Chaitra Narayan
Visiting Fellow
National Centre for Radio Astrophysics
Tata Institute of Fundamental Research
Savitribai Phule Pune University Campus,
Pune 411 007
Maharashtra, INDIA
Tata Institute of Fundamental Research
Savitribai Phule Pune University Campus,
Pune 411 007
Maharashtra, INDIA
Status: Left
Main Research Areas: The interstellar medium; Star formation; Structure of Galaxies; Dark matter distribution.
Biography:
Chaitra completed her B.Sc. and M.Sc. from Bangalore University in 1996 and 1998, respectively. She then joined the Joint Astronomy Programme (JAP) at the Indian Institute of Science, Bangalore, where she worked on the vertical structure of disk galaxies for her doctoral thesis. She obtained her Ph.D. in 2005 and then moved to the Astronomy Institute of Ruhr University, Bochum, as a Humboldt post-doctoral fellow. Following this, she worked as a post-doctoral fellow at the Max Planck Institute for Extraterrestrial Physics, Garching. She has also briefly worked on instrumentation at the ophthalmic camera company Forus Health, Bangalore, and on data science at Riverus Technologies, Pune, before joining NCRA-TIFR in 2019 as a post-doctoral fellow.Research description:
My work has mostly been on the vertical structure of stellar and neutral hydrogen (HI) components of disk galaxies. For my Ph.D. thesis, a semi-analytical model that solves for the vertical density distribution in a gravitationally-coupled three-component disk galaxy, was developed. This model has been successful at deriving the dark matter halo properties of normal spiral galaxies, dwarf galaxies, and low-surface brightness galaxies.
Constraining Dark Matter Halo properties using HI distribution in disk galaxies:
The HI disk thickness increases in the outer parts of a disk galaxy as the stellar surface density drops. This is known as HI flaring. A possible cause for flaring could be that the total gravitational force acting perpendicular to the disk plane decreases with radius. The contribution to the total perpendicular gravitational force comes mainly from the stellar disk, gas and the dark matter. While the stellar disk dominates within the optical region of a normal disk galaxy, the outer region is dominated by the dark matter. The disk and halo seem to dominate in different regions of a galaxy because of their different density distributions. Although both decrease from the center, the disk density decreases rapidly whereas the halo density decreases much more slowly, so that the halo extends to several times the size of the optical disk. Thus the halo plays a major role in determining the vertical disk structure beyond the optical region. This makes the outer Galactic HI layer sensitive to the mass and the density profile of the halo, and hence it can be used as a diagnostic to study the halo properties. We applied this technique to the Milky Way and find that a simple isothermal infinite halo of any shape, oblate or prolate, is not able to account for the observed flaring. Instead we show that a spherical halo with density falling faster than isothermal halo and faster than NFW halo in the outer region provides a better fit to the observed HI flaring as well as the observed rotation curve of our Galaxy.
The HI disk thickness increases in the outer parts of a disk galaxy as the stellar surface density drops. This is known as HI flaring. A possible cause for flaring could be that the total gravitational force acting perpendicular to the disk plane decreases with radius. The contribution to the total perpendicular gravitational force comes mainly from the stellar disk, gas and the dark matter. While the stellar disk dominates within the optical region of a normal disk galaxy, the outer region is dominated by the dark matter. The disk and halo seem to dominate in different regions of a galaxy because of their different density distributions. Although both decrease from the center, the disk density decreases rapidly whereas the halo density decreases much more slowly, so that the halo extends to several times the size of the optical disk. Thus the halo plays a major role in determining the vertical disk structure beyond the optical region. This makes the outer Galactic HI layer sensitive to the mass and the density profile of the halo, and hence it can be used as a diagnostic to study the halo properties. We applied this technique to the Milky Way and find that a simple isothermal infinite halo of any shape, oblate or prolate, is not able to account for the observed flaring. Instead we show that a spherical halo with density falling faster than isothermal halo and faster than NFW halo in the outer region provides a better fit to the observed HI flaring as well as the observed rotation curve of our Galaxy.
Selected publications:
1. The extended Planetary Nebula Spectrograph (ePN.S) early-type galaxy survey: The kinematic diversity of stellar halos and the relation between halo transition scale and stellar mass (C. Pulsoni et al. 2018, A&A, 618, A94) 2. The thickness of HI in galactic discs under MOdified Newtonian Dynamics: theory and application to the Galaxy (F. J. Sánchez-Salcedo, K. Saha, C. A. Narayan 2008, MNRAS, 385, 1585) 3. Constraints on the halo density profile using HI flaring in the outer Galaxy (C.A. Narayan, K. Saha, & C.J. Jog 2005, A&A, 440, 523) 4. The puzzle about the radial cut-off in galactic disks (C.A. Narayan & C.J. Jog 2003, A&A, 407, L59) 5. Origin of radially increasing stellar scaleheight in a galactic disk (C.A. Narayan & C.J. Jog 2002, A&A, 390, L35) 6. Vertical scaleheights in a gravitationally coupled, three-component Galactic disk (C.A. Narayan & C.J. Jog 2002, A&A, 394, 89) 7. Vertical distribution of Galactic disc stars and gas constrained by a molecular cloud complex (C.J. Jog & C. A. Narayan 2001, MNRAS, 327, 1021)
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