Missing the Fireworks in Massive Dusty Galaxies

Title: The Star Formation Rate of Massive Dusty Galaxies at Early Cosmic Times

Authors: Zacharias E. Escalante, Shardha Jogee, and Sydney Sherman

First Author’s Institution: The University of Texas at Austin

Status: Published in ASPCS, open access on ArXiv

The results of the first extragalactic surveys undertaken in the submillimeter wavelength regime in the early 2000s were dramatic and astonishing. Images acquired from staring blindly at the sky with the James Clerk Maxwell Telescope demonstrated that there exists a population of galaxies that shine with phenomenal luminosity at these long wavelengths.

After much debate, it was hypothesized that the reason for these “submillimeter galaxies” is massive reservoirs of dust. Generally, infrared emission is due to absorption and re-emission of light from star-formation. The amount of dust seen in these galaxies implies star-formation rates in excess of 1000 M*/yr! Their extreme dust emission even allows us to detect submillimeter galaxies in the early universe.

Fast-forward to 2020. Multitudes of focused observations of these submillimeter galaxies in other wavelength regimes (x-ray, ultraviolet, optical, and near-infrared) have bolstered our standing hypothesis of a population of dust-obscured, infrared-bright, and extremely star-forming galaxies. We have dubbed them “Dusty Star-Forming Galaxies”, or DSFGs. However, their existence continues to challenge traditional galaxy formation models and the cause of their immense dust content is unknown. Despite these open questions, they provide a valuable lens through which we can study the first two billion years of the universe.

Today’s Astrobite proposes that like viewing a fireworks display through the smoke, we might be missing some of the action.

Current galaxy surveys are primarily composed of ultraviolet and optical observations. These wavelengths are easily accessible from the ground and probed with some of the largest telescopes ever constructed with excellent angular resolution. Infrared and submillimeter light, however, suffer from a phenomenon known as confusion, which arises from poor angular resolution “blending” galaxies together. The result is that a galaxy seen in the infrared may correspond to several galaxies seen in the optical.

Figure 1. Comparison of fitting the observational data (dots) with spectral energy templates from the limited UV-infrared data (left) and full UV-FIR data (right). The dramatic effect of dust extinction can be seen in the attenuated (red) and unattenuated (blue dotted) templates on the right. Taken from Figures 1 and 2 of the paper.

This SED-fitting procedure was carried out in two ways. Firstly, they include only the observations which are typically included in galaxy surveys: ultraviolet through the infrared. Secondly, they fit the entire observational dataset including the FIR and submillimeter. The results for one galaxy is shown in Figure 1.

They then recover the star-formation rates corresponding to the best-fit templates. 

For the first fitting method with the limited dataset, the SED is corrected for dust based on how much of the UV emission has been attenuated away. However, this correction is highly uncertain since without the far-infrared data the amount of dust is unconstrained. Hence, the SFR for this UV-corrected measurement is also uncertain.

For the second fitting, the amount of dust and the UV attenuation are both well-constrained by the measurements: the missing UV flux and the increased far-infrared flux. This enables a direct constraint on the SFR from both the UV emission and then UV absorption and FIR re-emission by dust.

Figure 2. Comparison of the star-formation rates (SFR) derived from the limited UV-corrected dataset and the full UV-FIR dataset. The one-to-one line is shown in blue. Taken from Figure 3 of the paper.

Comparing these two methods reveals a concerning disagreement. As shown in Figure 2, the inclusion of infrared and submillimeter data results in estimates of 3.5 times more dust on average, and by more than an order of magnitude in 7 cases!

This study demonstrates that galaxy studies without infrared and submillimeter data may underestimate the star-formation furor of these cosmic fireworks. High confidence estimates of star-formation rates are crucial if we are to understand how the stellar mass in the universe was built up over time. Significant advancements will be enabled by the next-generation of far-infrared and submillimeter facilities (e.g. Origins, ELT), which will be better equipped to observe with higher angular resolution and greater survey speed. 

About John Weaver

I am a second year PhD student at the Cosmic Dawn Center at the University of Copenhagen, where I study the formation and evolution of galaxies across cosmic time with incredibly deep observations in the optical and infrared. I got my start at a little planetarium, and I've been doing lots of public outreach and citizen science ever since.

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