The Universe in Formation. Hubble Sees 6 Examples of Merging Galaxies

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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.

Newly released collage of six galaxy mergers used in the HiPEEC survey.
Top Row Left to Right: NGC 3256, 1614, 4195 Bottom Row Left To Right: NGC 3690, 6052, 34
– Credit ESA/Hubble/NASA

Cosmic Ballet

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.

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.

Computer simulation of the future merger between the Milky Way and Andromeda c – NASA

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.

(Top Left in the Collage) NGC 3256 is about 100 million light-years from Earth and provides an ideal target in which to investigate starbursts that have been triggered by galaxy mergers. Credit – ESA/Hubble/NASA

(Top Middle of the Collage) NGC 1614 is 211 million light years away. It has a bright centre and two inner spiral arms with an outer structure that consists principally of a large one-sided curved extension of one of these arms to the lower right, and a long, almost straight tail that emerges from the nucleus and crosses the extended arm to the upper right.
Image and description Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

(Top Right in Collage) – NGC 4194, 129 million light years away, is also known as the Medusa merger. An early galaxy consumed a smaller gas-rich system, throwing out streams of stars and dust out into space. These streams, seen rising from the top of the merger galaxy, resemble the writhing snakes that Medusa, a monster in ancient Greek mythology, famously had on her head in place of hair, lending the object its intriguing name. The Medusa merger is located about 130 million light-years away in the constellation of Ursa Major (The Great Bear).
Image and Description Credit: ESA/Hubble & NASA, A. Adamo

(Bottom Left of Collage) This system consists of a pair of galaxies, dubbed IC 694 and NGC 3690, 130 million light years away, which made a close pass some 700 million years ago. As a result of this interaction, the system underwent a fierce burst of star formation. In the last fifteen years or so six supernovae have popped off in the outer reaches of the galaxy, making this system a distinguished supernova factory.
Image and Description Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

(Bottom Middle of Collage) Located in the constellation of Hercules, about 230 million light-years away, NGC 6052 is a pair of colliding galaxies. They were first discovered in 1784 by William Herschel and were originally classified as a single irregular galaxy because of their odd shape. However, we now know that NGC 6052 actually consists of two galaxies that are in the process of colliding.
Image and Description Credit: ESA/Hubble & NASA, A. Adamo et al.

(Bottom Right of Collage) Lying in the constellation Cetus (The Sea Monster) at a distance of 250 million light years, NGC 34’s outer region appears almost translucent, pin pricked with stars and strange wispy tendrils. This image shows the galaxy’s bright centre, a result of this merging event that has created a burst of new star formation and lit up the surrounding gas. Image and Description Credit: ESA/Hubble/NASA A. Adamo et al.

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.

Star clusters in NGC 3256 isolated from other structures in the image
Credit: Figure 2 in Adamo et al 2020

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.

One of the closest star forming regions to Earth, the Great Orion Nebula. There are approximately 700 stars forming in the nebula which weighs in at 2000 solar masses, much smaller than the star forming regions seen in colliding galaxies.
Credit: Matthew Cimone Trottier Observatory

“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.

A zoom-in cutout of the inner kiloparsec of NGC1614. The cluster positions are colour-coded accordingly to their mass (left) and age (right). The circular dashed lines note the clusters contained with R(80%). Credit: Fig 8 Adamo et al. 2020

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.

Omega Centauri Globular Cluster in the Milky Way. The largest known Globular Star Cluster in our Galaxy containing about 10 million stars located in the constellation Centaurus at a distance of 17,000 light years. Credit: ESO CC by 4.0

Andromeda’s Collision with the Milky Way – Fraser Cain Universe Today

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.