> For the complete documentation index, see [llms.txt](https://docs.caterpillarproject.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.caterpillarproject.org/technical/basic-approach.md).

# Overview & Basic Approach

## Overview

The general approach of Caterpillar was that of the zoom-in technique adopted by many other groups, albeit with a few differences. we performed a much, much larger search for optimal computer parameters to ensure that contamination volumes are as large as possible without great cost. We also implemented iterative unbinding in `ROCKSTAR` which we found to be critical in identifying halos at the highest resolution simulations, particularly on highly radial orbits.

![Caterpillar halos were drawn from a parent simulation and re-simulated at high resolution.](/files/-LeT6dFdnstBIBeUAiPz)

## Considerations

### 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 $$10^{12} M\_\odot$$ halos with 10,000 particles so as to construct well defined lagrangian volumes. This resulted in us selecting a resolution of 1024^3 or a particle mass of $$8.72 \times 10^{12} M\_\odot/h$$ .

### 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.

## Halo Properties

{% hint style="info" %}
Nearly all of the following can be found in our flagship paper [Griffen et al. (2015)](https://iopscience.iop.org/article/10.3847/0004-637X/818/1/10/pdf).
{% endhint %}

### Halo Profiles

![](/files/-LeT98E6qzWFBq0mFQqx)

### Accretion History

Lower Resolution Runs

![](/files/-LeT944MyHmSvbmxVUko)

### Subhalos

![](/files/-LeT9BftFTd6EWcU1P43)

### Convergence

![](/files/-LeTC3FZKSLKJi6lq8oh)

![](/files/-LeT9mNgFu0tUjHB0ExQ)

### Mass-Concentration

![](/files/-LeTA1m70jKTSnlia5zd)

### Relaxed

![](/files/-LeT7FZXBR5y3t-jZ3eO)

### Lower Resolution Runs

![](/files/-LeT9XW_8403-p2oWMvg)

![](/files/-LeT9UUgnvQrtEHwME8s)