Zusammenfassung
We present the early science results from a blind search of the extragalactic
HI 21-cm absorption lines at z $łeqslant$ 0.09 with the drift-scan observation
of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We carried
out the search using the data collected in 643.8 hours by the ongoing Commensal
Radio Astronomy FasT Survey (CRAFTS), which spans a sky area of 3155 deg$^2$
and covers 44827 radio sources with a flux density greater than 12 mJy. Due to
the radio frequency interference (RFI), only the relatively clean data in the
frequency range of 1.3-1.45 GHz are used in the present work. Under the
assumption of $T_s/c_f$ = 100 K, the total completeness-corrected comoving
absorption path length spanned by our data and sensitive to Damped Lyman
$\alpha$ Absorbers (DLAs) are $\Delta X^inv$ = 8.33$\times10^3$ ($\Delta
z^inv = 7.81\times10^3$) for intervening absorption. For associated
absorption, the corresponding values are $\Delta X^asc$ = 12.8 ($\Delta
z^asc = 11.9$). Three known HI absorbers (UGC 00613, 3C 293 and 4C +27.14)
and two new HI absorbers (towards NVSS J231240-052547 and NVSS J053118+315412)
are detected blindly. We fit the HI profiles with multi-components Gaussian
functions and calculate the redshift (0.063, 0.066), width, flux density,
optical depth and HI column densities for each absorption. Our results
demonstrate the power of FAST in blindly searching HI absorbers. For absorption
towards NVSS J231240-052547, the optical counterparts are faint and currently
lack existing spectra. The most likely interpretation is that a radio-loud
active galactic nucleus (AGN) is faint in the optical as the background source,
with a faint optical absorber in between. NVSS J053118+315412 exhibits an
associated absorption with a complex profile, which may suggest unsettled gas
structures or gas accretion onto the supermassive black hole (SMBH).
Nutzer