# Tutorial on Conditional Abundance Matching¶

Conditional Abundance Matching (CAM) is a technique that you can use to model a variety of correlations between galaxy and halo properties, such as the dependence of galaxy quenching upon both halo mass and halo formation time, or the dependence of galaxy disk size upon halo spin. This tutorial explains CAM by applying the technique to a few different problems.

## Basic Idea Behind CAM¶

CAM is designed to answer questions of the following form: does halo property A correlate with galaxy property B? The Halotools approach to answering such questions is via forward modeling: a mock universe is created in which the A–B correlation exists; comparing the mock universe to the real one allows you to evaluate the success of the A–B correlation hypothesis.

Forward-modeling the galaxy-halo connection requires specifying some statistical distribution of the galaxy property being modeled, so that Monte Carlo realizations can be drawn from the distribution. CAM uses the most ubiquitous approach to generating Monte Carlo realizations, inverse transformation sampling, in which the statistical distribution is specified in terms of the cumulative distribution function (CDF), $${\rm CDF}(z) \equiv {\rm Prob}(< z).$$ Briefly, the way this work is that once you specify the CDF, you only need to generate a realization of a random uniform distribution, and pass the values of that realization to the CDF inverse, $${\rm CDF}^{-1}(p),$$ which evaluates to the variable $$z$$ being painted on the model galaxies. See the Transformation of Probability tutorial for pedagogical derivations associated with inverse transformation sampling, and the monte_carlo_from_cdf_lookup function for a convenient one-liner syntax.

In ordinary applications of inverse transformation sampling, the use of a random uniform variable guarantees that the output variables $$z$$ will be distributed according to $${\rm Prob}(z),$$ and that each individual $$z$$ will be purely stochastic. CAM generalizes this common technique so that $${\rm Prob}(z)$$ is still recovered exactly, and moreover $$z$$ exhibits residual correlations with some other variable, $$h$$. Operationally, the way this works is that rather than evaluating $${\rm CDF}^{-1}(p)$$ with random uniform variables, instead you evaluate with $$p = {\rm CDF}(h) = {\rm Prob}(< h),$$ introducing a monotonic correlation between $$z$$ and $$h$$. In most applications, $$h$$ is some halo property like mass accretion rate, and $$z$$ is some galaxy property like star-formation rate. In this way, the galaxy property you paint on to your halos will trace the distribution $${\rm Prob}(z)$$, such that above-average values of $$z$$ will be painted onto halos with above average values of $$h$$, and conversely.

Finally, the “Conditional” part of CAM is that this technique naturally generalizes to introduce a galaxy property correlation while holding some other property fixed. For example, at fixed stellar mass, it is natural to hypothesize that star-forming galaxies live in halos that are rapidly accreting mass, and that quiescent galaxies live in halos that have already built up most of their mass. In this kind of CAM application, we have: $${\rm Prob}(z)\rightarrow{\rm Prob}(<SFR\vert M_{\ast})$$, and $${\rm Prob}(h)\rightarrow{\rm Prob}(<\dot{M}_{\rm sub}\vert M_{\rm sub})$$. That is, SFR at fixed stellar mass is hypothesized to correlate with halo accretion rate at fixed (sub)halo mass.

## Quick Demonstration¶

This tutorial gives many different examples of how to apply CAM when modeling the galaxy-halo connection, including several different variations of the technique. Before going into the details, we’ll first give a brief demo of a simple example.

Suppose we live in a very simple universe in which every galaxy lives on the star-forming sequence, with SFR distributed like a log-normal. Let’s create a Monte Carlo realization of such a galaxy population:

import numpy as np
ngals = int(1e4)
log_mstar = np.random.uniform(10, 12, ngals)
galaxy_mstar = 10**log_mstar
mean_log_sfr = np.interp(log_mstar, [10, 11, 12], [0, 1, 2])
log_sfr = np.random.normal(loc=mean_log_sfr, scale=0.2, size=ngals)
galaxy_sfr = 10**log_sfr


Now let’s suppose that some simple stellar-to-halo mass relation accurately maps stellar mass onto dark matter halos, and that SFR is just determined by the CAM hypothesis that halos with large accretion rates host galaxies with large star-formation rates. First, we grab a halo catalog and map stellar mass onto it:

from halotools.sim_manager import CachedHaloCatalog
halocat = CachedHaloCatalog(simname='bolplanck', redshift=0)
halo_mass = halocat.halo_table['halo_mvir']
from halotools.empirical_models import Behroozi10SmHm
model = Behroozi10SmHm(redshift=0)
halo_mstar = model.mc_stellar_mass(prim_haloprop=halo_mass)
halo_acc_rate = halocat.halo_table['halo_dmvir_dt_100myr']


Now we show how to map star-formation rate onto these halos using CAM.

from halotools.empirical_models import conditional_abunmatch
nwin = 101
halo_sfr = conditional_abunmatch(halo_mstar, halo_acc_rate, galaxy_mstar, galaxy_sfr, nwin)


The rest of this tutorial will help you understand the details behind this example, as well as other applications that involve different variations of the technique.

## Worked Examples¶

CAM applications take on a slightly different form depending on whether or not you can analytically evaluate the inverse CDF of the galaxy property you are modeling. So the examples in this tutorial are divided into two categories:

### Modeling a galaxy property with a simple analytical distribution¶

Many galaxy properties are well-described by straightforward statistical distributions. For example, if your distribution can be approximated by a log-normal or power law, then the functions implemented in scipy.stats can be used to analytically evaluate the inverse CDF. Each of the following tutorials gives an example of how to apply CAM in such a situation:

### Modeling a galaxy property without a known analytical distribution¶

In many cases, evaluating the inverse CDF analytically is intractible, and it can only be numerically tabulated from some sample data. The examples below illustrate a few CAM applications for such galaxy properties: