Audio narration by the author is available above
10 billion years ago, galaxies of the Universe were ablaze with the light of newly forming stars. This epic phase of history is known as “Cosmic Noon” – the height of all star creation. Galaxies like our Milky Way aren’t creating stars at nearly the rates they were in the ancient past. However, there is a time when galaxies in the present can explode with star formation – when they collide with each other. This recently published collage of merging galaxies by the Hubble HiPEEC survey (Hubble imaging Probe of Extreme Environments and Clusters) highlights six of these collisions which help us understand star formation in the early Universe.
An international research team led by Dr. Angela Adamo studied these six Hubble targets, captured between 2008 and 2020, to understand star formation rates in the chaotic conditions of galaxy collisions. Ancient star forming galaxies are not distorted and twisted like the mergers we see in the local galaxy. They are large disk galaxies we’re more familiar with. But these local collisions serve as a nearby laboratory replicating conditions of the early Universe.
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The Milky Way creates between 1.5 to 3 solar masses (mass of our own Sun) worth of stars each year. Colliding galaxies can create upwards of 100 solar masses per year. These six mergers are all in various stages of collision. Galaxies, for all their hundreds of billions of stars, are mainly empty space. It’s actually possible for two galaxies to merge and yet no two individual stars collide with each other. Rather galaxies pass through one another several times until they finally coalesce. Eventually the nuclei of both galaxies merge to become one larger galaxy – an epic cosmic dance routine over billions of years.
Earliest in the merging phase are NGC 3256, 3690, and 6052 with 3690 still showing two distinct galactic nuclei. NGC 34, 1614, and 4194 are the most advanced with NGC 34 at the final stage of coalescence.
Shining Through the Shroud
The six targets chosen are within 80 Mpc (Megaparsec = 3.26 million light years) where Hubble can resolve large star forming clusters within each of the colliding galaxies. For photography nerds, resolution scales for the closest of the 6 targets are 6 parsecs per pixel while the more distant are 10 parsecs per pixel. Light years and light years in one tiny point of light. The targets were selected because they are oriented face-on meaning Hubble can scan the entire surface of the galaxy for star forming clusters. The clusters themselves are enshrouded by massive clouds of dust and gas, the raw material for star formation.
Interstellar dust causes “extinction” a process where light is literally extinguished as it’s absorbed. However, star forming clusters are powerful sources of infrared light and a particular red light known as Hydrogen-Alpha created by young massive stars blasting hydrogen gas with their intense radiation. Both infrared and H-Alpha can cut through the shroud to be observed by Hubble. Merging galaxies are ablaze with infrared light. Classified as “Luminous Infrared Galaxies” (LIRG) they are brighter in infrared than the entire light spectrum of other galaxies. The image data from Hubble is processed to isolate star forming clusters by age and mass filtering out both foreground stars from our own galaxy and the light of distant background galaxies.
Every Last Drop
Researchers discovered enormous star forming clusters within the merging systems – far larger than found in our own galaxy. The largest young star clusters in the Milky Way can reach tens of thousands of solar masses. As galaxies merge, more and more massive clusters form – the largest created in the later stages of coalescence. NGC 34 features a cluster upwards of 20 million solar masses that is 100 million years old. The younger mergers have a greater percentage of clusters less than 10 million years old indicating the rate of star formation has been steadily increasing.
“These systems are among the most efficient environments to form star clusters in the local Universe”Adamo et al 2021
The raw material to form stars is interstellar hydrogen gas. Galaxies contained a greater abundance and density of this gas in the past which was consumed during Cosmic Noon. Hydrogen remains in galaxies like the Milky Way but the gas isn’t nearly as concentrated resulting in lower rates of star formation and smaller star clusters. The merging of galaxies create tidal forces through gravity that funnel this remaining gas into high density, concentrated regions resulting in massive star forming clusters and a cascade of star formation – a “starburst.”
The researchers designate a radius in each merger called R80 where 80% of star formation is occurring. Within that 80% radius the mergers are the most efficient at forming star clusters. This radius also contains the youngest star clusters around 10 million years old. As these clusters age, they are displaced by the tidal forces of the collision and drift to different regions of the galaxies.
Ancient Collision Scars
While the Milky Way doesn’t feature massive young clusters of stars, our galaxy does have very large old clusters of stars known as Globular Clusters. The largest we’ve observed in the Milky Way is called Omega Centauri which contains about 10 million stars weighing in at 4 million solar masses that are billions of years old. The origin of globular clusters is not entirely known but they are thought to originate from the Milky Way’s own collision with other galaxies in the past or formed during Cosmic Noon. The HiPEEC researchers suggest that the young massive star clusters observed in these mergers may evolve into globular clusters and call on future research to study this possibility. The team also recommends revisiting these 6 mergers with the upcoming James Webb Space Telescope which will see infrared targets with even more resolution and sharpness than Hubble.
The Milky Way is also destined for a future merger when it collides with our neighbouring Andromeda Galaxy to become…”Milkomeda” (we have 4.5 billion years to come up with a better name). The merger will trigger our own future surge in star formation. Anyone around in the Milky Way will see the sky transform with the glow of giant star forming clusters across the entire galaxy. In the meantime, enjoy these stunning mergers we see now, courtesy of Hubble.
More to Explore:
Hubble Showcases 6 Galaxy Mergers | ESA/Hubble (Full Sized Versions of all the Images)
[2008.12794] Star cluster formation in the most extreme environments: Insights from the HiPEEC survey (arxiv.org) (Original Research Paper Open Access)