We report results from a Giant Metrewave Radio Telescope search for "associated" redshifted HI 21 cm absorption from 24 active galactic nuclei (AGNs), at 1.1 < z < 3.6, selected from the Caltech-Jodrell Bank Flat-spectrum (CJF) sample. 22 out of 23 sources with usable data showed no evidence of absorption, with typical 3σ optical depth detection limits of ≈ 0.01 at a velocity resolution of ≈ 30 km s −1 . A single tentative absorption detection was obtained at z ≈ 3.530 towards TXS 0604+728. If confirmed, this would be the highest redshift at which HI 21 cm absorption has ever been detected.Including 29 CJF sources with searches for redshifted HI 21 cm absorption in the literature, mostly at z < 1, we construct a sample of 52 uniformly-selected flat-spectrum sources. A Peto-Prentice two-sample test for censored data finds (at ≈ 3σ significance) that the strength of HI 21 cm absorption is weaker in the high-z sample than in the low-z sample; this is the first statistically significant evidence for redshift evolution in the strength of HI 21 cm absorption in a uniformly selected AGN sample. However, the two-sample test also finds that the HI 21 cm absorption strength is higher in AGNs with low ultraviolet or radio luminosities, at ≈ 3.4σ significance. The fact that the higher-luminosity AGNs of the sample typically lie at high redshifts implies that it is currently not possible to break the degeneracy between AGN luminosity and redshift evolution as the primary cause of the low HI 21 cm opacities in high-redshift, high-luminosity active galactic nuclei.

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We present high resolution Hi 21cm Giant Meterwave Radio Telescope (GMRT) observations of the superthin galaxy FGC1540 with a spatial resolution of 10 × 8 and a spectral resolution of 1.73 kms −1 and an rms noise of 0.9 mJy per beam. We obtain its rotation curve as well as deprojected radial Hi surface density profile by fitting a 3-dimensional tilted ring model directly to the Hi data cubes by using the publicly-available software, Fully Automated Tirrific (FAT). We also present the rotation curve of FGC1540 derived from its optical spectroscopy study using the 6-m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. We use the rotation curve, the Hi surface density profile together with Spitzer 3.6 µm and the SDSS i-band data to construct the mass models for FGC1540. We find that both the Pseudo-isothermal (PIS), as well as Navarro-Frenk-White (NFW) dark matter (DM) halos, fit the observed rotation curve equally well. The PIS model indicates a compact dark matter halo (R C /R D < 2), with the best-fitting core radius (R C ) approximately half the exponential stellar disc scale length (R D ), which is in agreement with the mass models of superthin galaxies studied earlier in the literature. Since the vertical thickness of the galactic stellar disc is determined by a balance between the net gravitational field and the velocity dispersion in the vertical direction, the compact dark matter halo may be primarily responsible in regulating the superthin vertical structure of the stellar disc in FGC1540 as was found in case of the superthin galaxy UGC7321.