Nearby Galaxies

(Jayaram N. Chengalur, Nimisha Kantharia, Dipanjan Mitra, Subhashis Roy, Yogesh Wadadekar, Sushma Kurapati, Omkar S. Bait, Preetish K. Mishra, Souvik Manna, Former members: Narendra Nath Patra, Aritra Basu)

“Normal” galaxies are quiescent systems that do not produce extremely energetic emission. In fact, the Milky Way is a good example of a normal galaxy! The formation and evolution of these galaxies in the Universe remains an open area in cosmology. Some of the topics of research at NCRA-TIFR include morphological evolution of galaxies, high-resolution studies of the radio-far infrared correlation in galaxies, the magnetic field and diffusion of cosmic rays in nearby galaxies, radio continuum and neutral hydrogen studies of dwarf galaxies and compact galaxy groups, as well as studies of the disk-halo connection in galaxies.

Recent Results
Angular momentum of dwarf galaxies
Mass and specific angular momentum are two fundamental physical parameters of galaxies. Kurapati et al. (2018) use high-resolution HI 21cm observations and broad band photometry to measure the baryonic mass (M) and baryonic specific angular momentum (j) of 11 dwarf galaxies that lie in the Lynx-Cancer void. They find that the specific angular momentum of void dwarf galaxies is similar to that of other dwarf galaxies in average density environments. However, all dwarf galaxies (regardless of environment) have significantly higher specific angular momentum than expected from an extrapolation of the relation between specific angular momentum and baryonic mass for large spiral galaxies. The figure shows the difference between the observed specific angular momentum of dwarf galaxies and the specific angular momentum expected from the bulge-less spiral relation, as a function of baryonic mass. The elevation in specific angular momentum occurs for dwarf galaxies with masses lower than roughly a billion solar masses. Galaxies above this mass threshold have relatively low baryonic specific angular momenta, following the relation obtained for massive galaxies with zero bulge fraction. Interestingly, the above mass threshold is very similar to the mass below which galaxy discs start to become systematically thick. Kurapati et al. examine the possibility that both these effects, viz. the thickening of disks and the increase in specific angular momentum, are results of feedback from star formation. Such feedback would preferentially remove the low angular momentum gas from the central parts of dwarfs (thus increasing the specific angular momentum of the system) and also inject mechanical energy into the system, leading to thicker discs. They find however, that the observed amount of observed star formation in their sample galaxies is insufficient to produce the observed increase in the specific angular momentum. It hence appears that some other, as yet unknown mechanism, plays a role in producing the observed enhancement in specific angular momentum.
The angular momentum content of gas-rich dwarf galaxies
A galaxy's spin is intricately connected to its morphology --- spiral galaxies spin faster and hence are thinner whereas elliptical galaxies have lower specific angular momentum and are puffier. The mass and the angular momentum of a galaxy are related via their evolutionary history. Various researchers in the past have reported a power-law scaling relation between the mass and the specific angular momentum of large spiral galaxies. Chowdhury and Chengalur used archival GMRT, VLA and WSRT HI 21cm data of five gas-rich dwarf galaxies and found that the specific angular momentum in these smaller, less massive, dwarf galaxies is significantly higher than that expected from the earlier studies of spiral disks. The figure shows the location of these dwarf galaxies in the specific angular momentum - mass plane, and compares them with the distribution of spiral galaxies. All the five gas-rich dwarf galaxies lie outside the 95% probability band of the relation for spiral galaxies. The chance probability that the dwarf galaxies belong to the same angular momentum - mass distribution as the spirals is less than one part in a million. The authors suggest two mechanisms through which the dwarfs may acquire their higher specific angular momentum: (i) preferential outflow of low angular momentum gas due to stellar feedback, and (ii) cosmic cold mode accretion, which is known to dominate in less massive galaxies.