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We examine how HI and metal absorption lines within low-redshift galaxy halostrace the dynamical state of circumgalactic gas, using cosmologicalhydrodynamic simulations that include a well-vetted heuristic model forgalactic outflows. We categorize inflowing, outflowing, and ambient gas basedon its history and fate as tracked in our simulation. Following our earlierwork showing that the ionisation level of absorbers was a primary factor indetermining the physical conditions of absorbing gas, we show here that it isalso 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 severalGyr, while high-ionisation metal absorbers (e.g. OVI) generally trace materialthat was deposited by outflows many Gyr ago. Inflowing gas is dominated byenriched material that was previously ejected in an outflow, hence accretion atlow redshifts is typically substantially enriched. Recycling wind material ispreferentially found closer to galaxies, and is more dominant in lower-masshalos since high-mass halos have more hot gas that is able to support itselfagainst infall. Low-mass halos also tend to re-eject more of their accretedmaterial, owing to our outflow prescription that employs higher mass loadingfactors for lower-mass galaxies. Typical HI absorbers trace unenriched ambientmaterial that is not participating in the baryon cycle, but stronger HIabsorbers arise in cool, enriched inflowing gas. Instantaneous radial velocitymeasures of absorbers are generally poor at distinguishing between inflowingand outflowing gas, except in the case of very recent outflows. These resultssuggest that probing halo gas using a range of absorbers can provide detailedinformation about the amount and physical conditions of material that isparticipating in the baryon cycle.

Tracing Inflows and Outflows with Absorption Lines in Circumgalactic Gas