Observations of the Initial Formation and Evolution of Spiral galaxies at $1 < z < 3$ in the CANDELS fields

TL;DR

This study identifies and characterizes a significant population of proto-spiral and clumpy galaxies at redshifts 1 to 3, revealing their evolution, properties, and likely formation mechanisms in the early universe.

Contribution

It provides the first detailed observational analysis of proto-spiral galaxies at high redshift, including their number density, sizes, star formation rates, and evolutionary trends.

Findings

High number density of massive spiral-like galaxies at z>1

Number of spirals increases by a factor of 10 from z=2.5 to z=0.5

These galaxies have large sizes and high star formation rates at z>2

Abstract

Many aspects concerning the formation of spiral and disc galaxies remain unresolved, despite their discovery and detailed study over the past $150$ years. As such, we present the results of an observational search for proto-spiral galaxies and their earliest formation, including the discovery of a significant population of spiral-like and clumpy galaxies at $z>1$ in deep \textit{Hubble Space Telescope} CANDELS imaging. We carry out a detailed analysis of this population, characterising their number density evolution, masses, star formation rates and sizes. Overall, we find a surprisingly high overall number density of massive $M_{*} >10^{10}\mathrm{M}_{\odot}$ spiral-like galaxies (including clumpy spirals) at $z > 1$ of $0.18\,{\rm per}\, \mathrm{arcmin}^{-2}$. We measure and characterise the decline in the number of these systems at higher redshift using simulations to correct for redshift effects in identifications, finding that the true fraction of spiral-like galaxies grows at lower redshifts as $\sim$ $(1+z)^{-1.1}$. This is such that the absolute numbers of spirals increases by a factor of $\sim 10$ between $z = 2.5$ and $z = 0.5$. We also demonstrate that these spiral-like systems have large sizes at $z>2$, and high star formation rates, above the main-sequence, These galaxies represent a major mode of galaxy formation in the early universe, perhaps driven by the spiral structure itself. We finally discuss the origin of these systems, including their likely formation through gas accretion and minor mergers, but conclude that major mergers are an unlikely cause.