Abstract
We examine how HI and metal absorption lines within low-redshift galaxy halos
trace the dynamical state of circumgalactic gas, using cosmological
hydrodynamic simulations that include a well-vetted heuristic model for
galactic outflows. We categorize inflowing, outflowing, and ambient gas based
on its history and fate as tracked in our simulation. Following our earlier
work showing that the ionisation level of absorbers was a primary factor in
determining the physical conditions of absorbing gas, we show here that it is
also a governing factor for its dynamical state. Low-ionisation metal absorbers
(e.g. MgII) tend to arise in gas that will fall onto galaxies within several
Gyr, while high-ionisation metal absorbers (e.g. OVI) generally trace material
that was deposited by outflows many Gyr ago. Inflowing gas is dominated by
enriched material that was previously ejected in an outflow, hence accretion at
low redshifts is typically substantially enriched. Recycling wind material is
preferentially found closer to galaxies, and is more dominant in lower-mass
halos since high-mass halos have more hot gas that is able to support itself
against infall. Low-mass halos also tend to re-eject more of their accreted
material, owing to our outflow prescription that employs higher mass loading
factors for lower-mass galaxies. Typical HI absorbers trace unenriched ambient
material that is not participating in the baryon cycle, but stronger HI
absorbers arise in cool, enriched inflowing gas. Instantaneous radial velocity
measures of absorbers are generally poor at distinguishing between inflowing
and outflowing gas, except in the case of very recent outflows. These results
suggest that probing halo gas using a range of absorbers can provide detailed
information about the amount and physical conditions of material that is
participating in the baryon cycle.
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