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Circumgalactic medium simulations

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ECCA Research

Simulations of the circumgalactic medium

The successes of Lyman-α forest simulations suggests applying them to test models of galaxy formation by constraining the expected impact of forming galaxies on the IGM. Observational studies of the environment of galaxies at redshifts 3 < z < 4 show an enhancement in the amount of HI absorption at comoving distances r < 5 Mpc/h (Adelberger et al. 2003). Enhanced absorption is expected, as demonstrated by numerical simulations in the context of ΛCDM cosmologies (Kollmeier et al. 2006), which agree well with measurements of the total absorption over scales r > 1 - 2 Mpc/h. Within the inner 1 Mpc/h, however, discrepancies are found. Cold inflows are predicted to bring in material that results in an overproduction of stars within the inner 500 kpc (Dekel et al. 2009); as a result, semi-analytic galaxy formation models predict a galaxy luminosity function in conflict with observations. It has been suggested that feedback by supernovae and active nuclei within the forming galaxies may resolve the conflict by suppressing the cooling (Bower et al. 2012). If this is the case, then the associated winds and jets should disturb the circumgalactic medium (CGM) on scales of up to 300 - 500 kpc (proper) (Kollmeier et al. 2006, Oppenheimer & Davé 2008).

Direct observational evidence of winds is still lacking. Kinematical evidence is necessary to reveal the impact of a wind on the CGM. Until recently, existing spectra were not able to resolve the Lyman-α forest near the galaxies, so that the kinematic state of the circumgalactic medium was unknown. The Keck Baryonic Structure Survey (KBSS) has since set out to provide a detailed description of the HI distribution around 886 star-forming galaxies in haloes of mass 1012 h-1 M in the redshift range 2.0 < z < 2.8, where the impact on the IGM should be greatest. They have measured the HI absorption along lines of sight to bright QSOs with impact parameters exceeding 50 kpc (proper) from the galaxies, quantifying the net amount of absorption along with the properties of the Lyman-α forest, focussing on systems with HI column densities down to 1014.5 cm-2, well within the Lyman-α forest (Rakic et al. 2012, Rudie et al. 2012). They find a strong correlation of the IGM properties with distance from the galaxies, revealing three regimes: within 300 kpc (proper), there is a systematic increase in the velocity widths of the absorption systems, suggestive of turbulence or heating, and a rise in the HI column densities, with the highest HI column density systems occurring three orders of magnitude more often than in the IGM at large; between 300 kpc and 2 Mpc the peak amount of HI in absorbers shows a plateau elevated above the mean cosmic value recovered at distances exceeding 3 Mpc. A comparison of the incidence of the column density systems along and transverse to the lines of sight to the galaxies shows the characteristic compression signal in redshift space of infall for the lower column density absorbers (< 1014.5 cm-2) and elongation characteristic of relaxation for the higher column density systems.

We use the STFC distributed computing facility DiRAC and facilities available through the Partnership for Advanced Computing in Europe ( PRACE ) to perform cosmological numerical simulations to predict the expected properties of the Lyman-α forest in the environment of star-forming galaxies in the context of a ΛCDM cosmology. Our primary goal is to establish the gaseous flow pattern around the galaxies and the spatial extent to which winds impact on the CGM through comparison with the observations.

Peculiar velocity field
Since the measured distance between the galactic haloes and the IGM is in velocity space, it is important to converge on the velocity field. Figure 1 shows the convergence of the peculiar velocities of the haloes. Sufficient numbers of haloes are found to demonstrate convergence of the internal velocity dispersions over the halo masses required (bottom panel). The cosmic peculiar velocities of the haloes (top panel) have note yet converged, as is expected because of the known large-scale contribution of density fluctuations to the peculiar velocity field. A much larger 120 Mpc (comoving) simulation is expected to provide convergence on the relative peculiar velocities between the haloes and the IGM on the relevant scales of up to 4 Mpc (comoving).


Adelberger K.L., Steidel C.C., Shapley A.E., Pettini M., 2003, ApJ, 584, 45

Bower R.G., Benson A.J., Crain R.A., 2012, MNRAS, 422, 2816

Dekel A., Birnboim Y., Engel G., Freundlich J., Goerdt T., Mumcuoglu M., Neistein E., Pichon C., Teyssier R., Zinger E., 2009, Nature, 457, 451

Kollmeier J.A., Miralda-Escudé J., Cen R., Ostriker J.P., 2006, ApJ, 638, 52

Oppenheimer B.D., Davé R., 2008, MNRAS, 387, 577

Rakic O., Schaye J., Steidel C.C., Rudie G.C., 2012, ApJ, 751, 94

Rudie G.C., Steidel C.C., Trainor R.F., Rakic O., Bogosavljević M., Pettini M., Reddy N., Shapley A.E., Erb D.K., Law D.R., 2012, ApJ, 750, 67

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