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Astronomy

The view from the periphery

Surveys identify dwarf galaxies and stellar nurseries in far reaches of the Milky Way

La Silla, Chile: the Milky Way (bright band on the left) and two neighboring galaxies, the Large and Small Magellanic Clouds

Yuri Belestky / ESO La Silla, Chile: the Milky Way (bright band on the left) and two neighboring galaxies, the Large and Small Magellanic CloudsYuri Belestky / ESO

The well-known picture of the Milky Way, the galaxy in which our solar system is located, shows a disk in which milky waves of stars, dust and gas are spread out in a spiral around a central, elongated core.  With the emergence of more precise and powerful means of observation, this image is becoming clearer and more complex, as shown by three papers published in February and March 2015 that analyzed remote regions of the Milky Way.

In one, a group in the Astronomy Department at the Federal University of Rio Grande do Sul (UFRGS) identified two star clusters embedded in a gas cloud located about 16,000 light-years below the galactic plane.  The clusters were named Camargo 438 and Camargo 439, for Denilso Camargo, one of the members of the team that also included Charles Bonatto, Eduardo Bica and Gustavo Salerno.  In previous studies, they had identified 437 other clusters.  All of them, however, were closer to the galaxy’s plane.

One of the new clusters has 33 stars, and the other contains 42.  They are both quite young by astronomical standards: they are 2 million years old (the Sun is 5 billion years old and the Milky Way about 13 billion).  This indicates that the gas cloud in which they are located is a stellar nursery, from which new stars will continue to be born.  “This is the first time that star formation has been detected so far from the galactic disk,” says Camargo.

The clusters were identified based on images from the NASA Wise Space Telescope launched in 2009 to make infrared observations.  “It’s not easy to detect stars in dust clouds because the ultraviolet radiation emitted by high-mass stars is absorbed by the dust, which then re-emits the radiation in the infrared spectrum,” says Camargo.  “These detectors are making important contributions to astronomy.”

Near neighbor: on the far left, the dwarf galaxy Reticulum II, a satellite of the Milky Way identified by DES; on the left, the stars that make up Reticulum II

Images ESONear neighbor: on the far left, the dwarf galaxy Reticulum II, a satellite of the Milky Way identified by DES; on the left, the stars that make up Reticulum IIImages ESO

Most stars are born in star clusters, within the giant molecular clouds that populate the galactic disk, especially its arms.  These clouds are normally in equilibrium.  But perturbations—such as collisions with other clouds, shock waves from exploding supernovae and encounters with spiral arms—can destabilize them and cause them to collapse under the influence of their own gravity.  During the collapse the cloud fragments and generates denser regions, with higher mass, in which stars and planets are formed.

The Milky Way contains two stellar populations.  The first contains most of the stars in the galaxy, concentrated in the plane of the disk, in a band about 100,000 light-years long and 3,000 light years wide.  Stars form here continuously.  The second star population inhabits the halo—a more external region that envelops the disk.  In the halo, the stars are grouped into globular clusters.  This population is made up of old stars, on the order of billions of years, which have low metal content, features suggesting that they formed when the galaxy was young—there is evidence that the Milky Way is home to stars almost as old as the universe itself.

The UFRGS researchers identified the new star clusters forming in a region in which stellar formation was unexpected.  This discovery represents a paradox: how can new stars emerge in a region that is home to the oldest stars in the galaxy?  “One possibility is that the phenomenon we call cannibalism is taking place,” explains Walter Maciel, of the University of São Paulo (USP), who investigates the chemical composition of structures in the Milky Way and did not participate in this study.

Cannibalism in this context is the absorption of stars and gas clouds belonging to other galaxies, but which are pulled towards the Milky Way through gravitational attraction.  According to Maciel, this hypothesis was first presented earlier this century to explain the observation that stars in the Milky Way moved at speeds for which there seemed to be no explanation.  “Today there are at least a half dozen well-documented cases in which it is believed that the Milky Way cannibalized other galaxies,” he says.

There is another hypothesis.  In the past, stars in the galactic plane could have followed the stellar evolution process and reached the supernova stage.  When this happens, a violent explosion takes place that spews gas and dust great distances.  Part of the ejected material is attracted by the gravitational pull of the Milky Way and returns to the fold.  This mechanism is called a galactic chimney or fountain.  “This possibility is more likely, while cannibalism is rare,” says Maciel.

He underscores, however, an interesting feature in the results obtained by Camargo’s group.  “The cases of cannibalism seen previously involved dwarf galaxies.  He [Camargo] found star clusters, which are much smaller structures.”  It is possible, then, that the cloud in which the two clusters are located is associated with some as yet unknown galaxy.

Camargo agrees that either of the hypotheses could explain the origin of the clusters and it is impossible to know which is correct at this point.  But he points out that, even if the dust came from outside the galaxy, the stars are so young that they could only have formed in the Milky Way.  “There may be more dwarf galaxies orbiting the Milky Way than we are aware of,” he says.  “I think that our galaxy formed by swallowing smaller ones on its periphery, and that this process is continual.”

Studying dark energy
Another discovery on the periphery of the galaxy came as a result of the first year of the Dark Energy Survey (DES) project, which brought together 120 researchers from five countries.  In March 2015 the researchers announced the identification of eight new star systems at least 100,000 light-years from the Sun, orbiting the Milky Way like satellites.

Begun in 2013, the DES project is expected to map one eighth of the sky in great detail in order to shed light on dark energy.  The Brazilian branch of the project, DES-Brazil, participated in the discovery and Brazilian astronomer Basílio Santiago, also at UFRGS, coordinates the international work group studying the Milky Way (DES-MW).

The detection of objects located so far away, using the visible light spectrum, was possible thanks to the camera used in the project.  DECam, installed at the Cerro Tololo Inter-American Observatory in Chile, has a resolution equivalent to 570 million pixels, about 10 times greater than that of the most powerful cameras on the market.  It is capable of capturing minute quantities of light, permitting the observation of very distant stars.  “It is the world’s most efficient instrument in operation for producing high sensitivity images,” says Santiago.  DECam became operational in 2013 and, in 2014, published the first catalog of celestial light-emitting bodies identified by DES for the scientific community.

It is not yet clear if the eight new satellite systems are star clusters or dwarf galaxies.  The most likely scenario is that most, or even all, are the latter.  If this is the case, the number of dwarf galaxies orbiting the Milky Way could rise from 27 to 35.

The researchers believe that studying the dwarf galaxies could help them understand the nature of dark matter, one of the mysteries of contemporary astronomy.  In addition to size, an important difference between star clusters and dwarf galaxies is the fact that the latter are rich in dark matter.  “They are low-density galaxies.  If the matter they contain were limited to that in their stars, they would already have fallen apart a long time ago due to the tidal forces that our galaxy exerts on them.  This indicates that there is more matter there, but not in the form of stars,” says Santiago.

The list of unanswered questions is longer.  Current cosmological models suggest that the Milky Way should have thousands of dwarf galaxies around it, not just the three dozen already identified—with this number assuming that the recently discovered stars will also be classified as dwarf galaxies.  “This apparent discrepancy needs to be resolved.  Some people think that there are hundreds of them to be discovered and that this is only the tip of the iceberg,” says Santiago.

The data DES intends to collect in the coming years could help to confirm or refute these expectations.  Hundreds of terabytes of information will be stored in the catalogs of sources extracted from images.  In order to analyze this massive amount of data, DES-Brazil has developed a scientific portal with the support of the Interinstitutional e-Astronomy Laboratory (LineA).  LineA’s mission is to help Brazilian groups participate in surveys like DES and more ambitious experiments, such as the Large Synoptic Survey Telescope (LSST), which will capture images of more than half the southern sky in greater depth.  “It is essential that Brazilians continue to participate in international surveys such as the LSST,” says Santiago.

Another study focusing on the discrepancy between the predicted and detected number of dwarf galaxies in the neighborhood of the Milky Way has identified four types of Cepheid variable stars 180,000 light-years from the center of the galaxy.  The research project, in which Roberto Saito, of the Federal University of Sergipe, participated, was based on data from Vista, the largest infrared telescope in the world, located at the European Southern Observatory (ESO) in Chile.

The study, accepted for publication in the Astrophysical Journal, sought objects below the plane of the galaxy.  This is no easy task, as the dust and gas in the disk make observations difficult.  The Cepheids were 4,000 light-years away from the galactic plane.  “Also, in this case, it was the infrared telescope which allowed us to see the objects,” says Maciel.

It was by studying these stars that we have been able to discover that there are other galaxies in the Universe and that it is expanding.  Periodically, the volume of these stars increases and decreases.  This pulsation is accompanied by an oscillation in brightness visible from Earth.  By establishing the relationship between the star’s period and the variations in brightness, the distance between the star and Earth can be calculated.

According to Saito and other authors, the large distance suggests that the newly-discovered Cepheids belong to some dwarf galaxy of unknown dimensions.  “It is a good hypothesis, but there is no known galaxy there.  Further studies have to be carried out in order to prove this hypothesis,” says Maciel.

One of the strategic decisions that would allow Brazilians to continue to study the Milky Way is the country’s participation in ESO, today threatened by lack of funds.  “Important projects proposed by Brazilians are not being allotted observation time with the telescopes, and this is why government support is necessary for the growth of astronomy in Brazil,” states Camargo.  “This is an opportunity for Brazil to move out of the periphery and take its place at the center of cutting-edge scientific knowledge production.”

Scientific articles
CHAKRABARTI, S. et al. Clustered Cepheid variables 90 kiloparsec from the galactic centerAstrophysical Journal.  In production.
CAMARGO, D. et al. Discovery of two embedded clusters with Wise in the high galactic latitude cloud HRK 81.4−77.8Monthly Notices of the Royal Astronomical Society.  V. 448, p. 1930-6. 2015.

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