Zooming In

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 parameter 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 re-sampled. 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.
MUSIC Parameter File
1
# H1079897_EX_Z127_P7_LN7_LX14_O4_NV4.conf
2
[setup]
3
boxlength            = 100
4
zstart               = 127
5
levelmin             = 7
6
levelmin_TF          = 10
7
levelmax             = 14
8
padding              = 7
9
overlap              = 4
10
region               = ellipsoid
11
hipadding            = 1.05
12
region_point_file    = /n/home01/bgriffen/data/caterpillar/ics/lagr/H1079897NRVIR4
13
align_top            = no
14
baryons              = no
15
use_2LPT             = no
16
use_2LLA             = no
17
periodic_TF          = yes
18
​
19
[cosmology]
20
Omega_m              = 0.3175
21
Omega_L              = 0.6825
22
Omega_b              = 0.049
23
H0                   = 67.11
24
sigma_8              = 0.8344
25
nspec                = 0.9624
26
transfer             = eisenstein
27
​
28
[random]
29
seed[10]              = 34567
30
seed[11]              = 1079897
31
seed[12]              = 2159794
32
seed[13]              = 3239691
33
seed[14]              = 4319588
34
​
35
[output]
36
format               = gadget2_double
37
filename             = ./ics
38
gadget_num_files     = 8
39
gadget_spreadcoarse  = yes
40
​
41
[poisson]
42
fft_fine             = yes
43
accuracy             = 1e-05
44
pre_smooth           = 3
45
post_smooth          = 3
46
smoother             = gs
47
laplace_order        = 6
48
grad_order           = 6
Copied!
Halo Selection
We selected halos with the following environmental requirements:
halos mass between  0.7≤Mvir≤3×1012M⊙0.7 \leq M_{vir} \leq 3 \times 10^{12} M_\odot0.7≤Mvir​≤3×1012M⊙​
  (6564 candidates)
no halos larger than 7×1013M⊙7 \times 10^{13} M_\odot7×1013M⊙​
 within 7 Mpc
no halos larger than 7×1012M⊙7 \times 10^{12} M_\odot7×1012M⊙​
  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.
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
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.
Resolution Levels
Aquarius Level
MUSIC levelmax
Effective Resolution
​104h−3M⊙10^4 h^{-3} M_\odot104h−3M⊙​
 
​104h−3M⊙10^4 h^{-3} M_\odot104h−3M⊙​
 
Force Softening ϵ\epsilonϵ
 (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.
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 inter-particle separation. We adopt the formula: boxwidth/lx^2/80 but stagger the force softening 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 (units of Mpc/h):
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
Temporal Resolution
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
Geometry
Details
BA
Original MUSIC bounding box (e.g . the exac t boun ding box of lagr volu me).
BB
1.2 bounding box extent
BC
1.4 bounding box extent
BD
1.6 bounding box extent
CA
Convex Hull Volume
EA
Original MUSIC Ellipsoid (e.g . the exact bounding box of Lagrangian volume).
EB
1.1 padding
EC
1.2 padding
EX
1.05 padding
We did this for both 4 and 5 times the virial radius at z = 0 (marked by the letter 4 or 5 at the end of the abbreviated geometry). Making a total of ~18 test halos per Caterpillar halo. Our requirement was that there was no contamination (particle type 2) within 1 Mpc of the host at the LX11 level.
We also looked at how the geometry of the Lagrangian volume affected the contamination radius. As outlined in Griffen et al. (2015), we did not find any correlation with geometry and overall level contamination. Every simulation requires its own tailored geometry to achieve our contamination requirements.
The size of the Lagrangian volumes were also another challenge to overcome. If a halo had LX11 ICs which were larger than 300mb, we found that we could not run these at LX14 on national facilities. The size and distance became our two biggest obstacles when running the Caterpillar suite.
Our ROCKSTAR catalogues only use the high-resolution particles. This means that there will be halos in the outskirts of the simulation which are contaminated. These are shown clearly below. Be sure not to just take all halos within the ROCKSTAR catalogues as some of them will be contaminated (underestimated masses, wrong profiles etc.). As a safety, one should only take halos which are within the contamination distance. This changes as a function of redshift so make sure you update your cut for each snapshot. The plots below are for z = 0.
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