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This page contains a single entry from the blog posted on February 8, 2012 10:35 AM.

The previous post in this blog was PhD Defense - Sanjit Karmakar.

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PhD Proposal Defense - Neetika Sharma

Neetika successfully defended her PhD Proposal on February 8, 2012.

Diagnostics of Photoionized Gas in Seyfert Galaxies

As their name implies Active Galactic Nuclei (AGN) reside in the centers of many (10%) galaxies. They are active in the sense that their spectra exhibit a broad (from radio to gamma-rays) non-stellar continuum which exceeds the luminosity of the host galaxy. Furthermore at some wavelengths intensity variations have been observed on timescales less than a day (minutes in extreme cases). The ultimate cause of this tremendous outflow of energy in the form of radiation is widely believed to be due to the accretion of matter onto a supermassive black hole (BH ~ 106-109 times the mass of the Sun). Unfortunately since all this activity takes place in such a relatively small region (<< 3lyr) the central engine of even the closest AGN cannot be imaged directly with current technology (eg. The central region of Circinus, the nearest active galaxy at a distance of about 40 million trillion miles, has an angular size of about 0.2" that is just resolvable with VLT of spatial resolution 0.1"-0.5" ). Nevertheless spectroscopic observations can help us constrain the conditions of the gas very close to the BH. Such observations reveal that, as expected from considerations of the angular momentum of the infalling matter, such material in the circumnuclear regions of an AGN is not in a spherically-symmetric distribution. The uni fied model of AGN comprises of di fferent gas components surrounding the central engine and hence that leads to the classi fication of AGN depending upon their oreintations with respect to the observer on the Earth.

My thesis focuses on constraining the physical conditions in the NLR of Seyfert galaxies. I propose to compare observational data with the predictions of a publically available software tool CLOUDY in order to further constrain the conditions within the circumnuclear regions of a number of nearby Seyfert Galaxies. Speci fically I propose to use archival data in the soft X-ray band (6-38Å) obtained by the grating spectrometers onboard the XMM-Newton and Chandra observatories for this purpose. Some Seyfert galaxies (Seyfert 2 galaxies) emit a multitude of emission lines due to H-like and He-like ions of the cosmically abundant elements (C, N, O, Ne, Mg...etc). These line intensities are far in excess of those predicted by models of the gas in the standard Uni ed Model of Active Galactic Nuclei (AGN). It has been suggested that the lines are due to an additional amount of circumnuclear material. Some of this material is actually thought be co-located with the so-called Narrow-line region (NLR) of AGN. This has been dubbed the Xray NLR (XNLR) and is probably co-located with the regular NLR. Due to the intense continuum radiation, the XNLR gas is thought to be photoionized. Understanding this X-ray emitting component of gas is important, as it will help us understand the overall structure and dynamics of the circumnuclear regions of AGN.