From IRAS to IRS: Evolution of the Most Luminous Galaxies in the Universe

TL;DR

This study analyzes the evolution and obscuration of luminous galaxies across a broad redshift range using Spitzer IRS data, revealing significant dust obscuration and luminosity evolution up to z~3.

Contribution

It provides a comprehensive infrared spectroscopic analysis of diverse galaxy populations, highlighting the evolution of luminosity and dust obscuration in the most luminous galaxies over cosmic time.

Findings

Luminosity of starbursts and AGN increases as (1+z)^{2.5} up to z~2.5.

Most starbursts exhibit severe dust obscuration, with median UV correction factors of ~50.

Ultraviolet-selected starbursts are less obscured and appear more luminous than previously estimated.

Abstract

We summarize observations with the Spitzer Infrared Spectrograph (IRS) of 571 starbursts (strong PAH emission features), 128 obscured AGN (strong silicate absorption), and 39 unobscured AGN (silicate emission). Sources range in luminosity from 10^{8} to 10^{14} solar luminosities and continuously in redshift for 0 < z < 3. The most luminous starbursts and AGN evolve as (1+z)^{2.5} to z ~ 2.5; no clear evidence is found that this evolution ceases beyond z = 2.5. Dust obscuration in starbursts is determined by comparing PAH luminosity with ultraviolet luminosity and indicates severe obscuration in most starbursts, even those selected in the ultraviolet; the median ratio (intrinsic ultraviolet/observed ultraviolet) is ~ 50 for infrared selected starbursts and ~ 8 for ultraviolet selected starbursts. Obscuration increases with bolometric luminosity, but starbursts which appear most luminous in the ultraviolet are those with the least obscuration. This result indicates that extinction corrections are significantly underestimated for ultraviolet selected sources, suggesting that galaxies at z ~> 2 are more luminous than deduced only from rest frame ultraviolet observations.