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Postdoctoral Research
Postdoctoral Research
The intra-cluster medium (ICM) in the centers of galaxy clusters is heavily influenced by the "feedback’’ from supermassive black holes (SMBHs) jets, which prevents catastrophic cooling and suppresses star formation. In addition to SMBH feedback, other processes such as type Ia supernovae and "sloshing’' as a result of merging substructures. All these processes can potentially drive turbulence in the ICM. Due to the limited spatial and spectral resolutions of X-ray telescopes, it has been rather challenging to observe turbulence in the hot ICM directly. Recently, we developed a new method to measure turbulence in the hot ICM using multiphase filaments as tracers. These filaments are ubiquitous in cluster centers and can be observed at very high resolution using optical and radio telescopes. We study the kinematics of the filaments by measuring their velocity structure functions (VSF) over a wide range of scales in the centers of ten well-observed galaxy clusters. The motions of the filaments are turbulent in all clusters. There is a clear correlation between features of the VSFs and the activities of the SMBHs, suggesting that SMBHs are the main driver of turbulent gas motions in the centers of galaxy clusters. In all systems, the VSF is steeper than the classical Kolmogorov expectation and the slopes vary from system to system, similar to our previous findings. Several theoretical explanations have been proposed and one of them is that the VSFs we have measured so far mostly reflect the motion of the driver (SMBH jets) rather than the cascade of turbulence. We show that in Abell 1795, the VSF of the outer filaments far from the SMBH flattens on small scales to a slope consistent with classical Kolmogorov. This suggests that in the central tens of kpc, the ICM is dominated by bulk motions induced by SMBH feedback rather than turbulence cascade. The cascade is only detectable on small scales farther out with current telescope resolution. The inferred level of turbulent heating is low, consistent with most numerical simulations.
Image credit: Ganguly et al. (2023, submitted), The Nature of the Motions of Multiphase Filaments in the Centers of Galaxy Clusters.
Background image credit: The image was produced by the Wide Field Imager of the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile.
Doctoral Research
Doctoral Research
My PhD research is focused on the study of the origin and dynamics of outflows/winds in different astrophysical systems such as low mass X-ray binaries (LMXB) and active galactic nuclei (AGN). I have done this by solving the non-adiabatic gas equations using the state of the art MHD code ATHENA++ to obtain the outflow dynamics resulting from irradiation by an AGN SED, followed by computing synthetic absorption line profiles and identifying the effects of outflow dynamics on them.
Synthetic Absorption Line Profiles from Thermally-driven Outflows Around AGN
The requirement of thermal driving places AGN outflows at parsec scales from the supermassive black hole (SMBH) at the center, thereby meeting the criteria for the so-called “warm absorber” outflows. Warm absorbers occupy a critical part of the thermal equilibrium curve (the trajectory over which the net cooling rate balances the net heating rate in the gas). We investigated the parameter space that would generate different entropy modes (waves in a non-isentropic fluid flow) eventually leading to the formation of clumps in such outflows. By adding perturbations to smooth wind solutions, clumps can be formed due to thermal instability and their effects on synthetically generated line profiles can be investigated. Our hydrodynamical models are generated using the magnetohydrodynamical code ATHENA++, coupled with self-consistent photoionization calculations using the code XSTAR ([3],[8]). The line profiles due to clumpy outflows provided us with the important result that secondary absorption features may or may not appear depending on two factors: (i) the medium is highly clumped leading to small interclump spacing, and (ii) the local cloud velocity exceeds the local thermal width leading to stronger/weaker absorptions at certain velocities (see Fig. 1). Our parsec-scale thermally driven outflows have typical temperatures of T ∼ 105 K and critical ionization parameter x ∼ 100 ergs cm s-1 ([2],[7]). This makes them viable models for warm absorbers. Future X-ray missions like XRISM and ATHENA would help distinguish absorption features upto a few km s-1, thus, providing a test for our synthetic profiles for WAs.
Follow this link to some cool time evolution movies of line profiles of these 1D outflows!!
Wind-driven Relaxation Cycles and State Transitions
In this project, I studied self-regulating windy disk models. These models have direct applications in low mass X-ray binaries and active galactic nuclei. The Compton-heated disk wind model first proposed by [1] discussed Compton heating not only as an important mechanism in driving matter from an accretion disk but also as the cause of accretion instability leading to oscillations in the mass accretion rates [6] and in turn, the disk luminosity. We wrote a python code that numerically simulated these model and studied the parameter space in which these accretion disk instabilities decayed, were stable and overgrowed [4]. We additionally assumed that the wind-launching radius was allowed to change depending on the mass accretion rate (see Fig 2). Thus, we developed a thermal-radiative model of such winds whose existence have been verified by several observations. The saturation of variation in mass loss rates in our simplistic radial model suggests that large amplitudes required for state transitions are not produced by such models.
Background image credit: [ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)]
Image credit: Ganguly et al. (2021), On Synthetic Absorption Line Profiles of Thermally Driven Winds from Active Galactic Nuclei, https://doi.org/10.3847/1538-4357/abf939
Image credit: Ganguly & Proga (2020), On the Wind-driven Relaxation Cycle in Accretion Disks, https://doi.org/10.3847/1538-4357/ab6aa0
References
References
[1] Begelman, M. C., C. F. McKee, and G. A. Shields (1983, August). Compton heated winds and coronae above accretion disks. I. Dynamics. ApJ 271, 70–88.
[2] Dannen, R. C., D. Proga, T. Waters, and S. Dyda (2020, April). Clumpy AGN Outflows due to Thermal Instability. ApJL 893(2), L34.
[3] Dyda, S., R. Dannen, T. Waters, and D. Proga (2017, June). Irradiation of astrophysical objects - SED and flux effects on thermally driven winds. MNRAS 467(4), 4161–4173.
[4] Ganguly, S. and D. Proga (2020, February). On the Wind-driven Relaxation Cycle in Accretion Disks. ApJ 890(1), 54.
[5] Ganguly, S., D. Proga, T. Waters, R. C. Dannen, S. Dyda, M. Giustini, T. Kallman, J. Raymond, J. Miller, and P. Rodriguez Hidalgo (2021, June). On Synthetic Absorption Line Profiles of Thermally Driven Winds from Active Galactic Nuclei. ApJ 914(2), 114.
[6] Shields, G. A., C. F. McKee, D. N. C. Lin, and M. C. Begelman (1986, July). Compton-heated Winds and Coronae above Accretion Disks. III. Instability and Oscillations. ApJ 306, 90.
[7] Waters, T., D. Proga, and R. Dannen (2021, June). Multiphase AGN Winds from X-Ray-irradiated Disk Atmospheres. ApJ 914(1), 62.
[8] Waters, T., D. Proga, R. Dannen, and T. R. Kallman (2017, May). Synthetic absorption lines for a clumpy medium: a spectral signature for cloud acceleration in AGN? MNRAS 467(3), 3160–3171.
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