Galaxy Formation by Cosmic Strings and Cooling of Baryonic Matter

Abstract

Today we have various kinds of scenarios for the formation of the large-scale structure in the universe (galaxy formation). Nevertheless, it is without doubt that none of them are satisfactory. Recently, much work has been done ·on galaxy formation by cosmic strings, which may be formed at a phase transition in the early universe/),2) especially by closed strings (IoopS).2)-5) A characteristic feature of galaxy formation scenarios by string loops is that virialized objects are formed at an early stage, whose feature may explain large redshifts of quasars. However, the evolution of baryonic matter (e.g., the formation of luminous pattern) has not been investigated sufficiently in these scenarios. In the present paper, we study this problem, taking into consideration the cooling processes of baryonic matter. For a cosmic model, we take a spatially fiat (Q=l) universe composed of cold dark matter with !!oM (~O.9) and baryonic matter with SJs (~O.l). Before discussing the evolution of baryonic matter, we sum up the accumulation of dark matter by string loops according to Refs. 3) and 4). It depends upon the mass ms (or rs=ms/27rfJ.) of loops. We assume spherically symmetric accumulation, approximat­ ing gravitational effect of a loop to the surrounding matter by that of a spherical shell with radius rs and mass ms. For a line density of strings, fJ., we take,GfJ.~10-7 as a standard value, which is a favourable value to explain the features ot' galaxies such as the masses and the rotation velocities. 3 ) (We sometimes use the unit c = 1.) The density distribution formed around a loop is given as follows: (a) ms> mSCl(t) ~ (GfJ.)3!2G- 1 t~~3tI/3~2 x 10 9 ( GfJ./10-7)3/2(t/to) 1/3 M0 , p(r, t)~ p(t)(rJir(t)/r)2; rs< r< rJir(t), ms/d; r< rs , (b) mSC2= mSCl(teq ) ~ 2 x 107( GfJ./10- 7 )3/2 M0 < ms < mSCl(t) , p(r, t)~ p(t)(rJlr(t)/r)9/4; rb< r< rJlr(t), p(t)(rJlr(t)/r)2; rs< r< rb, ms/d; r< rs, (c) mSC3~(GfJ.)2G-lt eq~4 X 10 4 (GfJ./10- 7 )2M 0 < ms< mSc2, p(r, t)~ p(t)(rJlr(t)/r)9/4; rc< r< rJlr(t), p(teq); r< rc ,