 |
| Image: Colour composite image of Centaurus A, revealing the lobes and jets emanating from the active galaxy’s central black hole. This is a composite of images obtained with three instruments, operating at very different wavelengths. The 870-micron submillimetre data, from LABOCA on APEX, are shown in orange. X-ray data from the Chandra X-ray Observatory are shown in blue. Visible light data from the Wide Field Imager (WFI) on the MPG/ESO 2.2 m telescope located at La Silla, Chile, show the background stars and the galaxy’s characteristic dust lane in close to "true colour". Credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray) |
A maser, an acronym for 'microwave amplification by stimulated emission of radiation', is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The maser was the forerunner of the laser: they work by the same principle. The difference is that the masers generates electromagnetic waves at microwave, radio and infrared frequencies, while the laser works at optical frequencies. The laser (originally called the 'optical maser') is the acronym of 'Light Amplification by Stimulated Emission of Radiation'.
Maser-like stimulated emission has also been observed in nature from interstellar space, and it is frequently called 'superradiant emission' to distinguish it from laboratory masers. Such emission is observed from molecules such as water (H2O), hydroxyl radicals (OH), methanol (CH3OH), formaldehyde (CH2O), and silicon monoxide (SiO). Water molecules in star-forming regions can undergo a population inversion and emit radiation at about 22.0 GHz, creating the brightest spectral line in the radio universe. Some water masers also emit radiation from a vibrational mode at a frequency of about 96 GHz.
Extremely powerful masers, associated with Active Galactic Nuclei (AGN), are known as megamasers and are up to a million times more powerful than stellar masers.
An active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion - and possibly all - of the electromagnetic spectrum. A galaxy hosting an AGN is called an active galaxy. The central engine is believed to contain a super massive black hole in the range 104-1010 Msun, which converts the potential energy of matter in an accretion process to radiation and particle outflow. AGN are the most luminous persistent sources of electromagnetic radiation in the universe.
A detailed studies of luminous water masers, typically associated with the nuclear activity, allow us to investigate the innermost regions of AGN, with an impact on the still debated Unified Model for this class of objects.
The Unified Model of Active Galactic Nuclei is based on the premise the all AGN are intrinsically the same object and that the observed dichotomy between broad line ('type 1') and narrow line ('type 2') AGN is only due to the orientation relative to a dusty toroidal structure surrounding the nuclear engine (the so called 'torus').
 |
| Image: Schematic representation of our understanding of the AGN phenomenon in the unified scheme. The type of object we see depends on the viewing angle, whether or not the AGN produces a significant jet emission, and how powerful the central engine is. Broad emission lines are produced in clouds orbiting above the accretion disc. They are located typically within the zone between 0.01 to 0.1 pc. The accretion disk and the broad-line region is surrounded by a thick dusty torus. Narrow emission lines are produced in clouds located much farther from the central engine, typically between 0.3 and 30 pc. Credit: Marie-Luise Menzel. |
According to this scheme, objects viewed face-on in which the line of sight to the nucleus is clear are classified as type 1 AGN, while those observed edge-on are recognized as type 2 because the torus obscures the broad line region (BLR).
Although the existence of an axisymmetric absorber and its crucial role in determining the observed diversity among AGN has been widely confirmed its composition and geometrical structure, as well as its dynamical origin, are not yet fully understood.
In a recent paper (Castangia et al. 2016), the authors detect a new luminous water maser in the lenticular S0 galaxy IRAS15480-0344, whose origin, associated with an accretion disc or a nuclear outflow/jet, needs to be assessed.
A lenticular galaxy is a type of galaxy which is intermediate between an elliptical galaxy and a spiral galaxy in galaxy morphological classification schemes. Lenticular galaxies are disc galaxies (like spiral galaxies) which have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. As a result, they consist mainly of aging stars (like elliptical galaxies).
 | |
| Image: An example of lenticular Galaxy - NGC 5866 Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA) |
The megamaser observed in the lenticular S0 galaxy IRAS15480-0344 has a total single-dish isotropic luminosity of ~ 200 Lsun and a profile composed of two main features: a broad line and a narrow one.
The different line profiles and the spatial separation between the two features in the spectrum of IRAS15480-0344
suggests a composite origin for the maser. The large linewidth and the strong spectral variability indicate that the broad component might originate from the interaction between a radio jet and ambient molecular clouds. Based on its small linewidth and the absence of high velocity features, instead, the authors favour an outflow origin for the narrow component. This scenario is consistent with the hypothesis of the presence of strong nuclear outflows recently invoked to explain the main characteristics of field S0 galaxies.
▪ Castangia et al, 2016 A&A- Water masers in Compton-thick AGN I. Detailed study of the new water megamaser in IRAS 15480-0344 (arXiv)
▪ Beckman & Shrader, 2013 - The AGN phenomenon: open issues
Related Posts
 |
| QUASARS AND THE COSMIC REIONIZATION |
 |
| ORIGIN OF RADIO EMISSION IN RADIO QUIET QUASARS |
Comments
Post a Comment