Spectra of distant QSOs show metal absorption lines
by common elements (e.g., C, O, Si) as far as we can look back in time. At some point, these elements
must have been released from the stars that created them, to the intergalactic medium.
The location relative to galaxies, the physical state of the metal-enriched gas, and the level of enrichment were still highly uncertain until the mid-1990s, when
my colleagues Haehnelt and Steinmetz and I decided to add metals to cosmological hydro-simulations, to study the relation between QSO metal absorption lines and the
properties of the underlying cosmic gas distribution. This was the beginning of
a strand of research, then rather primitive, that tries to track the life cycle
of metals in the galactic environment. Our simulation did not include the actual
process that had dispersed the metal to the IGM in the first place (implementing galactic
feedback was technically impossible then) but we assumed that the metals get dispersed to the IGM
homogeneously at very redshift. By the time we observe them from their absorption lines
in background QSO spectra
(around redshift 3) they are settling in and around galaxies with the rest of the IGM. Comparing the properties of observed and simulated metal absorption systems
we found that very realistic absorption lines (e.g., CIV, SIV, SiII, OVI) can be produced
from the in-falling gas, suggesting that this is the main mode in which we observe
QSO metal absorption systems.
Left: The formation of metal absorption lines in two lines of sight passing through in-falling
gas in the vicinity of the progenitors of a future Milky-Way-sized galaxy.
The absorption lines from common ions are shown in the top panel, the density and temperature in the second and third panel (versus velocity along
the line of sight).
The bottom
panel shows the peculiar velocity, .i.e., the local velocity relative to the large-scale Hubble flow. The
peculiar velocity exhibits the characteristic S-shaped reversal
that indicates gravitational infall from both sides. In the situation on the right,
there is a narrower zone of velocity reversal in the center, embedded in a ramp
like increase in the velocity along the line-of-sight. Apparently, there is local infall
within a filament that appears to stretch faster than the Hubble flow
(from Rauch, Haehnelt and Steinmetz 1997).
More recently, and fed by the expectation that we will ultimately be able to see
the processes that disperse metals from galaxies directly, the notion of a circumgalactic
medium ("CGM") has become popular, reflecting the belief that the properties of intergalactic gas can be better explained with reference to a nearby
galaxy. However, nearly all of the (often strong) correlations between gas and galaxies
can be attributed to gravitational effects, i.e., by both galaxies and gas congregating in the same dark matter potential wells.
Establishing a causal link between feedback from an individual galaxy and the properties of the surrounding gas (e.g., when explaining the chemical enrichment of the gas as the result of an ongoing galactic outflow) has remained difficult, and may not make much sense in a hierarchical universe where the choice of the nearest, brightest galaxy depends on the detection threshold, and where the enrichment processes may pre-date
any currently observed feedback. To date, observations are consistent with the idea that the gas in most of the volume of the CGM is just in-falling
IGM, and only a relative small zone close to a galaxy is affected by galactic feedback.
Haehnelt, Martin G.; Steinmetz, Matthias; Rauch, Michael: C IV Absorption from Galaxies in the Process of Formation, 1996, ApJ, 465, 95
Rauch, Michael; Haehnelt, Martin G.; Steinmetz, Matthias:
QSO Metal Absorption Systems at High Redshift and the Signature of Hierarchical Galaxy Formation, 1997, ApJ, 481,601
