Massive galaxies grew faster than dark matter models predicted, study finds
Astronomers mapping 10 billion years of cosmic history discovered that the biggest galaxies are more abundant than dark matter theory suggests they should be. The finding, based on the deepest infrared survey to date, could reshape how we understand galaxy formation and has implications for computational models used across astrophysics and cosmology research.
Originaltitel: Euclid preparation LXXXVI. Cosmic Dawn Survey: Evolution of the galaxy stellar mass function across 0.2 < z ≤ 6.5 measured over 10 square degrees
<p>The Cosmic Dawn Survey pre-launch catalogues cover an e ffective 10.13 deg<sup>2</sup> area with uniform deep Spitzer/IRAC data (m ∼ 25 mag, 5σ), the largest area covered to these depths at IR wavelengths. We used these data to gain new insight into the growth of stellar mass across cosmic history by characterising the evolution of the galaxy stellar mass function through 0.2 < z ≤ 6.5. The total volume (0.62 Gpc<sup>3</sup>) represents an order of magnitude increase compared to previous works that explored z > 3 and significantly reduces cosmic variance, thus yielding strong constraints on the abundance of galaxies above the characteristic stellar mass (<em>M</em>*) across this ten billion year period. The evolution of the galaxy stellar mass function is generally consistent with results from the literature but now provides firm estimates of the number density where only upper limits were previously available. Contrasting the galaxy stellar mass function with the dark matter halo mass function suggests that massive galaxies ( <em>M</em> greater than or similar to 10<sup>11</sup> M<sub><img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Codot" data-classname="equation" data-title="" /></sub>) at z > 3.5 required integrated star-formation e fficiencies of <em>M</em>/(<em>M</em><sub>h</sub><em>f</em><sub>b</sub>) greater than or similar to 0.25-0.5, in excess of the commonly held view of a 'universal peak e fficiency' from studies on the stellar-to-halo mass relation. Such increased e fficiencies imply an evolving peak in the stellar-tohalo mass relation at z > 3.5 that can be maintained if feedback mechanisms from active galactic nuclei and stellar processes are ine ffective at early times. In addition, a significant fraction of the most massive quiescent galaxies are observed to be in place by z ∼ 2.5-3. The apparent lack of change in their number density by z ∼ 0.2 is consistent with relatively little mass growth from mergers. Utilising the unique volume, we find evidence of an environmental dependence of the galaxy stellar mass function all the way through z ∼ 3.5 for the first time, though a more careful characterisation of the density field is ultimately required for confirmation.</p>