VISTA Views the Sculptor Galaxy

Click To Enlarge

A spectacular new image of the Sculptor Galaxy (NGC 253) has been taken with the ESO VISTA telescope at the Paranal Observatory in Chile as part of one of its first major observational campaigns. By observing in infrared light VISTA’s view is less affected by dust and reveals a myriad of cooler stars as well as a prominent bar of stars across the central region. The VISTA image provides much new information on the history and development of the galaxy.

The Sculptor Galaxy (NGC 253) lies in the constellation of the same name and is one of the brightest galaxies in the sky. It is prominent enough to be seen with good binoculars and was discovered by Caroline Herschel from England in 1783. NGC 253 is a spiral galaxy that lies about 13 million light-years away. It is the brightest member of a small collection of galaxies called the Sculptor Group, one of the closest such groupings to our own Local Group of galaxies. Part of its visual prominence comes from its status as a starburst galaxy, one in the throes of rapid star formation. NGC 253 is also very dusty, which obscures the view of many parts of the galaxy (eso0902). Seen from Earth, the galaxy is almost edge on, with the spiral arms clearly visible in the outer parts, along with a bright core at its centre.

VISTA, the Visible and Infrared Survey Telescope for Astronomy, the latest addition to ESO’s Paranal Observatory in the Chilean Atacama Desert, is the world’s largest survey telescope. After being handed over to ESO at the end of 2009 (eso0949) the telescope was used for two detailed studies of small sections of the sky before it embarked on the much larger surveys that are now in progress. One of these “mini surveys” was a detailed study of NGC 253 and its environment.

As VISTA works at infrared wavelengths it can see right through most of the dust that is such a prominent feature of the Sculptor Galaxy when viewed in visible light. Huge numbers of cooler stars that are barely detectable with visible-light telescopes are now also seen. The VISTA view reveals most of what was hidden by the thick dust clouds in the central part of the disc and allows a clear view of a prominent bar of stars across the nuclear region — a feature that is not seen in visible light pictures. The majestic spiral arms now spread over the whole disc of the galaxy.

The spectacular viewing conditions VISTA shares with ESO’s Very Large Telescope (VLT), located on the next mountain peak, also allow VISTA images to be exceptionally sharp for a ground-based telescope.

With this powerful instrument at their command astronomers wanted to peel away some of the mysteries of the Sculptor Galaxy. They are studying the myriad of cool red giant stars in the halo that surrounds the galaxy, measuring the composition of some of NGC 253’s small dwarf satellite galaxies, and searching for as yet undiscovered new objects such as globular clusters and ultra-compact dwarf galaxies that would otherwise be invisible without the deep VISTA infrared images. Using the unique VISTA data they plan to map how the galaxy formed and has evolved.

A Cauldron of Newborn Stars

N11, the Bean Nebula
A cocoon of young stars and gas clouds
informally known as N 11 (the Bean Nebula) lies 160,000 light-years
away in the Large Magellanic Cloud. This portion of it, about 6
arcminutes across, was captured with the Hubble Space Telescope’s
Advanced Camera for Surveys. Click here for a larger view.
NASA / ESA / J. M. Apellániz

A Cauldron of Newborn Stars

Some of the night sky's most spectacular nebulas involve giant clouds of hydrogen gas (termed H II regions), where massive young stars have illuminated their surroundings with intense, ultraviolet-rich light. Once ionized, the gas glows with the unmistakable crimson of hydrogen-alpha emission at a wavelength of 656 nm.

Many of these brilliant cotton-candy whorls lie not in the Milky Way proper but rather 160,000 light-years away in the Large Magellanic Cloud.

A stunning example is LHA 120-N 11, known more informally as N 11 or the Bean Nebula. It ranks as the second largest star-forming region in the LMC, behind 30 Doradus, the Tarantula Nebula. (Historical footnote: LHA 120-N 11 was first cataloged in 1956 by the late Karl Henize, an astronomer-turned-astronaut.)

Recently Jesús Maíz Apellániz (Astrophysics Institute of Andalucía, Spain) used the Hubble Space Telescope's Advanced Camera for Surveys to capture this stunning, 6-arcminute-wide portion of the much-larger nebula.

It beautifully illustrates the sequence of events that occurs in such regions. Near the bottom is a cluster massive blue-white stars, spectral types O and B, whose stellar winds and radiation have pushed away the residual gas to create a relatively clear pocket. As this ejected gas moves outward, it collides with surrounding dense clouds, causing them to collapse and start to form new stars. These newborns then light up their surroundings.

This isn't the first time astronomers have targeted N 11 with HST. A series of images taken in 1999 with the observatory's Wide Field Planetary Camera 2 were made available in 2004 as part of the Hubble Heritage series. But the new ACS views reveal much finer details.

In addition to the June 22nd release of the Apellániz composite, you'll find a nice 50-second-long animation here that zooms in on the LMC and N 11.

Herschel's Cold, Wonderful Universe

Herschel space selescope
Launched on May 14, 2009, the Herschel Space Observatory boasts a primary mirror 3.5 meters across. To view faint far-infrared sources, the spacecraft uses a large Sun shield and liquid helium to keep its optics and detectors cold

A couple of weeks ago we celebrated the Hubble Space Telescope's 20 years of discovery, and today astronomers gathered in Noordwijk, The Netherlands, to recognize another milestone by an up-and-coming space observatory.

It's been almost a year since the European Space Agency launched its Herschel Space Observatory, which shared its ride to orbit with the Planck spacecraft. Both payloads are doing well: Planck has been busy compiling the most accurate map ever made of the cosmic microwave background (CMB), the relic radiation from the Big Bang.

But the focus at today's gathering was Herschel and its trove of amazing observations. The spacecraft boasts a mirror 11.5 feet (3.5 m) across, more than twice HST's light-gathering power. But Herschel looks almost exclusively at far-infrared and submillimeter-wave "light," from 55 to 670 microns wavelength. (Visible light is from 0.4 to 0.7 microns.) Such wavelengths show very cold objects "glowing" at only a few tens of degrees above absolute zero. Since a telescope's resolution decreases at long wavelengths, Herschel needs all that aperture to see fine-scale detail in its frigid targets.

It also needs help to keep its own "body heat" from swamping the faint signals from the depths of space. An onboard supply of liquid helium should keep the detectors hovering near absolute zero for another three years. Cooling the primary mirror is another story — it's too big to refrigerate, so it stays hidden behind a Sun shield to remain near -315°F (80 K).

Herschel carries three instruments: a pair of cameras (PACS and SPIRE) and a ultra-high-precision spectrometer (HIFI). The mission's one serious glitch came last August, when HIFI fell silent, but in January controllers switched to a set of backup electronics and it's been working fine since then.


Heavyweight star forming in RCW 120
Herschel’s observation of the interstellar bubble RCW 120, about 4,300 light-years away, has revealed an embryonic star in its rim (arrowed) that looks set to turn into one of the brightest, most massive stars in our galaxy.

Investigator Annie Zavagno's presentation, "The Dark Side of Star Formation," focused on the little-understood process that spawns beefy stars with at least 8 times the Sun's mass. These heavyweights are rarely found, both because just-forming stars exercise lots of self-control when gobbling up interstellar gas and because they exhaust their hydrogen fuel and die in just a few million years. Zavagno (Laboratoire d’Astrophysique, Marseilles) and her team have used Herschel to spot high-mass stars as they form, and it now seems that the trigger points occur along the margins of rapidly expanding ionized-hydrogen bubbles (called HII regions).

Meanwhile, the recently rejuvenated HIFI spectrometer has been tracking down concentrations of ionized water, which has a distinct and strong spectral signature. "Water is an excellent diagnostic tool to probe the chemical and physical structure of the interstellar medium," explains Alexander Tielens (Leiden University). In particular, he notes, water helps cool the gas and dust surrounding newborn stars by radiating infrared energy to space.


Views of Hosehead nebula in visible and infrared
The familiar visible-light view of the Horsehead nebula (left) compared to a far-infrared internal view from the Herschel Space Observatory, which reveals regions of intense star formation.

But what really caught my eye today was a side-by-side comparison of the famed Horsehead Nebula in Orion. it turns out that the dark "clouds" that give this showpiece its distinctive appearance in visible light are internally ablaze with star formation when spied in the far infrared. Steve Eales (University of Cardiff) showed the paired views to demonstrate the potential of the Herschel ATLAS (short for Astrophysical Terahertz Large Area Survey), which will map 550 square degrees of sky at five wavelengths. Eales expects the 600 hours of exposures to reveal some 250,000 galaxies.

Herschel is showing us the "cold universe" as never before, and these results demonstrate that we've only scratched the surface of what this spacecraft will be revealing in the months and years ahead.