Galaxy disc scaling relations: A tight linear galaxy -- halo connection challenges abundance matching

TL;DR

This study finds that galaxy formation parameters are simple, low-scatter functions of mass, challenging complex models and supporting a nearly complete baryon-to-star conversion in massive spirals, thus questioning abundance matching assumptions.

Contribution

It demonstrates that galaxy formation efficiencies and angular momentum retention are best described by linear, low-scatter relations, contradicting models with abundance matching priors.

Findings

Scatter in galaxy formation parameters is very small (~0.07 dex).

Linear models outperform complex double power law models.

Massive spirals likely convert nearly all halo baryons into stars.

Abstract

In $\Lambda$CDM cosmology, to first order, galaxies form out of the cooling of baryons within the virial radius of their dark matter halo. The fractions of mass and angular momentum retained in the baryonic and stellar components of disc galaxies put strong constraints on our understanding of galaxy formation. In this work, we derive the fraction of angular momentum retained in the stellar component of spirals, $f_j$, the global star formation efficiency $f_M$, and the ratio of the asymptotic circular velocity ($V_{\rm flat}$) to the virial velocity $f_V$, and their scatter, by fitting simultaneously the observed stellar mass-velocity (Tully-Fisher), size-mass, and mass-angular momentum (Fall) relations. We compare the goodness of fit of three models: (i) where the logarithm of $f_j$, $f_M$, and $f_V$ vary linearly with the logarithm of the observable $V_{\rm flat}$; (ii) where these values vary as a double power law; and (iii) where these values also vary as a double power law but with a prior imposed on $f_M$ such that it follows the expectations from widely used abundance matching models. We conclude that the scatter in these fractions is particularly small ($\sim 0.07$ dex) and that the linear model is by far statistically preferred to that with abundance matching priors. This indicates that the fundamental galaxy formation parameters are small-scatter single-slope monotonic functions of mass, instead of being complicated non-monotonic functions. This incidentally confirms that the most massive spiral galaxies should have turned nearly all the baryons associated with their haloes into stars. We call this the failed feedback problem.