emission in the nuclear region of NGC3393. The maser emission has a linear distribution oriented at a position angle of ~ -34°, perpendicular to both the kpc-scale radio jet and the axis of the narrow line region. The position-velocity diagram displays a red-blue asymmetry about the systemic velocity and the estimated dynamical centre, and is thus consistent with rotation. Assuming Keplerian rotation in an edge-on disk, we obtain an enclosed mass of (3.1 ±0.2) x 10^7 M_{\sun} within 0.36 ±0.02 pc (1.48 ±0.06 mas), which corresponds to a mean mass density of ~10^{8.2} M_{\sun} pc^{-3}. We also report the measurement with the Green Bank Telescope of a velocity drift, a manifestation of centripetal acceleration within the disk, of 5 ±1 km s^{-1} yr^{-1} in the ~3880 km s^{-1} maser feature, which is most likely located along the line of sight to the dynamical centre of the system. From the acceleration of this feature, we estimate a disk radius of 0.17 ± 0.02 pc, which is smaller than the inner disk radius (0.36 ± 0.02 pc) of emission that occurs along the midline (i.e., the line of nodes). The emission along the line of sight to the dynamical centre evidently occurs much closer to the centre than the emission from the disk midline, contrary to the situation in the archetypal maser systems NGC4258 and NGC1068. The outer radius of the disk as traced by the masers along the midline is about 1.5 pc.

Read more (2217kb, PDF)