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Galactic Pairs in the Early Universe

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Arp 274
Nearby pairs of merging galaxies, like these two spirals in Arp 274, are easy to identify. But how do we spot and interpret galactic duos in the distant, early universe? [NASA, ESA, M. Livio and the Hubble Heritage Team (STScI/AURA)]

In the spirit of Valentine’s Day, today we’ll be exploring apparent pairs of galaxies in the distant, early universe. How can we tell whether these duos are actually paired galaxies, as opposed to disguised singles?

Real Pair, or Trick of the Light?

epoch of reionization

In the schematic timeline of the universe, the epoch of reionization is when the first galaxies and quasars began to form and evolve. [NASA]

The statistics of merging galaxies throughout the universe reveal not only direct information about how galaxies interact, but also cosmological information about the structure of the universe. While we’ve observed many merging galaxy pairs at low redshift, however, it’s much more challenging to identify these duos in the early universe.

A merging pair of galaxies at high redshift appears to us as a pair of unresolved blobs that lie close to each other in the sky. But spotting such a set of objects doesn’t necessarily mean we’re looking at a merger! There are three possible scenarios to explain an observed apparent duo:

  1. It’s a pair of galaxies in a stage of merger.
  2. It’s a projection coincidence; the two galaxies aren’t truly near each other.
  3. It’s a single galaxy being gravitationally lensed by a foreground object. This strong lensing produces the appearance of multiple galaxies.
high-redshift galactic group

Hubble photometry of one of the three galaxy groups identified at z ~ 8, with the galaxies in the image labeled with their corresponding approximate photometric redshifts. [Adapted from Chaikin et al. 2018]

Hunting for Distant Duos

In a recent study led by Evgenii Chaikin (Peter the Great St. Petersburg Polytechnic University, Russia), a team of scientists has explored the Hubble Ultra Deep Field in search of high-redshift galaxies merging during the epoch of reionization, when the first galaxies formed and evolved.

Using an approach called the “dropout technique”, which leverages the visibility of the galaxies in different wavelength filters, Chaikin and collaborators obtain approximate redshifts for an initial sample of 7,000 objects. They find that roughly 50 have a redshift of z ~ 7, and 22 have a redshift of z ~ 8. None of the galaxies at z ~ 7 are in pairs, but the sample at z ~ 8 includes three groups for which the distance between galaxies is less than ~1 arcsecond.

But are these three pairs actual merging galaxies?

Conclusions from Statistics

simulated galaxies

Top: Gas density at z ~ 7.7 in the authors’ simulation output. Bottom: Mock observations of this output with Hubble’s WFC3 (left) and JWST’s NIRCam (right). [Adapted from Chaikin et al. 2018]

To answer this question, the authors next perform numerical simulations of galaxy formation and produce mock observations showing what the simulated field would look like in an equivalent deep Hubble exposure.

Based on their simulation statistics, Chaikin and collaborators argue that the three pairs at z ~ 8 do represent an unusually high merger fraction — but projection coincidences or lensing are far less likely scenarios to account for all three pairs. If the three pairs are indeed all merging galaxies, it could indicate that this Hubble field corresponds to a local overdensity at a redshift of z ~ 8.

Looking Ahead

The best way to improve on these measurements is to repeat this study with more advanced telescopes. Chaikin and collaborators demonstrate the superiority of the observations that the upcoming James Webb Space Telescope (JWST) will provide. They also point out the potential power of the Wide Field Infrared Survey Telescope (WFIRST) — currently under threat under the proposed 2019 federal budget — to extend the observational horizon well into the epoch of reionization.

Continued studies backed by the power of these future telescopes are sure to discover a wealth of additional distant galactic duos, helping us to characterize the universe in its early stages.

Citation

Evgenii A. Chaikin et al 2018 ApJ 853 81. doi:10.3847/1538-4357/aaa196