2.1 Hubble Space Telescope

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2 2.1 Hubble Space Telescope More than a decade after launch, The Hubble Space Telescope (HST) continues to produce excellent scientific results and stunning imagery. In the past year, the active instruments were the infrared camera NICMOS, the Wide Field Planetary Camera 2 (WFPC2) and the Fine Guidance Sensors (FGS). They are all performing nominally. The Space Telescope Imaging Spectrograph (STIS), the only spectrograph on board the HST, unfortunately ceased operating in August 2004, and the Advanced Camera for Survey ceased operations in January We are now working towards Servicing Mission 4, currently planned for Summer During this mission, two new instruments will be installed: the Wide Field Camera 3 (WFC3) and the Cosmic Origin Spectrograph (COS). Repairs will be attempted to restore both STIS and ACS. Observations with HST have impacted every area of current astronomical research. A few highlights of the results obtained during the year 2007 are presented here. Introduction Astronomers have long thought that globular star clusters had a single star-formation episode early in their lives and then settled into a quiet existence. Observations by the Hubble Space Telescope, however, are showing that this idea may be too simple. The Hubble analysis of the massive globular cluster NGC 2808 provides evidence that star birth occurred again and again, with three generations of stars forming very early in the cluster s life. Globular clusters are the homesteaders of our Milky Way Galaxy, born during our galaxy s formation. They are compact swarms of typically hundreds of thousands of stars held together by gravity. The standard picture of a globular cluster is that all of its stars formed at the same time, in the same place, and from the same material, and they have co-evolved for billions of years. This is the cornerstone on which much of the study of stellar populations has been built. So astronomers were very surprised to find several distinct populations of stars in NGC All these stars were born within 200 million years, very early in the life of the 12.5-billion-year-old massive cluster. Finding multiple stellar populations in a globular cluster so close to home has deep cosmological implications, say the researchers. The astronomers used Hubble s Advanced Camera for Surveys to measure the brightness and colour of the cluster stars. Hubble s exquisite resolution allowed the astronomers to sort out the different stellar populations. The Hubble measurements showed three distinct populations, with each successive generation appearing slightly bluer. This colour difference suggests that successive generations contain a slightly different mix of some chemical elements. One possibility is that the successively bluer colour of the stellar populations indicates that the amount of helium increases with each generation of stars. Perhaps massive star clusters like NGC 2808 hold onto enough gas to ignite a rapid succession of stars. The star birth would be driven by shock waves from supernovae and stellar winds from giant stars, which compress the gas and make new stars. The gas would be increasingly enriched in helium from previous generations of stars more massive than the Sun. Astronomers commonly assume that globular clusters produce only one stellar generation because the energy radiating from the first batch of stars would clear out most of the residual gas needed to make more stars. But a hefty cluster like NGC 2808, which is two to three times more massive than a typical globular cluster, may have enough gravity to hang onto that gas, which has been enriched by helium from the first stars. Of the approximately 150 known globular clusters in our Milky Way Multiple stellar generations in a globular cluster For further information, see 9

3 Forming stars in the Small Magellanic Cloud Figure In a simplified version of the Hertzsprung-Russell diagram, this graph schematically plots the brightness of the stars in globular cluster NGC 2808 (along the vertical axis) against stellar colour and temperature (along the horizontal axis). The cooler a star is, the redder it appears, and it diminishes in brightness. The bluer stars are to the left; redder stars are to the right; the brightest stars are near the top. The three curves represent the three different populations of stars that are present in NGC Figure Image of the Small Magellanic Cloud starforming region NGC 602, taken with the Advanced Camera for Surveys. Filters that isolate visible and infrared light were combined with a filter that samples the hydrogen and nitrogen emission from the glowing clouds. Star formation started at the centre of the region, and propagated outwards. Newborn stars have been found along the dusty ridges. Galaxy, NGC 2808 is one of the most massive, containing more than 1 million stars. Another possible explanation for the multiple stellar populations is that NGC 2808 may only be masquerading as a globular cluster. The stellar grouping may have been a dwarf galaxy that was stripped of most of its material due to gravitational capture by our galaxy. Although the astronomers search is only in its infancy, multiple stellar populations may be a typical occurrence in other massive clusters. Forming stars in the Small Magellanic Cloud A new generation of bright, blue, newly-formed stars have been found blowing a cavity in the centre of a star-forming region in the Small Magellanic Cloud. At the heart of the star-forming region lies star cluster NGC 602. The high-energy radiation blazing out from the hot young stars is sculpting the inner edge of the outer portions of the nebula, slowly eroding it and eating into the material beyond. The diffuse outer reaches of the nebula prevent the energetic outflows from streaming away from the cluster. 10

4 Ridges of dust and gaseous filaments are seen towards the northwest (in the upper-left part of Fig ) and towards the southeast (the lower right-hand corner). Observations taken with the Spitzer Space Telescope in infrared have shown that star formation is presently ongoing along the ridges of the star-forming region, and several just born stars are found there. Elephant trunk-like dust pillars point towards the hot blue stars and are tell-tale signs of their eroding effect. In this region it is possible with Hubble to trace how the star formation started at the centre of the cluster and propagated outward, with the youngest stars still forming today along the dust ridges. The Small Magellanic Cloud is roughly light-years from Earth. Its proximity to us makes it an exceptional laboratory for performing in-depth studies of star formation processes and their evolution in an environment slightly different from our own Milky Way. Dwarf galaxies such as the Small Magellanic Cloud, with significantly fewer stars compared to our own galaxy, are considered to be the primitive building blocks of larger galaxies. The study of star formation within this dwarf galaxy is particularly interesting to astronomers because its primitive nature means that it lacks a large percentage of the heavier elements that are forged in successive generations of stars through nuclear fusion. The galaxy I Zwicky 18 appears to look older the more astronomers study it. The galaxy s youthful appearance was identified some 40 years ago through observations at the Palomar Observatory. Those studies showed that the galaxy erupted with star formation billions of years after its galactic neighbours. Galaxies resembling I Zwicky 18 s youthful appearance are typically found only in the early Universe. Astronomers were thrilled that a newly-forming galaxy like I Zwicky 18 could be studied nearby to learn about galactic evolution, which is normally only observable at great distances. New Hubble data have quashed that possibility. The telescope found faint, older stars contained within the galaxy, suggesting its star formation started at least 1 billion years ago and possibly as much as 10 billion years ago. The galaxy, therefore, may have formed at the same time as most other galaxies. Although the galaxy is not as youthful as was once believed, it is certainly developmentally challenged and unique in the nearby Universe. Spectroscopic observations with ground-based telescopes have shown that I Zwicky 18 is almost exclusively composed of hydrogen and helium, the main ingredients created in the Big Bang. Heavier elements are forged within the cores of stars and blasted into space when the stars die. The galaxy s primordial makeup suggests that its rate of star formation has been much lower than that of other galaxies of similar age. The galaxy has been studied with most of NASA s telescopes, including the Spitzer Space Telescope, the Chandra X ray Observatory, and the Far Ultraviolet Spectroscopic Explorer (FUSE). However, it remains a mystery why I Zwicky 18 formed so few stars in the past, and why it is forming so many new stars right now. The Hubble data also suggest that I Zwicky 18 is 59 million light-years from Earth, almost 10 million light-years more distant than previously believed. While this is still in our own backyard by extragalactic standards, the galaxy s larger-thanexpected distance may explain why astronomers have had difficulty detecting older, fainter stars within the galaxy until now. In fact, the faint, old stars in I Zwicky 18 are almost at the limit of Hubble s resolution and sensitivity. The true age of a galaxy 11

5 Figure This image of I Zwicky 18 was taken with the Advanced Camera for Surveys. I Zwicky 18 is classified as a dwarf irregular galaxy and is much smaller than our Milky Way Galaxy. The concentrated bluish-white knots embedded in the heart of the galaxy are two major starburst regions where stars are forming at a furious rate. Astronomers discerned the new distance by observing flashing Cepheid variable stars in I Zwicky 18. The team determined the observed brightness of three Cepheids and compared them with the actual brightness predicted by theoretical models. These models were calculated specifically for I Zwicky 18 s deficiency in heavy elements, indicating the galaxy s stars formed before these elements were abundant in the Universe. This analysis allowed the astronomers to determine the galaxy s distance. The Cepheid distance also was validated through another distance indicator: the observed brightness of the brightest red stars older than 1 billion years. Cepheid variable stars have been studied for decades and have been instrumental in the determination of the scale of our Universe. This is the first time, however, that variable stars with so few heavy elements have been found. Lego block galaxies in the early Universe The Hubble Space Telescope and Spitzer Space Telescope together have discovered nine of the smallest, faintest, most compact galaxies ever observed in the distant Universe. Blazing with the brilliance of millions of stars, each of the newlydiscovered galaxies is a hundred to a thousand times smaller than our Milky Way Galaxy. These are among the lowest-mass galaxies ever directly observed in the early Universe. The conventional model for galaxy evolution predicts that small galaxies in the early Universe evolved into the massive galaxies of today by coalescing. These nine Lego-like building block galaxies initially detected by Hubble most likely contributed to the construction of the Universe as we know it. Astronomers were surprised to find that the galaxies estimated masses were so small, and the Spitzer Space Telescope was called upon to make precise 12

6 Figure In this image of the Hubble Ultra Deep Field, several objects are identified as the faintest, most compact galaxies ever observed in the distant Universe. They are so far away that we see them as they looked less than one billion years after the Big Bang. Blazing with the brilliance of millions of stars, each of the newly-discovered galaxies is a hundred to a thousand times smaller than our Milky Way Galaxy. determinations of their masses. The Spitzer observations confirmed that these galaxies are some of the smallest building blocks of the Universe. These young galaxies offer important new insights into the Universe s formative years, just one billion years after the Big Bang. Hubble detected sapphire-blue stars residing within the nine pristine galaxies. The youthful stars are just a few million years old and are in the process of turning Big Bang elements (primarily hydrogen and helium) into heavier elements. The stars have probably not yet begun to pollute the surrounding space with elemental products forged within their cores. While blue light seen by Hubble shows the presence of young stars, it is the absence of infrared light in the sensitive Spitzer images that was conclusive in showing that these are truly young galaxies without an earlier generation of stars. The galaxies were first identified by their prominent and energetic light emission from glowing hydrogen. Three of the galaxies appear to be slightly disrupted: rather 13

7 14 than being shaped like rounded blobs, they appear stretched into tadpole-like shapes. This is a sign that they may be interacting and merging with neighbouring galaxies to form larger, cohesive structures. The galaxies were observed in the Hubble Ultra Deep Field (HUDF) with Hubble s Advanced Camera for Surveys and the Near Infrared Camera and Multi- Object Spectrometer. Observations were also carried out with Spitzer s Infrared Array Camera and the European Southern Observatory s Infrared Spectrometer and Array Camera.

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