Abstract
It is commonly believed that galaxies use, throughout Hubble time, a very small fraction of the baryons associated with their dark matter halos to form stars. This so-called low star formation efficiency f⋆ ≡ M⋆/fbMhalo, where fb ≡ Ωb/Ωc is the cosmological baryon fraction, is expected to reach its peak at nearly L* (at efficiency ≈20%) and decline steeply at lower and higher masses. We have tested this using a sample of nearby star-forming galaxies, from dwarfs (M⋆ ≃ 107 M⊙) to high-mass spirals (M⋆ ≃ 1011 M⊙) with HI rotation curves and 3.6 μm photometry. We fit the observed rotation curves with a Bayesian approach by varying three parameters, stellar mass-to-light ratio Υ⋆, halo concentration c, and mass Mhalo. We found two surprising results: (1) the star formation efficiency is a monotonically increasing function of M⋆ with no sign of a decline at high masses, and (2) the most massive spirals (M⋆ ≃ 1−3 × 1011 M⊙) have f⋆ ≈ 0.3−1, i.e. they have turned nearly all the baryons associated with their halos into stars. These results imply that the most efficient galaxies at forming stars are massive spirals (not L* galaxies); they reach nearly 100% efficiency, and thus once both their cold and hot gas is considered in the baryon budget, they have virtually no missing baryons. Moreover, there is no evidence of mass quenching of the star formation occurring in galaxies up to halo masses of a few × 1012 M⊙.
Highlights
In our Universe, only about one-sixth of the total matter is baryonic, while the rest is widely thought to be in form of non-baryonic, collisionless, non-relativistic dark matter (e.g. Planck Collaboration VI 2018)
A considerable advantage of this method is that each system can be studied individually and halo masses, along with their associated uncertainties, can be determined in great detail for each object. We show that this approach leads to a coherent picture of the relation between stellar and halo mass in late-type galaxies, which in turn profoundly affects our perspective on the star formation efficiency in the high-mass regime
We modelled the rotation curves and we measured the posterior distributions of Υdisc, Mhalo, and c for all the 158 SPARC galaxies with inclination on the sky higher than 30◦
Summary
In our Universe, only about one-sixth of the total matter is baryonic, while the rest is widely thought to be in form of non-baryonic, collisionless, non-relativistic dark matter (e.g. Planck Collaboration VI 2018). Persic et al 1996; McConnachie 2012; Cappellari et al 2013; Desmond & Wechsler 2015; Read et al 2017; Katz et al 2017; hereafter K17) Amongst all these determinations there is wide consensus on the overall shape of the relation and, in particular, on the fact that the ratio of stellar-to-halo mass f = M / fb Mhalo (sometimes called star formation efficiency), is a non-monotonic function of mass with a peak ( f ≈ 0.2) at Mhalo ≈ 1012 M (roughly the mass of the Milky Way). Efficiencies of the order of 20% are still relatively low, implying that most baryons are still undetected even in these systems
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