galactic evolution Archives

June 12, 2013December 23, 2015 by Tammy Plotner

Stacking Galactic Signals Reveals A Clearer Universe

Jacinta studies distant galaxies like those shown in this image from the Hubble Space Telescope, using the new 'stacking' technique to gather information only available through radio telescope observations. Credit: NASA, STScI, and ESA.

Very similar to stacking astronomy images to achieve a better picture, researchers from the International Centre for Radio Astronomy Research (ICRAR) are employing new methods which will give us a clearer look at the history of the Universe. Through data taken with the next generation of radio telescopes like the Square Kilometer Array (SKA), scientists like Jacinta Delhaize can “stack” galactic signals en masse to study one of their most important properties… how much hydrogen gas is present.

Probing the cosmos with a telescope is virtually using a time machine. Astronomers are able to look back at the Universe as it appeared billions of years ago. By comparing the present with the past, they are able to chart its history. We can see how things have changed over the ages and speculate about the origin and future of the vastness of space and all its many wonders.

“Distant, younger, galaxies look very different to nearby galaxies, which means that they’ve changed, or evolved, over time,” said Delhaize. “The challenge is to try and figure out what physical properties within the galaxy have changed, and how and why this has happened.”

According to Delhaize a vital clue to solving the riddle lay in hydrogen gas. By understanding how much of it that galaxies contained will help us map their history.

“Hydrogen is the building block of the Universe, it’s what stars form from and what keeps a galaxy ‘alive’,” said Delhaize.

“Galaxies in the past formed stars at a much faster rate than galaxies now. We think that past galaxies had more hydrogen, and that might be why their star formation rate is higher.”

Jacinta Delhaize with CSIRO's Parkes Radio Telescope during one of her data collecting trips. Credit: Anita Redfern Photography. — Jacinta Delhaize with CSIRO’s Parkes Radio Telescope during one of her data collecting trips. Credit: Anita Redfern Photography.

When it comes to distant galaxies, they don’t give up their information easily. Even so, it was a task that Delhaize and her supervisors were determined to observe. The faint radio signals of hydrogen gas were nearly impossible to detect, but the new stacking method allowed the team to collect enough data for her research. By combining the weak signals of thousands of galaxies, Delhaize then “stacked” them to create a stronger, averaged signal,

“What we are trying to achieve with stacking is sort of like detecting a faint whisper in a room full of people shouting,” said Delhaize. “When you combine together thousands of whispers, you get a shout that you can hear above a noisy room, just like combining the radio light from thousands of galaxies to detect them above the background.”

However, it wasn’t a slow process. The researchers engaged CSIRO’s Parkes Radio Telescope for 87 hours and surveyed a large region of galactic landscape. Their work collected signals from hydrogen over a vast amount of space and stretched back over two billion years in time.

“The Parkes telescope views a big section of the sky at once, so it was quick to survey the large field we chose for our study,” said ICRAR Deputy Director and Jacinta’s supervisor, Professor Lister Staveley-Smith.

Stacking up a clearer picture of the Universe from ICRAR on Vimeo.

As Delhaize explains, observing such a massive volume of space means more accurate calculations of the average amount of hydrogen gas present in particular galaxies at a certain distance from Earth. These readings correspond to a given period in the history of the Universe. With this data, simulations can be created to depict the Universe’s evolution and give us a better understanding of how galaxies formed and evolved with time. What’s even more spectacular is that next generation telescopes like the international Square Kilometre Array (SKA) and CSIRO’s Australian SKA Pathfinder (ASKAP) will be able to observe even larger volumes of the Universe with higher resolution.

“That makes them fast, accurate and perfect for studying the distant Universe. We can use the stacking technique to get every last piece of valuable information out of their observations,” said Delhaize. “Bring on ASKAP and the SKA!”.

Original Story Source: International Centre for Radio Astronomy Research.

December 17, 2011April 26, 2016 by Jon Voisey

How Can Growing Galaxies Stay Silent?

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Beginning around 2005, astronomers began discovering the presence of very large galaxies at a distance of around 10 billion lightyears. But while these galaxies were large, they didn’t appear to have a similarly large number of formed stars. Given that astronomers expect galaxies to grow through mergers and mergers tend to trigger star formation, the presence of such large, undeveloped galaxies seemed odd. How could galaxies grow so much, yet have so few stars?

One of the leading propositions is that the galaxies have undergone frequent mergers, but each one was very small and didn’t encourage large scale star formation. In other words, instead of mergers between galaxies of similar size, large galaxies developed quickly and early in the universe, and then tended to accumulate through the integration of minor, dwarf galaxies. While this solution is straightforward, testing it is difficult since the galaxies in question are at vast distances and detecting the minor galaxies as they are devoured would require exceptional observations.

Seeking to test this hypothesis, a team of astronomers led by Andrew Newman from the California Institute of Technology combined observations from Hubble and the United Kingdom Infra-Red Telescope (UKIRT), to search for these diminutive companions. The team examined over 400 galaxies that didn’t display signs of active star formation (called “quiet” galaxies) in search of possible companion galaxies from distances of 10 billion light years to a relatively close 2 billion lightyears in order to determine how this minor merger rate has evolved over time.

From their study, they determined that around 15% of quiet galaxies had a nearby counterpart that had at least 10% the mass of the larger galaxy. This took into account the possibility that some galaxies may have been more distant but along the line of sight by ensuring that both galaxies had similar redshifts. Over time, the partner galaxies became rarer suggesting that they were becoming rarer as more were consumed by the larger brethren. Using this as a rate at which mergers must occur, the team was able to answer the question of whether or not these minor mergers could account for the galaxy growth discovered six years earlier.

For galaxies closer than a distance of roughly 8 billion light years, the rate of minor mergers was able to completely explain the overall growth of galaxies. However, for the growth rate of galaxies at times earlier than this, such minor mergers could only account for around half of the apparent growth.

The team proposes several reasons this may be the case. Firstly, many of the basic assumptions could be flawed. Teams may have overestimated the sizes of the massive galaxies, or underestimated the rate of star formation. These key properties were often derived from photometric surveys which are not as reliable as spectroscopic observations. In the future, if better observations can be made, these values may be revised and the problem may resolve itself. The other option is that there are simply additional processes at work that astronomers have yet to understand. Either way, the question of how growing galaxies avoid advertising their growth is unanswered.

October 16, 2011December 24, 2015 by Jon Voisey

Is M85 Missing a Black Hole?

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The conventional wisdom of galaxies is that they should have a central massive black hole (CMBH). The presence of such objects has been confirmed in our own galaxy as well as numerous other galaxies, including the Andromeda galaxy (M31) and even some dwarf galaxies. The mass of these objects, several million times the mass of the Sun, has been found to be related to many properties of galaxies as a whole, indicating that their presence may be critical in the formation and evolution of galaxies as a whole. As such, finding a massive galaxy without a central black hole would be quite surprising. Yet a recent study by astronomers from the University of Michigan Ann Arbor seems to have found an exception: The well known M85.

To determine the mass of the CMBH, the team used the spectrograph on board the Hubble Space Telescope to examine the pull the central object had on stars in the nearby vicinity. The higher this mass is, the more quickly the stars should orbit. This orbital velocity is detected as a shift in the color of the light, blue as the stars move towards us, red as they move away. The amount the light is shifted is dependent on just how fast they move.

Doppler shift of gas and dust caused by M84's supermassive black hole. Image Credit: Gary Bower, Richard Green (NOAO), the STIS Instrument Definition Team, and NASA

This technique has been used previously in other galaxies, including another large elliptical of similar brightness in the Messier catalog, M84. This galaxy had its CMBH probed by Hubble in 1997 and was determined to have a mass of 300 million solar masses.

When this method was applied to M85 the team did not discover a shift that would be indicative of a black hole with a mass expected for a galaxy of such size. Using another, indirect method of determining the CMBH mass by looking at the the amount of overall light from the galaxy, which is generally correlated with black hole mass, would indicate that M85 should contain a black hole of 300 million to 2 billion solar masses. Yet this study indicates that, if M85 contains a central black hole at all, the upper limit for the black hole would be around 65 million solar masses.

This study is not the first to report a non-detection for the galaxy, a 2009 study led by Alessandro Capetti from Osservatorio Astronoimco di Torino in Italy, searched M85 for signs of radio emission from the black hole region. Their study was unable to detect any significant radio waves from the core which, if M85 had a significant black hole, should be present, even with a small amount of gas feeding into the core.

Overall, these studies demonstrate a significant shortcoming in secondary methods of black hole mass estimation. Such indirect methods have been previously used with confidence and have even been the basis for studies drawing the connection between galaxy evolution and black hole mass. If cases like M85 are more common that previously thought, it may prompt astronomers to rethink just how connected black holes and a galaxies properties really are.

June 25, 2011December 24, 2015 by Jon Voisey

More to Meets the Eye in M33

The spiral galaxy M33 is one of the largest galaxies in our local group. This spiral galaxy is moderately tilted when viewed from Earth, displaying a lack of a distinct central bulge but prominent spiral arms. It has only one potential companion galaxy (the Pisces Dwarf) and its spiral arms are so pristine, they have been thought to be unperturbed by the accretion of dwarf galaxies that constantly occurs in the Milky Way and Andromeda galaxy. Yet these features are what has made M33 so hard to explain. Since larger galaxies are expected to form from the merger of smaller galaxies it is expected that M33 should show some scars from previous mergers. If this picture is true, where are they?

The role of galaxy accretion in our own galaxy was first revealed in 1994 with the discovery of the Sagittarius stellar stream. With the completion of the first Sloan Digitised Sky Survey, many more tidal streams were revealed in our own galaxy. Modeling of the kinematics of these streams suggested they should last billions of years before fading into the rest of the galaxy. Deep imaging of the Andromeda galaxy revealed stellar streams as well as a notable warping of the disc of the galaxy.

Yet M33 seems to lack obvious signs of these structures. In 2006, a spectroscopic study analyzed the bright red giants in the galaxy and found three distinct populations. One could be attributed to the disc, one to the halo, but the third was not immediately explicable. Could this be the relic of an ancient satellite?

Another potential clue on missing mergers was discovered in 2005 when a radio survey around M33 was conducted with the Arecibo telescope. This study uncovered large clouds with a thousand to a million solar masses worth of raw hydrogen suspended around the galaxy. Might these be incomplete dwarf galaxies that never merged into M33? A new study uses the Subaru telescope atop Mauna Kea to study these regions as well as the outskirts of M33 to better understand their history.

The team, led by Marco Grossi at the Observatório Astronómico de Lisboa in Portugal, did not find evidence of a stellar population in these clouds suggesting they were not likely to be galaxies in their own right. Instead, they suggest that these clouds may be analogous to hydrogen clouds around the Milky Way and Andromeda which are “often found close to stellar streams or disturbances in the stellar disc” where gas is pulled from a former satellite galaxy through tidal or ram-pressure stripping. This would constitute another piece of indirect evidence that M33 once underwent mergers of some sort.

Outside of these clouds, in the outskirts of the galaxy, the team uncovered a diverse population of stars beyond the main disc. The overall metallicity of these stars was lower, but it also included some younger stars. At such a distance, these young stars would not be expected unless accreted.

While this finding doesn’t fully answer the question of how M33 may have formed, it does reveal that this galaxy has likely not evolved in the isolation previously assumed.

June 24, 2011December 24, 2015 by Jon Voisey

Globular Clusters and the Age-Metallicity Relation

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Globular Clusters have a story to tell. These dense clumps of thousands of stars are relics of the early history of our galaxy, preserving information of the galaxy’s properties from their formation. Knowing this, astronomers have used globular clusters for nearly 30 years to probe how our galaxy has evolved. New observations from Hubble, add surprising new insight to this picture.

One of the advantages to studying clusters, is that the large number of stars allows astronomers to accurately determine some properties of the constituent stars far better than they could if the stars were isolated. In particular, since clusters all form in a short span of time, all stars will have the same age. More massive stars will die off first, peeling away from the main sequence before their lower mass brothers. How far this point, where stars leave the main sequence, has progressed is indicative of the age of the cluster. Since globular clusters have such a rich population of stars, their H-R diagrams are well detailed and the turn-off becomes readily apparent.

Using ages found in this manner, astronomers can use these clusters to get a snapshot of what the conditions of the galaxy were like when it formed. In particular, astronomers have studied the amount of elements heavier than helium, called “metals”, as the galaxy has aged. One of the first findings using globular clusters to probe this age-metallicity relationship was that there was a notable difference in the way the inner portion and the outer portion of the galaxy has evolved. Globular clusters revealed that the inner 15 kpc evolved heavier elements faster than the outer portions. Such findings allow for astronomers to test models of galactic formation and evolution and have helped to support models involving halos of dark matter.

While these results have been confirmed by numerous follow-up studies, the sampling of globular clusters is still somewhat skewed. Many of the globular clusters studied were part of the Galactic Globular Cluster Treasury project conducted using the Hubble Space Telescope’s Advanced Camera for Surveys (HST/ACS). In order to minimize the time spent using the much demanded telescope, the team was only able to target relatively nearby globular clusters. As such, the most distant cluster they could observe was NGC 4147 which is ~21 kpc from the galactic center. Other studies have made use of Hubble’s Wide Field Planetary Camera 2 and pushed the radius back to over 50 kpc from the galactic center. However, currently only 6 globular clusters with distances over 50 kpc have been included in this larger study. Interestingly, there has been a notable absence of clusters between 15 and 50 kpc, leaving a gap in the fuller knowledge.

This gap is the target of a recent study by a team of astronomers led by Aaron Dotter from the Space Telescope Science Institute in Maryland. In the new study, the team examines 6 globular clusters. Three of them (IC 4499, NGC 6426, and Ruprecht 106) are towards the inner edge of this range, lying between 15 and 20 kpc from the galactic center while the other three (NGC 7006, Palomar 15, and Pyxis) each lie around 40 kpc.

Again making use of the HST/ACS, the team found that all of the clusters were younger than globular clusters from the inner portions of the galaxy with similar metalicities. But three of the clusters, IC 4499, Ruprecht 106, and Pyxis were significantly younger to the tune of 1-2 billion years younger again supporting the picture that inner clusters had evolved faster. Additionally, this finding of a sharp difference helps to support the picture that the outer clusters underwent a different evolutionary process, aside from the rapid enrichment in the inner halo. One suggestion is that many of the outer halo clusters were originally formed in dwarf galaxies and later accreted into the Milky Way due to the timescales on which clusters in such smaller galaxies are thought to evolve.