The epoch of the Universe’s history known as cosmic noon—around redshift z~2, more than 10 billion years in the past—earned its moniker as it marks the peak of star formation activity in the Universe. During this particularly active time, some rare galaxies are found to assemble stars at rates exceeding 1000 solar masses per year, in stark contrast to the Milky Way’s meager average of 2 solar masses per year. In fact, there have been no objects found in the past 5 billion years that come anywhere close to the star formation rates of these hyperluminous galaxies seen in the first few billion years after the Big Bang.
Image of PJ0116-24, with stellar light traced by Hubble Space Telescope imaging at 1.6 μm and cold molecular gas shown in red. The light from the foreground galaxy has been removed to highlight the lensed object of interest.
The story behind this dramatic transformation of galaxies is complex, but it’s complicated even further by the fact that it is much easier to resolve small-scale spatial details in nearby galaxies than in the distant Universe. To illustrate this, a galaxy with a light travel time of 1 billion years appears nearly 6 times larger in the sky than a galaxy at a distance of 10 billion lightyears. Features that might be clearly resolved at redshift z~0.1 (1 billion years ago) would be completely blurred if the same galaxy were relocated to z~2. To draw an analogy, this difference in acuity would be like someone with 20/20 vision vs. someone who can barely read the letter at the top row of an optometrist’s chart!
Half of the stars in the Universe formed during cosmic noon, which by contrast endured for only a quarter of the Universe’s age, so there is good reason to seek a better understanding of this perplexing and overactive time period. With this recent work published in Nature Astronomy, we focus our attention on the hyperluminous galaxy PJ0116-24 at cosmic noon. Through our telescopes, this object hardly looks like a typical galaxy: through the phenomenon of gravitational lensing, its light rays pass by a more nearby galaxy on their way to us and are deflected by the mass of the foreground galaxy. The result is that the background is distorted and magnified into an annular shape known commonly as an Einstein ring. We take advantage of this sizable magnification to probe details that are otherwise too small even for most state-of-the-art telescopes—effectively turning an entire galaxy into a cosmic telescope.
Perhaps the most intriguing result of this analysis comes from the imaging of molecular and ionized gas in PJ0116-24, through infrared and millimeter-wave observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Enhanced Resolution Imager and Spectrograph (ERIS) of European Southern Observatory’s Very Large Telescope (VLT). The ionized gas originates from HII regions surrounding young, massive stars, and therefore is a tracer of the most recent star formation activity in the galaxy. On the other hand, the molecular gas probed by carbon monoxide (CO) constitutes the fuel for ongoing and future star formation. Together, we obtain a picture of this extreme galaxy’s recent past and even its near future.
These spectroscopic observations also reveal the kinematics of the gas, so we observe which directions different parcels of gas are moving and at what velocities. This facilitates a comparison between what we observe and what we expect for typical rotating disk galaxies. Large deviations from regular, circular motions suggest a very chaotic and turbulent interstellar medium, comprised of the gas and dust that fill the space between stars in galaxies. The presence of these complex motions typically favors a recent collision or interaction with another galaxy. Major mergers—collisions between galaxies with comparable masses—play an important role in driving star formation in the most active systems in the low-redshift Universe. Yet, many recent studies of gas kinematics in high-redshift starbursts have highlighted the fundamentally different mechanisms at play in extreme galaxies in the early Universe. Such galaxies are capable of evolving secularly in isolation, without perturbation from galaxy interactions, pointing to a scenario where gas is accreted from the galaxy’s surroundings onto a rotating disk—not entirely unlike the structure of our own Milky Way.
How then does such an object compress gas to form stars so catastrophically quickly? Theory-based simulations suggest that material accreting onto massive galaxies can become gravitationally unstable and fragment into clumps throughout the disk. The widespread production of stars in the rotating disk suggests that this precise phenomenon might be at play in PJ0116-24.
A position-velocity diagram for PJ0116-24, which demonstrates how gas within the galaxy is moving at greater velocity in its outer regions. Yellow contours and dashed curves show the best-fit rotating disk model; a close agreement between the data and the model suggests that both the molecular and ionized gas are generally confined to a rotating disk structure.
PJ0116-24 is a textbook example of a population referred to as “dusty star-forming galaxies” (a broader description of what were originally discovered in the 1990s as “submillimeter galaxies,” named for their discovery wavelength). Much of their prodigious star formation that takes center stage in observations at (sub)millimeter wavelengths is hidden behind curtains of interstellar dust, which obscures and reddens their light (especially at optical wavelengths). Without some conception of how this dust is concentrated in galaxies like PJ0116-24, our view of them would be biased in favor of the regions that have the least amount of dust in the way. With the spectra taken by ERIS, we can compare the strengths of emission lines Hα and Hβ (originating from gas ionized by star formation) with the ratio that we would expect in the absence of attenuating dust. We find a ratio–referred to as the Balmer decrement–that is essentially unparalleled in previous studies, even at the high-mass end. This suggests that we are peering through an extremely thick shroud of dust that makes the light 15 times fainter. This would not be possible without the boost provided by gravitational lensing and without the sensitivity of the novel ERIS instrument; this represents an exciting new frontier in our study of high-redshift starbursts.
The ratio in Hα-to-Hβ brightness vs. stellar mass of PJ0116-24. Along with G165, another lensed member of the PASSAGES sample, these objects have much higher Balmer decrements than objects of similar mass, indicating greater dimming from interstellar dust. Amplification from lensing makes it possible to detect these lines in dust-obscured galaxies that would otherwise be very faint.
It is difficult to conceptualize just how extreme these high-redshift galaxies are. We estimate that an average 1500 Suns’ worth of stars are created each year in PJ0116-24, which is an astounding 750 times more than the Milky Way galaxy produces today. Not just that, but PJ0116-24 managed to already form 3 times the number of stars that exist in our galaxy, despite being observed at a time that was only one-fourth of the Universe’s age. With the PASSAGES project (Planck All-Sky Survey to Analyze Gravitationally-lensed Extreme Starbursts), we are examining more than two dozen other lensed galaxies that are similarly monstrous. Via this sample, the Universe provides us with an ideal laboratory for better understanding the extreme limits of galaxy formation and evolution. Ultimately, this research is revealing the shockingly turbulent history of our Universe’s past and helping us to draw connections with the present-day cosmos.
Cover image: Artist’s depiction of the gravitationally lensed galaxy PJ0116-24 and what the undistorted galaxy might look like. Image credit: J. Cook-Chrysos, A. Vishwas (Cornell University), P. Kamieneski (Arizona State University)