Prof. Niranjan Thatte

Abstract:

We present a new stellar-dynamical measurement of the supermassive black hole (SMBH) mass in the nearby spiral galaxy NGC 4258 (M106), a critical benchmark for extragalactic mass measurements. We use archival James Webb Space Telescope (JWST) Near-Infrared Spectrograph (NIRSpec) integral field unit data (G235H/F170LP grating) to extract high-resolution two-dimensional stellar kinematics from the CO bandhead absorption features within the central 3″ × 3″. We extract the stellar kinematics after correcting for instrumental artifacts and separating the stellar light from the nonthermal active galactic nucleus (AGN) continuum. We employ Jeans anisotropic models to fit the observed kinematics, exploring a grid of 12 models to systematically test the impact of different assumptions for the point-spread function, stellar mass-to-light ratio profile, and orbital anisotropy. All 12 models provide broadly acceptable fits, albeit with minor differences. The ensemble median and 68% (1σ) bootstrap confidence interval of our 12 models yield a black hole mass of MBH=(4.08−0.33+0.19)×107 M⊙. This paper showcases the utility of using the full model ensemble to robustly account for systematic uncertainties, rather than relying on formal errors from a single preferred model, as has been common practice. Our result is just 5% larger than, and consistent with, the benchmark SMBH mass derived from water-maser dynamics, validating the use of NIRSpec stellar kinematics for robust SMBH mass determination. Our analysis demonstrates JWST’s ability to resolve the SMBH’s sphere of influence and deliver precise dynamical masses, even in the presence of significant AGN continuum emission.

Abstract:

Hydrocarbons play a key role in shaping the chemistry of the interstellar medium, but their enrichment and relation with carbonaceous grains and polycyclic aromatic hydrocarbons still lack clear observational constraints. Here we report on JWST NIRSpec + MIRI/MRS infrared observations (~3–28 μm) of the local ultra-luminous infrared galaxy (ULIRG) IRAS 07251−0248, which revealed the extragalactic detection of small gas-phase hydrocarbons, such as benzene (C₆H₆), triacetylene (C₆H₂), diacetylene (C₄H₂), acetylene (C₂H₂), methane (CH₄) and methyl radical (CH₃), as well as deep amorphous C–H absorptions in the solid phase. The unexpectedly high abundance of these molecules indicates an extremely rich hydrocarbon chemistry not explained by high-temperature gas-phase chemistry, ice desorption or oxygen depletion. Instead, the most plausible explanation is the erosion and fragmentation of carbonaceous grains and polycyclic aromatic hydrocarbons. This scenario is supported by the correlation between the abundance of one of their main fragmentation products, C₂H₂, and the cosmic-ray ionization rate for a sample of local ULIRGs. These hydrocarbons are outflowing at ~160 km s⁻¹, which may represent a potential formation pathway for hydrogenated amorphous grains. Our results indicate that IRAS 07251−0248 might not be unique but represents an extreme example of the commonly rich hydrocarbon chemistry prevalent in deeply obscured galactic nuclei.

Abstract:

Current spatially resolved kinematic measurements of supermassive black hole (SMBH) masses are largely confined to the local Universe (distances Mpc). We investigate the potential of the Extremely Large Telescope’s (ELT) first-light instruments, MICADO and HARMONI, to extend these dynamical measurements to galaxies at redshift . We select a sample of five bright, massive, quiescent galaxies at these redshifts, adopting their Sérsic profiles, from HST photometry, as their intrinsic surface brightness distributions. Based on these intrinsic models, we generate mock MICADO images using SimCADO and mock HARMONI integral-field spectroscopic data cubes using hsim. The HARMONI simulations utilize input stellar kinematics derived from Jeans Anisotropic Models (JAM). We then process these mock observations: the simulated MICADO images are fitted with Multi-Gaussian Expansion (MGE) to derive stellar mass models, and stellar kinematics are extracted from mock HARMONI cubes with pPXF. Finally, these derived stellar mass models and kinematics are used to constrain JAM dynamical models within a Bayesian framework. Our analysis demonstrates that SMBH masses can be recovered with an accuracy of 10 per cent. We find that MICADO can provide detailed stellar mass models with 1 hour of on-source exposure. HARMONI requires longer minimum integrations for reliable stellar kinematic measurements of SMBHs. The required on-source time scales with apparent brightness, ranging from 5–7.5 hours for galaxies at (F814W, 20–20.5 mag) to 5 hours for galaxies at (F160W, 20.8 mag). These findings highlight the ELT’s capability to push the frontier of SMBH mass measurements to , enabling crucial tests of SMBH-galaxy co-evolution at the top end of the galaxies mass function.

Abstract:

<jats:title>Abstract</jats:title> <jats:p>We have carried out a detailed analysis of the 3.4 μm spectral feature arising from Polycyclic Aromatic Hydrocarbons (PAH), using JWST archival data. For the first time in an external galaxy (NGC 6240), we have identified two distinct spectral components of the PAH 3.4 μm feature: a shorter wavelength component at 3.395 μm, which we attribute to short aliphatic chains tightly attached to the aromatic rings of the PAH molecules; and a longer wavelength feature at 3.405 μm that arises from longer, more fragile, aliphatic chains that are weakly attached to the parent PAH molecule. These longer chains are more easily destroyed by far-ultraviolet photons (&amp;gt;5eV) and PAH thermal emission only occurs where PAH molecules are shielded from more energetic photons by dense molecular gas. We see a very strong correlation in the morphology of the PAH 3.395 μm feature with the PAH 3.3 μm emission, the latter arising from robust aromatic PAH molecules. We also see an equally strong correlation between the PAH 3.405 μm morphology and the warm molecular gas, as traced by H2 vibrational lines. We show that the flux ratio PAH 3.395/PAH 3.405 &amp;lt; 0.3 corresponds strongly to regions where the PAH molecules are shielded by dense molecular gas, so that only modestly energetic UV photons penetrate to excite the PAHs. Our work shows that PAH 3.405 μm and PAH 3.395 μm emission features can provide robust diagnostics of the physical conditions of the interstellar medium in external galaxies, and can be used to quantify the energies of the photon field penetrating molecular clouds.</jats:p>

Abstract:

We present combined James Webb Space Telescope (JWST) NIRSpec and MIRI/MRS integral field spectroscopy data of the nuclear and circumnuclear regions of the highly dust obscured Seyfert 2 galaxy NGC 7582, which is part of the sample of active galactic nucleaus (AGN) in the Galaxy Activity, Torus and Outflow Survey (GATOS). Spatially resolved analysis of the pure rotational H lines (S(1)–S(7)) reveals a characteristic power-law temperature distribution in different apertures, with the two prominent southern star-forming regions exhibiting unexpectedly high molecular gas temperatures, comparable to those in the AGN powered nuclear region. We investigate potential heating mechanisms including direct AGN photoionization, UV fluorescent excitation from young star clusters, and shock excitation. We find that shock heating gives the most plausible explanation, consistent with multiple near- and mid-IR tracers and diagnostics. Using photoionization models from the PhotoDissociation Region Toolbox, we quantify the ISM conditions in the different regions, determining that the southern star-forming regions have a high density ( cm) and are irradiated by a moderate UV radiation field ( Habing). Fitting a suite of Paris-Durham shock models to the rotational H lines, as well as rovibrational 1-0 S(1), 1-0 S(2), and 2-1 S(1) H emission lines, we find that a slow ( km s−1) C-type shock is likely responsible for the elevated temperatures. Our analysis loosely favours local starburst activity as the driver of the shocks and circumnuclear gas dynamics in NGC 7582, though the possibility of an AGN jet contribution cannot be excluded.