Title:	Lyman-Alpha absorption lines from minipancakes
Authors:	Nath, Biman B.
Issue Date:	Jun-1997
Publisher:	American Astronomical Society
Citation:	Astrophysical Journal, 1997, Vol. 482, p621
Abstract:	Recent numerical simulations show that many Ly alpha absorption lines of column densities NH I <~ 1015 cm-2 are produced in transient minipancakes. Such pancakes are modeled here, approximating the initial perturbation leading to the formation of the pancake as a single sinusoidal wave. The density and temperature profiles of the gas in the pancake are determined in the case where cooling and heating rate of the gas is larger than the Hubble time, which is shown to hold for zc ~ 3, where zc is the collapse redshift. The Ly alpha absorption-line profiles for a line of sight through the pancake are then calculated. The absorption lines in general have wings signifying bulk motions in the gas. It is shown that the deviation from a single Voigt profile is large for small H I column density lines, in which the effect of bulk motions is large. For lines with NH I > 1013 cm-2, high temperatures tend to wash out the signatures of bulk motion. The analytical modeling of minipancakes associated with Ly alpha forest lines---with 1013 <~ NH I <~ 1015 cm-2---gives the corresponding mass scales. In structure formation models with cold dark matter and with reheated intergalactic matter (IGM), the gas in the IGM is inhibited from falling into dark matter potential wells with very small velocity dispersions. Structures with masses larger than this limit (the Jeans mass) can have an infall of gas and form minipancakes. It is shown here that, for typical values of cosmological parameters, absorption lines with NH I ~ 1014 cm-2 correspond to structures with baryonic mass Mb ~ 1010 Msun with an overdensity of ~10 at z ~ 3. The value of NH I can change by a factor of ~3 in the course of evolution of the pancake in time. It is also shown that there is an upper limit to NH I from a pancake due to the slow recombination rate and the importance of collisional ionization at high temperatures. Minipancakes do not give rise to Ly alpha lines with NH I >~ 1014.5 cm-2, for a temperature of the IGM of T bar =104 K, J-21 = 1, and Omega IGM ~ 0.03.
Description:	Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations)
URI:	http://hdl.handle.net/2289/4449
ISSN:	0004-637X 1538-4357 (Online)
Alternative Location:	http://adsabs.harvard.edu/abs/1997ApJ...482..621N http://arxiv.org/abs/astro-ph/9701054 http://dx.doi.org/10.1086/304161
Copyright:	1997 The American Astronomical Society
Appears in Collections:	Research Papers (A&A)

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