# Creating The Suite¶

## Initial Conditions¶

### Overview¶

We use the multi-scale cosmological initial conditions creator MUSIC. MUSIC is a computer program to generate nested grid initial conditions for high-resolution “zoom” cosmological simulations. A detailed description of the algorithms can be found in Hahn & Abel (2011). You can download the user’s guide here or obtain a copy of the code here. Any questions should be directed to Brendan Griffen and then if that fails, Oliver Hahn.

### Parameters¶

These are the parameter files which were used to generate the initial coditions for the Caterpillar project. We adopt the raw Planck (2013) cosmology and do not use 2nd order lagrangian perturbation theory. The user guide contains all the information required to understand the following parameter files. Within each simulation directory, there is a file named OUTPUTmusic which gives the log of the construction of the initial conditions.

### Parent Simulation¶

First we constructed a parent simulation of sufficient size to include thousands of Milky Way sized systems but also of sufficient resolution to construct good lagrangian volumes. We decided on ~10,000 particles per Milky Way-sized host and a 100 Mpc/h volume. Our level_max is 10 below, which means our parent simulation’s effective resolution is (210)3 = (1024)3 (i.e. a particle mass of 8.72 x 107 $$M_:raw-latex:odot/h$$.

If you have access to the MIT cluster (bigbang.mit.edu), you can access the parent simulation data here: /bigbang/data/AnnaGroup/caterpillar/parent/gL100X10/. Within this folder you’ll find the following file (within ics/):

# parentics.conf
[setup]
boxlength               = 100
zstart                  = 127
levelmin                = 10
levelmin_TF             = 10
levelmax                = 10
overlap                 = 4
ref_center              = 0.5, 0.5, 0.5
ref_extent              = 0.2, 0.2, 0.2
align_top               = yes
baryons                 = no
use_2LPT                = no
use_LLA                 = no
periodic_TF             = yes

[cosmology]
Omega_m                 = 0.3175
Omega_L                 = 0.6825
Omega_b                 = 0.049
H0                      = 67.11
sigma_8                 = 0.8344
nspec                   = 0.9624
transfer                = eisenstein

[random]
seed[10]                = 34567

[output]
filename                = ./ics

[poisson]
fft_fine                = yes
accuracy                = 1e-5
pre_smooth              = 3
post_smooth             = 3
smoother                = gs
laplace_order           = 6


### Zoom-in “Caterpillar” Simulations¶

Each simulation folder has a configuration file for MUSIC (e.g. H1079897_EX_Z127_P7_LN7_LX14_O4_NV4.conf) which was used to construct the initial conditions.

There are a few things to note about the zoom-in paramter files when compared to the parent volume parameter file. First is we now specify a region_point_file which defines the x,y,z (normalized to the box width) of the particles to be resampled. We have added the parameter hipadding which our own modification to allow for expanded regions. 1.05, for example, represents an expanded ellipsoid (by 5%). See Section 2.3 of Griffen et al. (2015) for a more detailed description of these geometries and their impact on contamination.

We also draw the reader’s attention to the seed values. Note that we do not set the seed values for any level lower than the parent volume (10) which makes MUSIC smooth out any levels lower than 10. The seed values at 11 are simply the halo numbers and then each level higher scales by a factor of 2 of this original number. This was required because the ICs were generated through our automatic pipeline and each simulation needs unique values to seed the random noise field. The levelmin_TF is also set to be the same as the parent volume (10). The padding and overlap parameters are the same for all simulations.

# H1079897_EX_Z127_P7_LN7_LX14_O4_NV4.conf
[setup]
boxlength            = 100
zstart               = 127
levelmin             = 7
levelmin_TF          = 10
levelmax             = 14
overlap              = 4
region               = ellipsoid
region_point_file    = /n/home01/bgriffen/data/caterpillar/ics/lagr/H1079897NRVIR4
align_top            = no
baryons              = no
use_2LPT             = no
use_2LLA             = no
periodic_TF          = yes

[cosmology]
Omega_m              = 0.3175
Omega_L              = 0.6825
Omega_b              = 0.049
H0                   = 67.11
sigma_8              = 0.8344
nspec                = 0.9624
transfer             = eisenstein

[random]
seed[10]              = 34567
seed[11]              = 1079897
seed[12]              = 2159794
seed[13]              = 3239691
seed[14]              = 4319588

[output]
filename             = ./ics

[poisson]
fft_fine             = yes
accuracy             = 1e-05
pre_smooth           = 3
post_smooth          = 3
smoother             = gs
laplace_order        = 6


## Parent Simulation¶

The parent simulation was setup primarily to find Milky Way-sized candidates at z = 0. All the parameters used for the construction of the simulation were oriented around this goal.

### Overview¶

Volume h^ 3 Mpc^3 # partic les $$m_ {dm}\ ) 10^7 h^ -1 \(M_ :raw-l atex: odot$$ $$m_ {dm}\ ) 10^7 \(M_ :raw-l atex: odot$$ $$:ra w-late x:ep silon _{dm}$$ pc
100^3 1024^3 9 12 2441

{{site.data.alerts.note}} The parent simulation initial conditions were run on an older version of MUSIC.{{site.data.alerts.end}}

### Volume¶

The volume of the parent simulation was selected to be 100 Mpc/h as this allows for roughly ~6500 Milky Way-sized (i.e. 10^12 Msol) systems to be found. After a gentle selection over local environment (i.e. making sure no halos were near clusters) 2122 candidates were used to select Caterpillar candidates.

### Mass Resolution¶

We required a resolution which allowed us to resolve 10^12 Msol halos with 10,000 particles so as to construct well defined lagrangian volumes. This resulted in us selecting a resolution of 10243 or a particle mass of 8.72 x 107 $$M_:raw-latex:odot/h$$.

### Halo Selection¶

We selected halos with the following environmental requirements:

• halos between 0.7 - 3 x 1012 $$M_:raw-latex:odot$$ (6564 candidates)
• no halos larger than 7 x 1013 $$M_:raw-latex:odot$$ within 7 Mpc
• no halos larger than 7 x 1012 $$M_:raw-latex:odot$$ within 2.8 Mpc (2122 candidates)

This is roughly in line with Tollerud et al. (2012), Boylan-Kolchin et al. (2013), Fardal et al. (2013), Pfiffel et al. (2013), Li & White (2008), van der Marel et al. (2012), Karachentsev et al. (2004) and Tikhonov & Klypin (2009). This avoids Milky Way-sized systems near clusters but does not make them overly isolated necessarily. Halos were also selected to not be preferentially near the very edge of the simulation volume as a matter of convenience. The first 24 Caterpillar halos are highlighted within the parent volume below.

### Temporal Resolution¶

The time steps were set to be log of the expansion factor, following a similar convention to that used by the Millenium and Millenium-II simulations. The following table shows the various measures for time/size at each snapshot.

Tip

Be sure to use the halo utility module (haloutils) in Python for quickly getting the temporal quantity for a given snapshot. See data access for more information.

Snap Scale Factor Redshift Time
0 0.0213 46.0000 0.0535
1 0.0290 33.5029 0.0851
2 0.0367 26.2557 0.1212
3 0.0444 21.5245 0.1613
4 0.0521 18.1929 0.2051
5 0.0598 15.7199 0.2522
6 0.0675 13.8114 0.3025
7 0.0752 12.2940 0.3557
8 0.0829 11.0586 0.4117
9 0.0906 10.0333 0.4704
10 0.0983 9.1687 0.5316
11 0.1060 8.4297 0.5952
12 0.1138 7.7909 0.6612
13 0.1215 7.2331 0.7294
14 0.1292 6.7419 0.7998
15 0.1369 6.3060 0.8723
16 0.1446 5.9166 0.9469
17 0.1523 5.5666 1.0234
18 0.1600 5.2503 1.1018
19 0.1677 4.9630 1.1821
20 0.1754 4.7011 1.2642
21 0.1831 4.4611 1.3481
22 0.1908 4.2406 1.4337
23 0.1985 4.0371 1.5210
24 0.2062 3.8489 1.6098
25 0.2139 3.6742 1.7003
26 0.2216 3.5117 1.7923
27 0.2294 3.3601 1.8858
28 0.2371 3.2184 1.9808
29 0.2448 3.0856 2.0772
30 0.2525 2.9608 2.1749
31 0.2602 2.8435 2.2741
32 0.2679 2.7330 2.3745
33 0.2756 2.6286 2.4762
34 0.2833 2.5299 2.5792
35 0.2910 2.4364 2.6834
36 0.2987 2.3477 2.7888
37 0.3064 2.2635 2.8953
38 0.3141 2.1835 3.0029
39 0.3218 2.1072 3.1116
40 0.3295 2.0346 3.2213
41 0.3372 1.9652 3.3321
42 0.3449 1.8990 3.4438
43 0.3527 1.8356 3.5565
44 0.3604 1.7750 3.6701
45 0.3681 1.7169 3.7846
46 0.3758 1.6612 3.9000
47 0.3835 1.6077 4.0161
48 0.3912 1.5563 4.1331
49 0.3989 1.5069 4.2508
50 0.4066 1.4594 4.3693
51 0.4143 1.4137 4.4884
52 0.4220 1.3696 4.6082
53 0.4297 1.3271 4.7287
54 0.4374 1.2861 4.8497
55 0.4451 1.2465 4.9714
56 0.4528 1.2083 5.0936
57 0.4605 1.1713 5.2163
58 0.4683 1.1356 5.3395
59 0.4760 1.1010 5.4632
60 0.4837 1.0675 5.5873
61 0.4914 1.0351 5.7118
62 0.4991 1.0037 5.8367
63 0.5068 0.9732 5.9620
64 0.5145 0.9437 6.0876
65 0.5222 0.9150 6.2135
66 0.5299 0.8871 6.3396
67 0.5376 0.8601 6.4660
68 0.5453 0.8338 6.5927
69 0.5530 0.8082 6.7195
70 0.5607 0.7834 6.8465
71 0.5684 0.7592 6.9737
72 0.5761 0.7357 7.1010
73 0.5838 0.7128 7.2284
74 0.5916 0.6905 7.3559
75 0.5993 0.6687 7.4835
76 0.6070 0.6475 7.6111
77 0.6147 0.6269 7.7387
78 0.6224 0.6067 7.8663
79 0.6301 0.5871 7.9939
80 0.6378 0.5679 8.1215
81 0.6455 0.5492 8.2490
82 0.6532 0.5309 8.3764
83 0.6609 0.5131 8.5038
84 0.6686 0.4956 8.6310
85 0.6763 0.4786 8.7581
86 0.6840 0.4619 8.8851
87 0.6917 0.4456 9.0119
88 0.6994 0.4297 9.1385
89 0.7072 0.4141 9.2649
90 0.7149 0.3989 9.3912
91 0.7226 0.3840 9.5172
92 0.7303 0.3694 9.6430
93 0.7380 0.3551 9.7685
94 0.7457 0.3410 9.8938
95 0.7534 0.3273 10.0188
96 0.7611 0.3139 10.1436
97 0.7688 0.3007 10.2680
98 0.7765 0.2878 10.3922
99 0.7842 0.2752 10.5160
100 0.7919 0.2627 10.6395
101 0.7996 0.2506 10.7627
102 0.8073 0.2386 10.8855
103 0.8150 0.2269 11.0081
104 0.8228 0.2154 11.1302
105 0.8305 0.2042 11.2520
106 0.8382 0.1931 11.3734
107 0.8459 0.1822 11.4944
108 0.8536 0.1715 11.6151
109 0.8613 0.1611 11.7354
110 0.8690 0.1508 11.8552
111 0.8767 0.1406 11.9747
112 0.8844 0.1307 12.0938
113 0.8921 0.1209 12.2124
114 0.8998 0.1113 12.3307
115 0.9075 0.1019 12.4485
116 0.9152 0.0926 12.5659
117 0.9229 0.0835 12.6828
118 0.9306 0.0745 12.7994
119 0.9383 0.0657 12.9155
120 0.9461 0.0570 13.0311
121 0.9538 0.0485 13.1464
122 0.9615 0.0401 13.2611
123 0.9692 0.0318 13.3755
124 0.9769 0.0237 13.4894
125 0.9846 0.0157 13.6028
126 0.9923 0.0078 13.7158
127 1.0000 0.0000 13.8283

## Zoom-in “Caterpillar” Halos¶

Note

The majority of the information about the zoom-in runs can be found in Griffen et al. (2016). Here we simply outline some details which were left out of the publication for the sake of brevity.

### Overview¶

~*Aq uarius * Level MUSIC level max Effect ive Resolu tion $$m_ {dm}\ ) 10^4 h^ -1 \(M_ :raw-l atex: odot$$ $$m_ {dm}\ ) 10^4 \(M_ :raw-l atex: odot$$ $$:ra w-late x:ep silon _{dm}$$ pc/h
1 15 32768^ 3 0.25 0.37 36
2 14 1638 4^3 2 3 76
3 13 8096^3 16 24 152
4 12 4096^3 128 190 228
5 11 2048^3 1025 1527 452

Each panel represents one single realization of the Cat-1 halo at different resolutions. The far left is an LX11 run and the far right is an LX14 run.

Note

The LX15 run has currently only been run for one of the halos and has been temporarily paused at z = 1. This will be finished with a few others once the main suite has been completed.

We have complete (modified) rockstar halo catalogues (together with consistent-trees merger trees) and z = 0 subfind catalogues.

### Force Softening¶

Softening was 1/80th the interparticle separation. We adopt the formula: boxwidth/lx^2/80 but stagger the force softenings for each higher level as 4 x base, 8 x base, 32 x base, 64 x base where base is the base force softening. For each of the zooms, this equates to:

In Gadget LX11 LX12 LX13 LX14
SofteningHalo 0.000610352 0.000305176 0.000152588 0.0000762939
SofteningDisk 0.002441406 0.001220703 0.000610352 0.000305176
SofteningBulge 0.004882813 0.002441406 0.001220703 0.000610352
SofteningStars 0.01953125 0.009765625 0.004882813 0.002441406
SofteningBndry 0.0390625 0.01953125 0.009765625 0.004882813

units of Mpc/h

### Temporal Resolution¶

Important

Timesteps are spaced logarithmically in expansion factor to z = 6, then linearly spaced in expansion factor down to z = 0. Always be aware of this as it could be strength and a weakness of your study.

This image shows the difference between the time step resolutions used in Caterpillar and those used in the Aquarius simulation. We wanted higher resolution at all redshifts for many purposes. At z > 6 we wanted to model Lyman-Werner radiation which requires timesteps of order the lifespan of Population III star formation. At low redshift we wanted timesteps of roughly 50-60 Myrs which is the disruption time scale of many small dwarf galaxies of the Milky Way. This also allows detailed modelling of the pericentric passages of infalling satellite systems, which is a crucial parameter for determining post-infall mass loss, for example.

### Contamination Study¶

A number of contamination studies have been carried out. This involves changing the Lagrangian geometry in some way to keep the contamination (distance to the nearest particle type 2 as far as possible) low whilst conserving CPU hours. Our selected test geometries were as follows

Geomet ry Deta il
BA Orig inal MUSI C boun ding box (e.g . the exac t boun ding box of lagr volu me).
BB 1.2 boun ding box exte nt
BC 1.4 boun ding box exte nt
BD 1.6 boun ding box exte nt
CA Conv ex Hull Volu me
EA Orig inal MUSI C elli psoi d (e.g . the exac t boun ding box of lagr volu me).