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Below is a brief introduction to the research projects (published or submitted) I have led as a graduate researcher. For a complete list of all the published projects I've been involved in, please click here.
Below is a brief introduction to the research projects (published or submitted) I have led as a graduate researcher. For a complete list of all the published projects I've been involved in, please click here.
Our work proposes a new method to study the nature of dark matter (DM) in the close vicinity of supermassive black holes (SMBH) at sub-pc scales. Semi-analytical models predict a significant enhancement of the DM density in the region of gravitational influence of the SMBH. For galaxies at cosmological scales, the orbits of individual stars and gas clouds under the deep potential well of the SMBH remain unresolved directly, which restricts the study of their DM distribution. Using AGN with reverberation mapping based mass determinations from gas clouds arising from different ionization conditions (and thus different radii), we are able to study the total enclosed mass as a function of distance from the center.
This allows us to study directly, for the first ever time in extragalactic sources, the dark matter distribution at sub-pc scales. We find that evidence for increasing mass is indeed found in some sources (five out of fourteen), constraining the parameters of the DM component at 1-2 σ level. Furthermore, these objects also show preference for a radial steepness of ∼ 1.6, consistent with the scenario expected for a dark matter spike mildly relaxed by stellar heating processes.
RM based measurements of the enclosed mass within different radii for 3C 390 (gray points). The best-fit DM model and constant mass profile are shown in purple and dashed-pink, respectively.
Confidence contours on the dark matter profile parameters—density and radial slope. In horizontal markings the expected profile indexes for different theoretical expectations are shown.
Quasar outflows can play a crucial role in the evolution of their host galaxies through various feedback processes. This effect is expected to be particularly important when the universe was only 2-3 billion years old, during the period known as cosmic noon. By utilizing existing observations from the Dark Energy Spectroscopy Instrument (DESI), we conduct a survey of high-ionization quasar outflows at cosmic noon, with the aim of doubling the current sample of such outflows with distance and energetics determination.
The detected outflows show complex kinematic structures with a wide range in blueshifted velocities (100 - 4600 km/s). We locate five outflows at distances between 240 - 5500 pc away from the central source, while only upper limits could be obtained for two outflows, placing them closer than 100 and 900 pc. From the combined sample of 15 high-ionization S IV outflows at cosmic noon, we find a high fraction (up to 46%) of them to be powerful enough to contribute significantly to multi-stage AGN feedback processes. Their energetics are also found to be consistent with spatially resolved emission outflows in a luminosity and redshift matched sample of quasars.
Mass outflow rates as a function of the AGN bolometric luminosity (adapted from Fig. 9 of Carniani et. al. 2015, with permission). The teal stars represent our combined S IV sample seen in absorption. The open and filled circles represent the mass outflow rates for ionized outflows determined from [O III] and Hβ emission lines, respectively. The squares denote molecular outflows.
Observational signatures of quasar outflows are studied using two main techniques: (a) Line of sight (Absorption) Spectroscopy and (b) Integral Field (Emission) Spectroscopy. However, they have never been used together to study the same outflow phase in a quasar, and thus we do not know if these techniques even trace the same gas.
This work (along with its companion paper: Zhao. et. al. 2025) presents the results of applying both these analysis methods to the same quasar outflow. For the first time ever, we find consistent distance and velocity determinations that show that we can study the same gas using both these techniques. This finding provides the long missing link needed to combine the strengths of the two methods and develop a deeper understanding of quasar outflows and their impact on the host galaxies.
Median velocity map of the blueshifted component of the ionized outflow seen in emission. (VLT/SINFONI)
Observed blueshifted absorption troughs for the Si II/II* transitions along with their Gaussian models (VLT/XShooter)
In this work, we analyse the VLT/UVES spectrum of SDSS J0932+0840 and identify several narrow and broad outflow components in absorption, with multiple ionization species including Fe II (singly ionized Iron), which puts it amongst a rare class of outflows known as FeLoBALs.
We study one of the outflow components to determine its physical characteristics. We do so by determining the total hydrogen column density, ionization parameter and number density (of both electrons and hydrogen independently) through detailed photoionization modeling. Finally, we study multiple epochs of SDSS spectra to reveal an interesting pattern of variability in the outflowing troughs, which we explain in terms of a possible change in the ionization state of the gas.
Left: Photoionization modeling of the population ratios for different excited states of Fe II.
Right: Variability in the absorption troughs observed roughly 8 years apart by SDSS.
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