 | This article is rated Stub-class on Wikipedia's content assessment scale. It is of interest to the following WikiProjects: | |||||||||||||
| ||||||||||||||
.jpg){kind=small}
According to Galaxies in the Universe by Sparke & Gallagher (2nd. ed.), pg. 393-395, DLA clouds are 20-50kpc across, and have temperatures of ≤ 3×105K. Dividing the DLA column density by the diameter yields a space density of nearly 1 particle per cubic centimeter, most resembling the warm ISM in the Milky Way. DLA clouds evidence rapid star formation in early epochs (z ≈ 4-2). Alpha process elements, like C & Mg, are common, and more widely dispersed than the Lyman-alpha absorbing H gas; and low levels of dust (dust-to-gas ≈ 0.03 Milky Way) are sometimes seen. Inexpertly, these facts imply rapid bursts of star-formation, and core-collapse SNII, in coalescing proto-galaxies. Indeed, the relative number of "irregular" or "unsettled" galaxies was much higher before (z > 2), decreasing as galaxies "settled" or "disked-down" during (z ≈ 2-1). Cp. http://phys.org/news/2012-10-trend-galaxy-evolution-video.html 66.235.38.214 (talk) 10:25, 11 November 2012 (UTC)
- According to Formation of Structure in the Universe by Dekel & Ostriker, pg. 116, DLA clouds have velocity dispersions of 100-300 km/s; and metallicities, of 0.01 solar, deriving from core-collapse SNII. Thus, DLA clouds resemble galaxy halos, not (proto-)disks, since the oldest disk stars in our Milky Way have metallicities ten times higher (0.1 solar). The number of DLA clouds dwindles away after redshift (z ≈ 3). Perhaps intense star-formation, observed in "settling" irregular galaxies from (z ≈ 3-1), inexorably ionized proto-galaxy halo gas, so dissociating the neutral hydrogen in DLA clouds, converting them into galaxy x-ray halos, by present epoch? 66.235.38.214 (talk) 23:45, 12 November 2012 (UTC)
- DLA clouds have average column number densities, of neutral atomic hydrogen, of ≈1021 cm-2. That corresponds to column mass densities of ≈1012 Msun Mlyr-2. Now, DLA clouds correspond to galaxy-sized cloud clumps. Recent observations show our Milky Way is enveloped in a large hot halo, of x-ray emitting gas, at millions of Kelvin, extending out hundreds of thousands of light-years. By present epoch, that hot halo has been heated & fully ionized, perhaps by eons of star-formation & galactic winds. But billions of years ago, it could have been cooler (tens of thousands of Kelvin), and could have become a DLA in Quasar spectra observed by far away aliens in a far away galaxy, for sake of speculative illustration. So our galaxy cloud clump is over a half-million light-years across, and contains a trillion solar masses of material. DLAs could correspond to large galaxies. If so, then would Quasar spectra stop red-shifting, as they passed through the gravitationally-bound galaxy, no longer expanding with the Hubble flow? If so, then Quasar spectra might "lose" up to a million years of red-shift, each time they passed through DLAs; and perhaps hundreds of thousands of years of red-shift, each time they passed through less-dense & presumably-smaller Lyman-alpha clouds. The correspondence of Lyman-alpha clouds & DLAs to small & large galaxies may further imply that Quasar spectra would be blue-shifted slightly propagating into the galaxies, and red-shifted propagating out from them? Perhaps a professional could take the time to update & appropriately improve the article? 66.235.38.214 (talk) 08:05, 13 November 2012 (UTC)
- According to Galaxy Formation & Evolution by Spinrad, pg. 78, Lyman-alpha clouds & DLAs (the densest of which may be "normal star-formation galaxies" & "QSO nuclear regions"), comprising the Lyman-alpha forest, are observed to obey approximate scaling relations, between neutral column density, and both metallicity (Z) & over-density (Ω). For a fiducial redshift of (z≈5), over-densities can be converted into number densities, which then imply, from the column density, a length scale. By both size & metallicity, Lyman-alpha clouds resemble globular star clusters; whereas DLAs resemble dwarfish high-redshift proto-galaxies:
- The association of Lyman-alpha forest absorbers, to galaxies & halo globulars, implies that galaxy environments have been the last repositories of neutral hydrogen in our universe. And so Cosmic Voids may have been the first regions to become ionized. Once space gas is heated & ionized into plasma, no further condensation or star-formation occurs; structure formation ceases. i have tried to crudely illustrate that concept, with the following figure:
- Qualitatively, neutral proto-galactic halos were bigger and/or more abundant at higher redshifts, and have slowly thinned out, fragmented, and/or dwindled away, to present epoch. Again, galaxy environments are seemingly the last repositories of neutral hydrogen in our universe. Perhaps a professional could take the time to incorporate these sources into the article? 66.235.38.214 (talk) 13:12, 13 November 2012 (UTC)
- According to Sparke & Gallagher, the number of Lyman-alpha forest clouds decreases with increasing column density N(HI) as N(HI)-3/2. DLAs range over three orders of magnitude in column density (1020-1023 cm-2), with few clouds observed above the upper cutoff. Separately, the Galaxy Luminosity Function is observed to range over around three orders of magnitude (10-3L* - L*), with few bright galaxies observed above the L* cutoff. Thus, there seems to be a clear correspondence, between censuses of DLAs, and censuses of galaxies — denser DLA clouds correspond to larger galaxies, et vice versa. Moreover, from Spinrad, only DLAs evidence secondary metal enrichment, above 0.01Zʘ; smaller Lyman-alpha clouds, corresponding to globular star clusters, evidence nearly no enrichment above that level, seemingly suggesting that they formed from a single "burst", whereas larger galaxies have continued to form stars and thereby "grow" [1]. DLA metallicities also range over three orders of magnitude (10-3Zʘ - Zʘ). And, since spatial density seemingly scales with column density, all DLAs correspond to similar-sized objects (≈10Kly); and their increasing densities translate directly into increasing masses (1-1000 billion Mʘ). Thus, the correspondence between DLAs ↔ galaxies seems plausible, with densities, masses, luminosities, & metallicities all scaling together, over three orders of magnitude.66.235.38.214 (talk) 05:41, 14 November 2012 (UTC)
- DLA clouds have average column number densities, of neutral atomic hydrogen, of ≈1021 cm-2. That corresponds to column mass densities of ≈1012 Msun Mlyr-2. Now, DLA clouds correspond to galaxy-sized cloud clumps. Recent observations show our Milky Way is enveloped in a large hot halo, of x-ray emitting gas, at millions of Kelvin, extending out hundreds of thousands of light-years. By present epoch, that hot halo has been heated & fully ionized, perhaps by eons of star-formation & galactic winds. But billions of years ago, it could have been cooler (tens of thousands of Kelvin), and could have become a DLA in Quasar spectra observed by far away aliens in a far away galaxy, for sake of speculative illustration. So our galaxy cloud clump is over a half-million light-years across, and contains a trillion solar masses of material. DLAs could correspond to large galaxies. If so, then would Quasar spectra stop red-shifting, as they passed through the gravitationally-bound galaxy, no longer expanding with the Hubble flow? If so, then Quasar spectra might "lose" up to a million years of red-shift, each time they passed through DLAs; and perhaps hundreds of thousands of years of red-shift, each time they passed through less-dense & presumably-smaller Lyman-alpha clouds. The correspondence of Lyman-alpha clouds & DLAs to small & large galaxies may further imply that Quasar spectra would be blue-shifted slightly propagating into the galaxies, and red-shifted propagating out from them? Perhaps a professional could take the time to update & appropriately improve the article? 66.235.38.214 (talk) 08:05, 13 November 2012 (UTC)
