ASTRONOMY

Snapshots of the universe

Four newly-released images of the universe as captured by an overhauled and upgraded Hubble Space Telescope, provide some cosmic surprises.

R. RAMACHANDRAN

THE Hubble Space Telescope (HST), launched in April 1990 as a joint project of the National Aeronautics and Space Administration (NASA) of the United States and the European Space Agency (ESA), has been pushing the frontiers of astronomy. Instruments aboard the low-earth orbiting telescope have been generating new science all these 12 years with astounding images of the cosmos. These images include beautiful ones of new galaxies and colliding galaxies and the birth and death of stars and even those that provide proof of the existence of black holes. However, the first new images of the universe - four spectacular and breathtaking ones released on April 30 - taken by the overhauled and upgraded HST suggest, as Ed Weiler, the associate administrator for space science at NASA put it, that "we haven't seen anything yet".

These images taken with the new Advanced Camera for Surveys (ACS), which according to Holland Ford of Johns Hopkins University (JHU), who headed the ACS project that began in December 1994, "are among the best images of the distant universe humans have ever seen". The ACS was installed in March during Servicing Mission 3B (SM3B), replacing the faint object camera (FOC), one of the original instruments of the HST. The new instrument, which has given the telescope a ten-fold greater observational efficiency, marks the beginning of a new era of exploration of the universe with the HST.

A spectacular image of a colliding galaxy, UGC 10214, dubbed the Tadpole Galaxy, located 420 million light years away in the constellation Draco. Unlike the usual images of stately galaxies, this one, with a long tail of stars and resembling a runaway pinwheel firework, captures the essence of a dynamic, restless and violent universe. Tadpole's distorted image was caused by a small interloper, a very blue compact galaxy visible in the upper left corner of the more massive Tadpole. Seen shining through the Tadpole's disk, it looks like a case of galactic hit-and-run, with the tiny intruder now leaving the scene of the accident. It is the strong gravitational forces from the interaction that created the long train of debris, consisting of stars and gas that stretch out more than 280,000 light years. Numerous young blue stars and star clusters, spewed out following the collision, are seen in the spiral arms, as well as in the "tidal" tail of stars. Each of these clusters represents the formation of about a million stars. Their colour is blue because they contain massive stars which are 10 times hotter and a million times brighter than the sun. The ACS made this observation on April 1 and 9. The colour image is constructed from three separate images taken in near-infrared, orange and blue filters.

The ACS is expected to open up new capability for discovery, especially of the early universe. The four images are evidence of the ACS' capability to discover celestial objects far beyond the reach of currently available instruments, and that too in a fraction of the time. It will enable the deepest images of the universe for the foreseeable future, until 2010 when the Hubble Project will come to an end and its successor, the next generation space telescope (NGST), trains its 8 metre diameter mirror (as against Hubble's 2.4 m) from a vantage point 1.5 million km from the earth (as against Hubble's orbit 600 km above) to catch the first light of the universe. The ACS has taken astronomers to the 'twilight zone' period when galaxies were just beginning to form out of the blackness following the cooling of the universe after the big bang.

The ACS is a camera of superlatives. It is expected to go beyond the sensitivity range of the largest ground-based telescope eventually to see the faintest of objects. It delivers a panoramic and high resolution image with an amazing 16 million picture elements per snapshot. A typical digital camera delivers two to four megapixels. The camera, which was designed at JHU and built by Ball Aerospace Technologies, is a phone booth-sized instrument and comprises three different cameras operating in three specialised channels: the wide field channel, the high resolution channel (with a smaller field of view) and the solar blind channel (which is sensitive to ultra-violet rays). The camera is equipped with a coronagraph (which is four to six times better than the existing imaging spectrograph) which blocks out the glare of quasars and stars so that fainter objects near them become observable. The wide wavelength range and its three modes of operation make the ACS a very versatile camera.

The image of NGC 2264, something closer to the Milky Way, known as Cone Nebula, a craggy looking mountain top of cold gas and dust that is located 2500 light years away in the constellation Monoceros. It is the "cousin" of the earlier iconic "pillars of creation" in the Eagle Nebula that the HST photographed in 1995. The picture shows the upper 2.5 million light years of the nebula, a height that equals 23 million round trips to the moon. The entire nebula is 7 million light years long. The object is so named because in ground-based images it has a conical shape. This giant pillar of gas and dust resides in a turbulent star-forming region. The pillars are incubators for developing stars. Over time, only the densest regions of the cone will be left. Inside these regions, stars and planets may form. Radiation from hot, young stars (located beyond the top of the image) has slowly eroded the nebula over millions of years. Ultra-violet light heats the edges of the dark cloud, releasing gas into the relatively empty region of surrounding space. The ACS took this image on April 2. The colour image is constructed from three separate images taken in blue, near-infrared and hydrogen-alpha filters.

The ACS can survey a field on the sky twice as large as that was covered in the legendary hubble deep field (HDF) surveys of 1995, to the same exposure depth, three to four times faster than the HST's workhorse camera that took the HDF, the wide field and planetary camera 2 (WFPC2). A deep "core sample" from the "twilight zone" period had taken about 10 days of dedicated observations with WFPC2 whereas the ACS would take less than three days. The ACS is expected to become Hubble's new workhorse. It is sensitive to wavelengths ranging from the ultra-violet to the far infrared (115-1050 nanometre).

Yet another spectacular collision between two spiral galaxies, dubbed "The Mice", thanks to the long tail of stars and gas emanating from each. Otherwise known as NGC 4676, it is the "celestial dance" of 'mouse' and 'mouse' located 30 million light years away in the constellation Coma Berenices. The image is the most detailed and most resolved that has so far been seen of these galaxies. This is what is likely to happen to the Milky Way several billion years later when it collides with the nearest large neighbouring galaxy in the constellation Andromeda (M31). According to computer simulation carried out by Joshua Barnes of the University of Hawaii and John Hibbard of the National Radio Astronomy Observatory (NRAO), what is seen is the collision after about 160 million years after their closest encounter. Running the simulations forward indicates that eventually the two galaxies will merge, forming a large, nearly spherical galaxy (known as elliptical galaxy). The Milky Way and Andromeda may also merge similarly. The picture is made from three images in blue, orange and near-infrared filters taken on April 7.

The ACS image of the Tadpole Galaxy reflects the dramatic gains over WFPC2 resulting from doubling area and resolution and demonstrates a five-fold improvement in sensitivity. The image brought an unexpected bonus in the form of the spectacular backdrop of an enormous number of galaxies - over 6,000 - like a wallpaper pattern beyond the Tadpole. It resembles a galaxy-filled HDF image taken by the HST in 1995. The difference, however, is that the ACS image was taken in one-twelfth the time taken by the original HDF. In fact, in blue light it shows even fainter objects than the HDF. The number of galaxies seen in the field of view is about twice the number in the HDF. Like the HDF, the ACS image stretches back to nearly the origins of time and contains a variety of shapes that are snapshots of galaxies throughout the universe's 13 billion-year evolution. The ACS images are so sharp that astronomers can identify "building blocks" of galaxies, colliding galaxies and extremely distant galaxies in the field - a superb sampler of galaxies.

What looks like a fantasy of colour, this is actually the image of the centre of the Omega Nebula, a hotbed of newly born stars blanketed by colourful glowing gas within an enormous dark hydrogen cloud. The region of the nebula shown here is about 3,500 times wider than the solar system. The area represents 60 per cent of the total view captured by the ACS. The nebula - also called the Swan Nebula and M17 - is located 5,500 light years away in the constellation Sagittarius. As in the case of its famous cousin Orion, the Swan Nebula is illuminated by ultra-violet radiation from young massive stars, located just beyond the upper right corner of the image. Each star is about six times hotter and 30 times more massive than the sun. The powerful radiation from these stars evaporates and erodes the dense cloud of gas within which the stars formed. The blistered walls of the hollow cloud shine primarily in the blue, green and red light emitted by excited atoms of hydrogen, nitrogen, oxygen and sulphur. Particularly striking is the rose-like feature, seen to the right of the centre, which glows in red light emitted by hydrogen and sulphur. This image was taken on April 1 and 2. The colour image is constructed from four separate images taken in blue, near-infrared, hydrogen-alpha and doubly ionised oxygen filters.

Servicing Mission SM3B was the fourth one to the HST, the third having been split into two. SM3A was conducted in December 1999. During the five most challenging spacewalks, or extra vehicular activities (EVAs), over an 11-day mission, the crew of seven astronauts upgraded the HST with the ACS, a new power unit and new rigid solar arrays (which together now generate 27 per cent more electric power) and an experimental cooling unit - cryocooler - for an infrared camera called near infrared camera and multi-object spectrometer (NICMOS). The new high-tech mechanical "refrigerator" pumped the heat out of the interior of NICMOS, achieving the target temperature for neon gas passing through the instrument of 700Kelvin (- 2030C). The temperature is now maintained within a few hundredths of a degree. The resuscitated NICMOS, with its new phase of self-regulated operation, is being checked out, and by early June the first infrared images taken with NICMOS since 1998 are expected to be released. Earlier, the instrument was being cooled by rapidly depleting blocks of solid nitrogen ice. According to the Space Telescope Science Institute (STScI), the telescope has been operating superbly since March after its refurbishing during SM3B.