Impact of baryon physics on dark matter structures: a detailed simulation study of halo density profiles
Mon. Not. R. Astron. Soc. 405, 2161–2178 (2010)
doi:10.1111/j.1365-2966.2010.16613.x
Impact of baryon physics on dark matter structures: a detailed simulation
study of halo density profiles
Alan R. Duffy,1,2,3 Joop Schaye,2 Scott T. Kay,1 Claudio Dalla Vecchia,2,4
Richard A. Battye1 and C. M. Booth2
1 Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL
2 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, the Netherlands
3 ICRAR, The University of Western Australia, M468, WA 6009, Australia
4 Max Planck Institute for Extraterrestial Physics, Giessenbachstraβe, D-85748 Garching, Germany
Accepted 2010 March 2. Received 2010 January 19
The back-reaction of baryons on the dark matter halo density profile is of great interest, not least
because it is an important systematic uncertainty when attempting to detect the dark matter.
Here, we draw on a large suite of high-resolution cosmological hydrodynamical simulations to
systematically investigate this process and its dependence on the baryonic physics associated
with galaxy formation. The effects of baryons on the dark matter distribution are typically not
well described by adiabatic contraction models. In the inner 10 per cent of the virial radius the
models are only successful if we allow their parameters to vary with baryonic physics, halo
mass and redshift, thereby removing all predictive power. On larger scales the profiles from
dark matter only simulations consistently provide better fits than adiabatic contraction models,
even when we allow the parameters of the latter models to vary. The inclusion of baryons
results in significantly more concentrated density profiles if radiative cooling is efficient and
feedback is weak. The dark matter halo concentration can in that case increase by as much as
30 (10) per cent on galaxy (cluster) scales. The most significant effects occur in galaxies at high
redshift, where there is a strong anticorrelation between the baryon fraction in the halo centre
and the inner slope of both the total and the dark matter density profiles. If feedback is weak,
isothermal inner profiles form, in agreement with observations of massive, early-type galaxies.
However, we find that active galactic nuclei (AGN) feedback, or extremely efficient feedback
from massive stars, is necessary to match observed stellar fractions in groups and clusters,
as well as to keep the maximum circular velocity similar to the virial velocity as observed
for disc galaxies. These strong feedback models reduce the baryon fraction in galaxies by a
factor of 3 relative to the case with no feedback. The AGN is even capable of reducing the
baryon fraction by a factor of 2 in the inner region of group and cluster haloes. This in turn
results in inner density profiles which are typically shallower than isothermal and the halo
concentrations tend to be lower than in the absence of baryons. We therefore conclude that
the disagreement between the concentrations inferred from observations of groups of galaxies
and predictions from simulations that was identified by Duffy et al. is not alleviated by the
inclusion of baryons.
Key words: hydrodynamics – gravitation – methods: numerical – galaxies: haloes – galaxies:
structure – dark matter.
1 I N T RO D U C T I O N
Increasingly powerful computers, combined with ever more efficient N-body codes, have permitted accurate numerical tests of
structure formation in a cold dark matter (CDM) universe (e.g.
E-mail:
C 2010 The Authors. Journal compilation
C 2010 RAS
Klypin, Primack & Holtzman 1996; Navarro, Frenk & White 1997,
hereafter NFW; Moore et al. 1999b; Bullock et al. 2001; Springel
et al. 2005b; Diemand et al. 2008; Gao et al. 2008). It is now an
established prediction that CDM haloes form a cuspy mass distribution that is close to a ‘universal profile’, independent of halo mass
(Navarro, Frenk & White 1996; NFW). In detail, however, the haloes
are not strictly self-similar (Navarro et al. 2010). Foremost amongst
these results has been the existence of power-law relationships in
ABSTRACT
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A. R. Duffy et al.
2008). However, the influence of baryons on these smaller, less
well-resolved objects are by no means certain. For example, the
substructure may exhibit greater resistance to tidal disruption due
to its deeper potential well in the presence of baryons, while at the
same time the substructure will typically suffer from increased dynamical friction with the main halo (Jiang et al. 2008). Recent work
by Macciò et al. (2006) found a factor of 2 increase in the number
of surviving satellites in a galaxy-sized halo, in a hydrodynamical
simulation relative to a DM-only simulation. This increased survival
rate in the inner regions (Weinberg et al. 2008) may enable satellite
infall to partly counteract the contraction of the DM halo, as these
objects can efficiently transfer angular momentum to the inner parts
of the DM halo, instead of being tidally disrupted at larger radii (for
recent considerations of this effect see Abadi et al. 2009; Pedrosa,
Tissera & Scannapieco 2009, 2010). The erasure of the central DM
cusp in galaxies by infalling satellites was proposed by El-Zant
et al. (2001) and for the case of clusters by El-Zant et al. (2004).
The effect of the baryons on the inner DM density profile is of
particular interest, as it is currently a significant source of uncertainty when making predictions for DM detection experiments. For
example, the expected γ -ray signal from self-annihilation of potential DM candidate particles (Springel et al. 2008b), which may
become detectable in the near future using γ -ray observatories such
2
. We further note that the total
as Fermi,1 is proportional to ρDM
mass density profile is also of interest, as it is this quantity that is
determined from strong lensing studies and is relevant for direct
comparison with such observations (for a review of the methodologies and uses of strong lensing, see Kochanek 2006).
The incorporation of baryons in a simulation is a challenging theoretical undertaking as the relevant scales of the physical processes
are rarely resolved in a cosmological simulation and approximations
are therefore necessary. The method by which stellar feedback, for
example, is incorporated can have a large influence on the resulting
baryonic distribution of the galaxy (e.g. Dalla Vecchia & Schaye
2008). For this reason, we have attempted to probe these effects
in a systematic way by examining the DM halo density profiles
from galaxies, groups and clusters, drawn from an extensive series
of cosmological simulations with that incorporate a wide variety
of prescriptions for baryonic processes. These form a subset of
the larger, overall simulation series known as the OverWhelmingly
Large Simulations (OWLS) project (Schaye et al. 2010).
Our work has a number of novel features that allow us to extend recent studies that have probed the dyn (...truncated)
