Open AccessThe H i detection of low column density clouds and galaxies
Author(s) -
Linder Suzanne M.,
Minchin Robert F.,
Davies Jonathan I.,
Baes Maarten,
Evans Rhodri,
Roberts Sarah,
Sabatini Sabina,
Smith Rodney
Publication year - 2004
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2004.08055.x
Subject(s) - physics , range (aeronautics) , lyman limit , column (typography) , galaxy , astrophysics , magnitude (astronomy) , population , intergalactic medium , geometry , redshift , materials science , mathematics , demography , connection (principal bundle) , sociology , composite material
The HIDEEP survey was carried out in an attempt to find objects having low inferred neutral hydrogen column densities, yet it found a distribution that was strongly peaked at 10 20.65 cm −2 . In an attempt to understand this distribution and similar survey results, we model H i profiles of gas discs and use simple simulations of objects having a wide range of H i properties in the presence of an ionizing background. We find that inferred column density ( N 0 H i ) values, which are found by averaging total H i masses over some disc area, do not vary strongly with central column density ( N max ) for detectable objects, so that even a population having a wide range of N max values will give rise to a strongly peaked distribution of N 0 H i values. We find that populations of objects having a wide range of model parameters give rise to inferred column density distributions around 10 20.6±0.3 cm −2 . However, populations of fairly massive objects having a wide range of central column densities work best in reproducing the HIDEEP data, and these populations are also consistent with observed Lyman limit absorber counts. It may be necessary to look two orders of magnitude fainter than HIDEEP limits to detect ionized objects having central column densities <10 20 cm −2 , but the inferred column densities of already detected objects might be lower if their radii could be estimated more accurately.