My Research Interests
My primary focus has been on studying how neutral gas survives and evolves outside galactic HI disks. I have explored various environments such as the intra-group medium, the large-scale structure as well as outskirts of the galactic disks. Depending on the conditions, we have traced the neutral gas in these environments with the 21cm HI line in absorption or emission (using GBT, VLA & VLBA) or with the Lyman alpha transition in the UV (STIS data).
Intra-group HI in Hickson Compact Groups, 2005-present
Hickson Compact Groups (HCGs) are groups of galaxies with high projected densities (comparable to cluster cores) and low velocity dispersion (about 250 km/s). Consequently, they are the sites of intense tidal interaction and galaxy evolution. These groups have a much smaller fraction of late-type (spiral and irregular) galaxies compared to that seen in the field (Rood & Williams 1989, ApJ, 339, 772; Kindl 1990. Ph.D. Thesis). Moreover, spiral galaxies associated with these groups tend to be deficient in HI, containing on an average only 40% of the HI for their optical morphology and luminosity types (Verdes-Montenegro et al. 2001, A&A, 377, 812 )
Frequent tidal encounters within the group potential rapidly disperse HI tidal debris. As a part of my Ph.D. thesis, I investigated the fate of the neutral gas in the hot intra-group medium. To do so we obtained Very Large Array (VLA) imaging data (programs AV275, AV285, AY86 and from NRAO archive) (Borthakur et al. in prep) as well as the Green Bank Telescope (GBT) spectra (program GY4) for a sample of 24 HCGs.
By comparing the spectra from the two instruments, we found significant HI excess in the GBT spectra (Borthakur et al. 2008, AIPC, 1035, 201). The excess gas is in the form of a faint diffuse neutral medium produced by the slow evolution of tidal debris in the intragroup medium (IGM). This represents an intermediate stage between tidal HI and ionized hydrogen in HI deficient systems. Further support is lent by the fact that the diffuse gas fraction increases with the evolutionary phase of the group. We showed theoretically that HI clouds of radius > 200 pc would survive in an IGM of 2 million Kelvin for more than the typical dynamical lifetime of a group. However, smaller clouds get evaporated and assimilated into the hot IGM in a much shorter timescale and thus resulting in HI deficiency.
21 cm HI Absorber in the Outer disk & Halos of Galaxies: 2007-present
Damped Lyman-alpha absorbers (DLAs) are systems with column density . They are believed to be the primarily depository (80%) of neutral gas for most part of the universe's history (Prochaska et al. 2005, ApJ, 635, 123) and are also thought to be the progenitors of modern day galaxies (Wolfe et al. 1986, ApJS, 61, 249). The connection between DLAs and the associated galaxies is crucial in understanding their role in galaxy evolution and star-formation. While the galaxies associated with the high-redshift DLA are hard to map, the origin and nature of the low-redshift DLAs can be studied in detail by identifying and imaging their host galaxies. However, at low redshift, very few DLAs have been identified so far. Part of the problem is that the Lyman-alpha transition occurs in the ultra-violet (UV) band and ground based observations for detecting DLAs
can be carried out only for high redshift systems (z > 1.7).
21 cm HI line transition of neutral hydrogen in the radio band provides a unique way of identifying these systems. It also gives important information on the cold gas component associated with DLAs. Using a recently discovered technique by Dr. Donald York and his team, small projected distance galaxy-pairs can be identified from composite SDSS spectra showing emission lines superimposed on the background QSO spectrum. In my thesis, I have carried out radio observations of such a galaxy-QSO pair with a projected distance of 2.5" ( 1.7 kpc in galaxy rest frame) using three NRAO facilities- the GBT, the VLA and the VLBA (shown in the figure). We detected a 21 cm H I absorber of column density at the redshift of the foreground galaxy. The H I absorber is dynamically similar to a single quiescent cloud and is consistent with the cold- neutral medium found in the ISM of galaxies. The galaxy is a low luminosity spiral with a star-formation rate of 0.001 Mo yr-1 kpc-2 . The foreground galaxy remains undetected in H I emission, thus suggesting that the sub-DLA systems and H I rich systems are not tracing the same underlying population.
Intragroup HI in two galaxy groups associated with a QSO absorption line system: 2008-present
Galaxy groups are expected to contain a substantial fraction of the baryons in the nearby universe (e.g., Fukugita et al. 1998, ApJ, 503, 518). The detection of substantial intragroup medium (IGM) in X-ray (Mulchaey 2000, ARA&A, 38, 289) provides a conclusive evidence of baryons associated with the group potential. However, most of the groups do not have an intra-group medium that is detectable in X-ray emission. This could be a result of the ISM being too cool, too metal poor or has a very low density. One way of detecting such a medium is through absorption lines spectroscopy. Using data from the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer and correlating with galaxy-group catalogs, a variety of absorption systems associated with low redshift groups can be identified.
Aracil et al. (2006, MNRAS, 367, 139) have identified a remarkable cluster of 13 H I Lyman alpha absorption lines spread over 1000 km/s which is associated with a galaxy-group at z = 0.0635. Similarly, cross correlating absorptions systems in the sightline PG1259+593 (Richter et al. 2004, ApJS, 153, 165), we established that a multicomponent, multiphase absorption system is associated with an isolated spiral-rich galaxy-group at z=0.0635. 
The detection of a multiple Lyman series lines along with metal lines including C III, C IV, O VI, and Si III, confirm the presence of a neutral IGM with significant metal enrichment. Such systems are unique as they give an insight into the neutral gas content in harsher environment that escapes detection in emission studies. To understand the connection between the cold gas (HI 21 cm ) and slightly warmer neutral gas in the IGM of galaxy-groups, we have mapped these two systems with the VLA in C and D configuration.
Connection between Lyman-alpha Absorbers and Galaxies: cross-correlation function: 2006-present
The Lyman alpha absorbers in the spectrum of background Quasars are highly sensitive tool to map the neutral baryonic content of the universe. Understanding how these absorbers are connected to the galaxies is crucial understanding their origin and role in galaxy evolution. Galaxies around low-redshift absorbers can be mapped in detail thus giving us the opportunity to study the connection between the absorbers and galaxies. As a part of my Ph.D. thesis, I explored the extent of association between the Lyman alpha population and galaxies using the statistical framework of "cross-correlation function". The absorption data was obtained from 10 QSO sightlines observed by the Space Telescope Imaging Spectrograph (STIS). The sightlines are chosen on the basis of QSO brightness and The sightlines are choosen on the basis of QSO brightness and SDSS coverage (up to DR7).
Our cross-correlation function shows strong correlation between galaxies and absorbers at a projected distance of 0.5 Mpc and a line of sight velocity difference of less that 35 km/s. The signal reduces as we increase the projected distance or the velocity difference. 
We also studied the variation of the strength of the cross-correlation function with galaxy properties such as - "red galaxies vs blue galaxies", "emission-line galaxies vs absorption-line galaxies" and "low stellar mass galaxies vs high stellar mass galaxies". Strongest correlation is seen between blue galaxies and absorbers. In addition, emission-line galaxies and low stellar mass galaxies show a much stronger correlated with absorbers than absorption-line galaxies and high stellar mass galaxies respectively. We also found that some of the correlations were lost if the blended absorbers were considered as a single system. This emphasizes that need of using high-resolution data for such studies and the importance of interpreting the results in terms of the quality of the data. Some of the initial results have been presented as a talk in the 4th UC Irvine Center for Cosmology Workshop in April 2008 and a journal paper is in preparation (Borthakur & Tripp 2009 in preperation).
Understanding Nulling, Drifting and Mode Changing in Pulsars: 2002-03
Pulsars are known for accurate regular pulses unmatched by any astronomical phenomenon. However, some of them do show variations in pulse profiles - phenomenon such as drifting of the sub-pulse, nulling and mode switching. I, under the supervision of Dr. Rathnasree (Nehru Planetarium, New Delhi), investigated the phenomenon of pulse drifting and nulling as different modes in pulsar emission using Arecibo Observatory data at 430 MHz and 1414 MHz for 10 pulsars. With the help of an iterative algorithm based on Karl Pearson's Coefficient of Correlation, we tried to separate the pulse profile into two stable modes. While most of the pulse profiles can be broken down into two stable modes, there are some exceptions. Most of my work focused on separating the modes of pulsars PSR 0611+22 and PSR 2303+30, which exhibited extensive mode switching. We also found that nulling and drifting observed in PSR 2303+30 can also be understood as emission modes of the pulsar. This research was presented at the Young Astronomers' Meet at the Indian Space Research Organization in Spring 2003.
Other research topics I had worked on -
- Imaging Double-double Radio Galaxy WN B0925+4200 to probe its environment: 2003
- Variation in Sunspots and the Solar magnetic field during a sunspot cycle: 2001-02
Talks