Nav: Home

Astronomers pin down galaxy collision rate with Hubble data

October 27, 2011

A new analysis of Hubble surveys, combined with simulations of galaxy interactions, reveals that the merger rate of galaxies over the last 8 billion to 9 billion years falls between the previous estimates.

The galaxy merger rate is one of the fundamental measures of galaxy evolution, yielding clues to how galaxies bulked up over time through encounters with other galaxies. And yet, a huge discrepancy exists over how often galaxies coalesced in the past. Measurements of galaxies in deep-field surveys made by NASA's Hubble Space Telescope generated a broad range of results: anywhere from 5 percent to 25 percent of the galaxies were merging.

The study, led by Jennifer Lotz of the Space Telescope Science Institute in Baltimore, Md., analyzed galaxy interactions at different distances, allowing the astronomers to compare mergers over time. Lotz's team found that galaxies gained quite a bit of mass through collisions with other galaxies. Large galaxies merged with each other on average once over the past 9 billion years. Small galaxies were coalescing with large galaxies more frequently. In one of the first measurements of smashups between dwarf and massive galaxies in the distant universe, Lotz's team found these mergers happened three times more often than encounters between two hefty galaxies.

"Having an accurate value for the merger rate is critical because galactic collisions may be a key process that drives galaxy assembly, rapid star formation at early times, and the accretion of gas onto central supermassive black holes at the centers of galaxies," Lotz explains.

The team's results are accepted for publication appeared in The Astrophysical Journal.

The problem with previous Hubble estimates is that astronomers used different methods to count the mergers.

"These different techniques probe mergers at different 'snapshots' in time along the merger process," Lotz says. "It is a little bit like trying to count car crashes by taking snapshots. If you look for cars on a collision course, you will only see a few of them. If you count up the number of wrecked cars you see afterwards, you will see many more. Studies that looked for close pairs of galaxies that appeared ready to collide gave much lower numbers of mergers than those that searched for galaxies with disturbed shapes, evidence that they're in smashups."

To figure out how many encounters happen over time, Lotz needed to understand how long merging galaxies would look like "wrecks" before they settle down and begin to look like normal galaxies again.

That's why Lotz and her team turned to highly detailed computer simulations to help make sense of the Hubble photographs. The team made simulations of the many possible galaxy collision scenarios and then mapped them to Hubble images of galaxy interactions.

Creating the computer models was a time-consuming process. Lotz's team tried to account for a broad range of merger possibilities, from a pair of galaxies with equal masses joining together to an interaction between a giant galaxy and a puny one. The team also analyzed different orbits for the galaxies, possible collision impacts, and how galaxies were oriented to each other. In all, the group came up with 57 different merger scenarios and studied the mergers from 10 different viewing angles. "Viewing the simulations was akin to watching a slow-motion car crash," Lotz says.

The simulations followed the galaxies for 2 billion to 3 billion years, beginning at the first encounter and continuing until the union was completed, about a billion years later.

"Our simulations offer a realistic picture of mergers between galaxies," Lotz says.

In addition to studying the smashups between giant galaxies, the team also analyzed encounters among puny galaxies. Spotting collisions with small galaxies are difficult because the objects are so dim relative to their larger companions.

"Dwarf galaxies are the most common galaxy in the universe," Lotz says. "They may have contributed to the buildup of large galaxies. In fact, our own Milky Way galaxy had several such mergers with small galaxies in its recent past, which helped to build up the outer regions of its halo. This study provides the first quantitative understanding of how the number of galaxies disturbed by these minor mergers changed with time."

Lotz compared her simulation images with pictures of thousands of galaxies taken from some of Hubble's largest surveys, including the All-Wavelength Extended Groth Strip International Survey (AEGIS), the Cosmological Evolution Survey (COSMOS), and the Great Observatories Origins Deep Survey (GOODS), as well as mergers identified by the DEEP2 survey with the W.M. Keck Observatory in Hawaii. She and other groups had identified about a thousand merger candidates from these surveys but initially found very different merger rates.

"When we applied what we learned from the simulations to the Hubble surveys in our study, we derived much more consistent results," Lotz says.

Her next goal is to analyze galaxies that were interacting around 11 billion years ago, when star formation across the universe peaked, to see if the merger rate rises along with the star formation rate. A link between the two would mean galaxy encounters incite rapid star birth.
-end-
In addition to Lotz, the coauthors of the paper include Patrik Jonsson of Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass; T. J. Cox of Carnegie Observatories in Pasadena, Calif.; Darren Croton of the Centre for Astrophysics and Supercomputing at Swinburne University of Technology in Hawthorn, Australia; Joel R. Primack of the University of California, Santa Cruz; Rachel S. Somerville of the Space Telescope Science Institute and The Johns Hopkins University in Baltimore, Md.; and Kyle Stewart of NASA's Jet Propulsion Laboratory in Pasadena, Calif.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI) conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.

NASA/Goddard Space Flight Center

Related Star Formation Articles:

Star formation project maps nearby interstellar clouds
Astronomers have captured new, detailed maps of three nearby interstellar gas clouds containing regions of ongoing high-mass star formation.
Scientists discover pulsating remains of a star in an eclipsing double star system
Scientists from the University of Sheffield have discovered a pulsating ancient star in a double star system, which will allow them to access important information on the history of how stars like our Sun evolve and eventually die.
Distant milky way-like galaxies reveal star formation history of the universe
Thousands of galaxies are visible in this radio image of an area in the Southern Sky, made with the MeerKAT telescope.
Cascades of gas around young star indicate early stages of planet formation
What does a gestating baby planet look like? New research in Nature by a team including Carnegie's Jaehan Bae investigated the effects of three planets in the process of forming around a young star, revealing the source of their atmospheres.
Massive exoplanet orbiting tiny star challenges planet formation theory
Astronomers have discovered a giant Jupiter-like exoplanet in an unlikely location -- orbiting a small red dwarf star.
ALMA pinpoints the formation site of planet around nearest young star
Researchers using ALMA (Atacama Large Millimeter/submillimeter Array) found a small dust concentration in the disk around TW Hydrae, the nearest young star.
Star formation burst in the Milky Way 2-3 million years ago
A team led by researchers of the Institute of Cosmos Sciences of the University of Barcelona and the Besançon Astronomical Observatory have found, analysing data from the Gaia satellite, that a severe star formation burst occurred in the Milky Way about to and three thousand million years ago.
The rise and fall of Ziggy star formation and the rich dust from ancient stars
Researchers have detected a radio signal from abundant interstellar dust in MACS0416_Y1, a galaxy 13.2 billion light-years away in the constellation Eridanus.
Lifting the veil on star formation in the Orion Nebula
Writing in 'Nature', an international research team including astronomers from Cologne describe their discovery that stellar wind from a newborn star in the Orion Nebula is preventing more stars from forming nearby.
Massive star's unusual death heralds the birth of compact neutron star binary
Carnegie's Anthony Piro was part of a Caltech-led team of astronomers who observed the peculiar death of a massive star that exploded in a surprisingly faint and rapidly fading supernova, possibly creating a compact neutron star binary system.
More Star Formation News and Star Formation Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Making Amends
What makes a true apology? What does it mean to make amends for past mistakes? This hour, TED speakers explore how repairing the wrongs of the past is the first step toward healing for the future. Guests include historian and preservationist Brent Leggs, law professor Martha Minow, librarian Dawn Wacek, and playwright V (formerly Eve Ensler).
Now Playing: Science for the People

#566 Is Your Gut Leaking?
This week we're busting the human gut wide open with Dr. Alessio Fasano from the Center for Celiac Research and Treatment at Massachusetts General Hospital. Join host Anika Hazra for our discussion separating fact from fiction on the controversial topic of leaky gut syndrome. We cover everything from what causes a leaky gut to interpreting the results of a gut microbiome test! Related links: Center for Celiac Research and Treatment website and their YouTube channel
Now Playing: Radiolab

The Flag and the Fury
How do you actually make change in the world? For 126 years, Mississippi has had the Confederate battle flag on their state flag, and they were the last state in the nation where that emblem remained "officially" flying.  A few days ago, that flag came down. A few days before that, it coming down would have seemed impossible. We dive into the story behind this de-flagging: a journey involving a clash of histories, designs, families, and even cheerleading. This show is a collaboration with OSM Audio. Kiese Laymon's memoir Heavy is here. And the Hospitality Flag webpage is here.