29 research outputs found
The Accuracy of Subhalo Detection
With the ever increasing resolution of N-body simulations, accurate subhalo
detection is becoming essential in the study of the formation of structure, the
production of merger trees and the seeding of semi-analytic models. To
investigate the state of halo finders, we compare two different approaches to
detecting subhaloes; the first based on overdensities in a halo and the second
being adaptive mesh refinement. A set of stable mock NFW dark matter haloes
were produced and a subhalo was placed at different radii within a larger halo.
SUBFIND (a Friends-of-Friends based finder) and AHF (an adaptive mesh based
finder) were employed to recover the subhalo. As expected, we found that the
mass of the subhalo recovered by SUBFIND has a strong dependence on the radial
position and that neither halo finder can accurately recover the subhalo when
it is very near the centre of the halo. This radial dependence is shown to be
related to the subhalo being truncated by the background density of the halo
and originates due to the subhalo being defined as an overdensity. If the
subhalo size is instead determined using the peak of the circular velocity
profile, a much more stable value is recovered. The downside to this is that
the maximum circular velocity is a poor measure of stripping and is affected by
resolution. For future halo finders to recover all the particles in a subhalo,
a search of phase space will need to be introduced.Comment: 9 pages, 7 figures, accepted for publication in MNRA
Black holes and galaxy environment in cosmological simulations
Understanding the formation and evolution of galaxies is one of the primary research goals of astronomy today. Galaxies are observed to have a range of masses, colours and morphologies, and various processes, including feedback, have been proposed to explain these differences. Some of these processes are related to the environment in which a galaxy resides. In this Thesis I present the results of three projects I have undertaken to help increase our understanding of galaxy formation. The first was to investigate the different methods of structure detection used in simulations. Placing an identical subhalo at different radii inside a larger halo demonstrated that subhalo mass recovery is radially dependent. Subhaloes closer to the centre of a halo are recovered smaller than haloes near the edge, but their peak circular velocity is less affected. The second project set about investigating different ways of measuring galaxy environment. Observationally galaxy environment is most commonly measured through nearest neighbours or fixed apertures, and these have different relationships to the underlying dark matter haloes. Fixed aperture measures are sensitive to halo mass and best probe the `large-scale environment' external to a halo. Meanwhile nearest neighbour measures are insensitive to halo mass and best probe the `local environment' internal to a halo. The final project involved implementing the Accretion Disc Particle (ADP) model of black hole growth within a cosmological, large volume simulation, including cooling, star formation and feedback. Comparing this method with a modified Bondi-Hoyle model allows for the investigation of how accretion rates affect feedback and galaxy properties. ADP suffers from the limited resolution of large-scale simulations and produces unphysically large accretion discs. Both models can reproduce the local black hole scaling relations, but produce black hole mass functions that do not agree with observations
Black holes and galaxy environment in cosmological simulations
Understanding the formation and evolution of galaxies is one of the primary research goals of astronomy today. Galaxies are observed to have a range of masses, colours and morphologies, and various processes, including feedback, have been proposed to explain these differences. Some of these processes are related to the environment in which a galaxy resides. In this Thesis I present the results of three projects I have undertaken to help increase our understanding of galaxy formation. The first was to investigate the different methods of structure detection used in simulations. Placing an identical subhalo at different radii inside a larger halo demonstrated that subhalo mass recovery is radially dependent. Subhaloes closer to the centre of a halo are recovered smaller than haloes near the edge, but their peak circular velocity is less affected. The second project set about investigating different ways of measuring galaxy environment. Observationally galaxy environment is most commonly measured through nearest neighbours or fixed apertures, and these have different relationships to the underlying dark matter haloes. Fixed aperture measures are sensitive to halo mass and best probe the `large-scale environment' external to a halo. Meanwhile nearest neighbour measures are insensitive to halo mass and best probe the `local environment' internal to a halo. The final project involved implementing the Accretion Disc Particle (ADP) model of black hole growth within a cosmological, large volume simulation, including cooling, star formation and feedback. Comparing this method with a modified Bondi-Hoyle model allows for the investigation of how accretion rates affect feedback and galaxy properties. ADP suffers from the limited resolution of large-scale simulations and produces unphysically large accretion discs. Both models can reproduce the local black hole scaling relations, but produce black hole mass functions that do not agree with observations
What are protoclusters? – Defining high-redshift galaxy clusters and protoclusters
We explore the structures of protoclusters and their relationship with high-redshift clusters using the Millennium Simulation combined with a semi-analytic model. We find that protoclusters are very extended, with 90 per cent of their mass spread across∼35 h−1 Mpc commoving at z =2 (∼30 arcmin). The ‘main halo’, which can manifest as a high-redshift cluster or group, is only a minor feature of the protocluster, containing less than 20 per cent of all protocluster galaxies at z = 2. Furthermore, many protoclusters do not contain a main halo that is massive enough to be identified as a high-redshift cluster. Protoclusters exist in a range of evolutionary states at high redshift, independent of the mass they will evolve to at z = 0. We show that the evolutionary state of a protocluster can be approximated by the mass ratio of the first and second most massive haloes within the protocluster, and the z = 0 mass of a protocluster can be estimated to within 0.2 dex accuracy if both the mass of the main halo and the evolutionary state are known. We also investigate the biases introduced by only observing star-forming protocluster members within small fields. The star formation rate required for line-emitting galaxies to be detected is typically high, which leads to the artificial loss of low-mass galaxies from the protocluster sample. This effect is stronger for observations of the centre of the protocluster, where the quenched galaxy fraction is higher. This loss of low-mass galaxies, relative to the field, distorts the size of the galaxy overdensity, which in turn can contribute to errors in predicting the z = 0 evolved mass
Streams Going Notts: The tidal debris finder comparison project
While various codes exist to systematically and robustly find haloes and
subhaloes in cosmological simulations (Knebe et al., 2011, Onions et al.,
2012), this is the first work to introduce and rigorously test codes that find
tidal debris (streams and other unbound substructure) in fully cosmological
simulations of structure formation. We use one tracking and three non-tracking
codes to identify substructure (bound and unbound) in a Milky Way type
simulation from the Aquarius suite (Springel et al., 2008) and post-process
their output with a common pipeline to determine the properties of these
substructures in a uniform way. By using output from a fully cosmological
simulation, we also take a step beyond previous studies of tidal debris that
have used simple toy models. We find that both tracking and non-tracking codes
agree well on the identification of subhaloes and more importantly, the {\em
unbound tidal features} associated with them. The distributions of basic
properties of the total substructure distribution (mass, velocity dispersion,
position) are recovered with a scatter of . Using the tracking code as
our reference, we show that the non-tracking codes identify complex tidal
debris with purities of . Analysing the results of the substructure
finders, we find that the general distribution of {\em substructures} differ
significantly from the distribution of bound {\em subhaloes}. Most importantly,
both bound and unbound {\em substructures} together constitute of the
host halo mass, which is a factor of higher than the fraction in
self-bound {\em subhaloes}. However, this result is restricted by the remaining
challenge to cleanly define when an unbound structure has become part of the
host halo. Nevertheless, the more general substructure distribution provides a
more complete picture of a halo's accretion history.Comment: 19 pages, 12 figures, accepted for publication in MNRA
Galaxies going MAD: The Galaxy-Finder Comparison Project
With the ever increasing size and complexity of fully self-consistent
simulations of galaxy formation within the framework of the cosmic web, the
demands upon object finders for these simulations has simultaneously grown. To
this extent we initiated the Halo Finder Comparison Project that gathered
together all the experts in the field and has so far led to two comparison
papers, one for dark matter field haloes (Knebe et al. 2011), and one for dark
matter subhaloes (Onions et al. 2012). However, as state-of-the-art simulation
codes are perfectly capable of not only following the formation and evolution
of dark matter but also account for baryonic physics (e.g. hydrodynamics, star
formation, feedback) object finders should also be capable of taking these
additional processes into consideration. Here we report on a comparison of
codes as applied to the Constrained Local UniversE Simulation (CLUES) of the
formation of the Local Group which incorporates much of the physics relevant
for galaxy formation. We compare both the properties of the three main galaxies
in the simulation (representing the MW, M31, and M33) as well as their
satellite populations for a variety of halo finders ranging from phase-space to
velocity-space to spherical overdensity based codes, including also a mere
baryonic object finder. We obtain agreement amongst codes comparable to (if not
better than) our previous comparisons, at least for the total, dark, and
stellar components of the objects. However, the diffuse gas content of the
haloes shows great disparity, especially for low-mass satellite galaxies. This
is primarily due to differences in the treatment of the thermal energy during
the unbinding procedure. We acknowledge that the handling of gas in halo
finders is something that needs to be dealt with carefully, and the precise
treatment may depend sensitively upon the scientific problem being studied.Comment: 14 interesting pages, 17 beautiful figures, and 2 informative tables
accepted for publication in MNRAS (matches published version
The structure and evolution of a forming galaxy cluster at z = 1.62
We present a comprehensive picture of the Cl 0218.3−0510 protocluster at z = 1.623 across 10 comoving Mpc. Using filters that tightly bracket the Balmer and 4000 Å breaks of the protocluster galaxies we obtain precise photometric redshifts resulting in a protocluster galaxy sample that is 89 ± 5 per cent complete and has a contamination of only 12 ± 5 per cent. Both star-forming and quiescent protocluster galaxies are located, which allows us to map the structure of the forming cluster for the first time. The protocluster contains six galaxy groups, the largest of which is the nascent cluster. Only a small minority of the protocluster galaxies are in the nascent cluster (11 per cent) or in the other galaxy groups (22 per cent), as most protocluster galaxies reside between the groups. Unobscured star-forming galaxies predominantly reside between the protocluster’s groups, whereas red galaxies make up a large fraction of the groups’ galactic content, so observing the protocluster through only one of these types of galaxies results in a biased view of the protocluster’s structure. The structure of the protocluster reveals how much mass is available for the future growth of the cluster and we use the Millennium Simulation, scaled to a Planck cosmology, to predict that Cl 0218.3−0510 will evolve into a 2.7+3.9 −1.7 × 1014M cluster by the present day
Major mergers going Notts: challenges for modern halo finders
Merging haloes with similar masses (i.e. major mergers) pose significant challenges for halo finders. We compare five halo-finding algorithms’ (ahf, hbt, rockstar, subfind, and velociraptor) recovery of halo properties for both isolated and cosmological major mergers. We find that halo positions and velocities are often robust, but mass biases exist for every technique. The algorithms also show strong disagreement in the prevalence and duration of major mergers, especially at high redshifts (z > 1). This raises significant uncertainties for theoretical models that require major mergers for, e.g. galaxy morphology changes, size changes, or black hole growth, as well as for finding Bullet Cluster analogues. All finders not using temporal information also show host halo and subhalo relationship swaps over successive timesteps, requiring careful merger tree construction to avoid problematic mass accretion histories. We suggest that future algorithms should combine phase-space and temporal information to avoid the issues presented
SubHaloes going Notts: The SubHalo-Finder Comparison Project
We present a detailed comparison of the substructure properties of a single
Milky Way sized dark matter halo from the Aquarius suite at five different
resolutions, as identified by a variety of different (sub-)halo finders for
simulations of cosmic structure formation. These finders span a wide range of
techniques and methodologies to extract and quantify substructures within a
larger non-homogeneous background density (e.g. a host halo). This includes
real-space, phase-space, velocity-space and time- space based finders, as well
as finders employing a Voronoi tessellation, friends-of-friends techniques, or
refined meshes as the starting point for locating substructure.A common
post-processing pipeline was used to uniformly analyse the particle lists
provided by each finder. We extract quantitative and comparable measures for
the subhaloes, primarily focusing on mass and the peak of the rotation curve
for this particular study. We find that all of the finders agree extremely well
on the presence and location of substructure and even for properties relating
to the inner part part of the subhalo (e.g. the maximum value of the rotation
curve). For properties that rely on particles near the outer edge of the
subhalo the agreement is at around the 20 per cent level. We find that basic
properties (mass, maximum circular velocity) of a subhalo can be reliably
recovered if the subhalo contains more than 100 particles although its presence
can be reliably inferred for a lower particle number limit of 20. We finally
note that the logarithmic slope of the subhalo cumulative number count is
remarkably consistent and <1 for all the finders that reached high resolution.
If correct, this would indicate that the larger and more massive, respectively,
substructures are the most dynamically interesting and that higher levels of
the (sub-)subhalo hierarchy become progressively less important.Comment: 16 pages, 7 figures, 2 tables, Accepted for MNRA