eso0142 — Valokuvan julkistus
The Eagle's EGGs
VLT ISAAC Looks for Young Stars in the Famous "Pillars of Creation"
20. joulukuuta 2001
Through imaging at infrared wavelengths, evidence has been found for recent star formation in the so-called "Pillars of Creation" in the Eagle Nebula (also known as Messier 16), made famous when the NASA/ESA Hubble Space Telescope (HST) obtained spectacular visible-wavelength images of this object in 1995. Those huge pillars of gas and dust are being sculpted and illuminated by bright and powerful high-mass stars in the nearby NGC 6611 young stellar cluster. The Hubble astronomers suggested that perhaps even younger stars were forming inside. Using the ISAAC instrument on the VLT 8.2-metre ANTU telescope at the ESO Paranal Observatory, European astronomers have now made a wide-field infrared image of the Messier 16 region with excellent spatial resolution, enabling them to penetrate the obscuring dust and search for light from newly born stars.
Two of the three pillars are seen to have very young, relatively massive stars in their tips. Another dozen or so lower-mass stars seem to be associated with the small "evaporating gaseous globules (EGGs)" that the Hubble astronomers had discovered scattered over the surface of the pillars.
These findings bring new evidence to several key questions about how stars are born. Was the formation of these new stars triggered as the intense ultraviolet radiation from the NGC 6611 stars swept over the pillars, or were they already there? Will the new stars be prematurely cut off from surrounding gas cloud, thus stunting their growth? If the new stars have disks of gas and dust around them, will they be destroyed before they have time to form planetary systems?
The famous "Pillars of Creation"
Hundreds of millions of people all over the world have admired those towering "Pillars of Creation" in Messier 16 (M16), also known as the Eagle Nebula, and located in the southern constellation of Serpens. It is one of the most famous NASA/ESA Hubble Space Telescope (HST) images - released in 1995, it has become an icon of modern astronomy, giving the viewer an extraordinary three-dimensional impression of scuba-diving through some leviathan undersea forest. These light-years long columns of gas and dust are being simultaneously sculpted, illuminated, and destroyed by the intense ultraviolet light from massive stars in the adjacent NGC 6611 young stellar cluster. Within a few million years, a mere twinkling of the universal eye, they will be gone forever.
But before they are, they have a chance to leave a longer-lasting legacy: a whole new generation of stars may be forming within them. Their formation may have been triggered by the immense power of the NGC 6611 stars, or perhaps they had already started to form quietly earlier on, only to be suddenly subjected to the ravages of an ionising storm front.
The real question is then: are there or are there not any new born stars inside those "Pillars of Creation"?
The Hubble Space Telescope view
When the HST turned to photograph M16 in 1995, it did so using its visible wavelength camera, WFPC-2. The Hubble astronomers  took data through three narrow-bandpass optical filters selecting emission lines from the ionised gas they knew to be present in the region. In doing so, they obtained an extraordinarily sharp view of the well-known pillars of cold gas and dust that are sometimes referred to as "elephant trunks" for obvious reasons.
Their image showed the light-years long pillars partly silhouetted against a bright nebular background, and revealed in exquisite detail the surface structure of the pillars as they are being transformed by ultraviolet radiation from massive, hot stars in the NGC 6611 cluster which lies just outside the area covered by the Hubble image.
A surprising finding made by the Hubble astronomers was that the pillars are covered with a large number (they counted 73) of small bumps and protrusions which in a few cases are almost completely detached from the pillars. With a typical angular size of only 0.5 arcsec, those objects had not been seen in previous ground-based photographs, and it took the exceptional acuity of Hubble to reveal them.
The astronomers dubbed these objects "evaporating gaseous globules", shortened to "EGGs" . They noted that one or two of these EGGs appeared to have stars right at their tips, and they suggested that perhaps the EGGs are formed as the advancing front of ionised gas driven by the hot NGC 6611 stars is slowed down by the presence of dense knots of gas and dust within the larger pillars. Within those knots then, they hypothesised a population of extremely young stars, still in the womb of their natal cloud but soon to be rudely exposed to a much harsher outside world.
However, there was a problem: since their images were taken at visible wavelengths which are relatively easily absorbed by the dust in the EGGs, the Hubble astronomers could not actually see inside the EGGs to test their theory.
The VLT looks inside the "Pillars"
What was needed then was a survey of the M16 region made at longer wavelengths and penetrating much more deeply through the dense dust. Such a survey should be sensitive enough to detect faint, low-mass young stars deeply embedded in the dusty EGGs. It should have excellent sub-arcsec angular resolution to unambiguously identify an object with a given EGG. And it should cover a wide field-of-view to probe all of the pillars and their surroundings.
Over the past twenty years, a number of surveys of M16 have been made at near-infrared, mid-infrared, and millimetre wavelengths. Unfortunately, none of them had this perfect combination of characteristics to answer the crucial question of whether or not there is a population of young stars inside the Eagle's EGGs .
However, this past austral autumn (April and May 2001), European astronomers  were able to image the Eagle Nebula at near-infrared wavelengths, using the infrared multi-mode ISAAC instrument on the 8.2-m VLT ANTU telescope at ESO's Paranal Observatory in Chile. By specifying that the observations be carried out in so-called "service mode", they ensured that the on-site ESO team could undertake their pre-defined programme under the necessary excellent observing conditions.
The results were well worth the effort! The ISAAC near-infrared images cover a 9 x 9 arcmin region, i.e., fourteen times the area seen in the famous Hubble visible image, in three broad-band colours and with sufficient sensitivity to detect young stars of all masses and - most importantly - with an image sharpness as good as 0.35 arcsec. Although this is still some way from the diffraction-limited performance of 0.07 arcsec or better that is now achieved with the adaptive optics system NAOS/CONICA on the VLT telescope, the ISAAC data cover a much wider field-of-view and, vitally, with enough image resolution to probe deep into the individual EGGs .
The ISAAC infrared images of Messier 16
The wide-field view of M16 (ESO Press Photo eso0142a) shows that there is much more to the region than is seen in the Hubble image. The first impression one gets is of an enormous number of stars. Those which are blue in the infrared image are either members of the young NGC 6611 cluster - whose massive stars are concentrated in the upper right (north west) part of the field - or foreground stars which happen to lie along the line of sight towards M16.
Most of the stars are fainter and more yellow. They are ordinary stars behind M16, along the line of sight through the galactic bulge, and are seen through the molecular clouds out of which NGC 6611 formed. Some very red stars are also seen: these are either very young and embedded in gas and dust clouds, or just brighter stars in the background shining through them.
Zooming in, ESO Press Photo eso0142b shows the region of the pillars covered by the Hubble image and its immediate surroundings. The pillars are still obvious, although appearing less prominent in places as one penetrates the thinner parts, getting closer to the goal of probing inside the pillars. ESO Press Video eso0142 shows how this appearance changes in a continuous dissolve between the Hubble visible wavelength view and its VLT infrared equivalent.
Hunting for new stars in the EGGs
ESO Press Photos eso0142c-e show even further close-ups of the heads of Columns 1 and 2, plus Column 4, seen in the wide-field ISAAC image (ESO Press Photo eso0142a) towards the lower left (south east). The young star in the head of Column 1 (ESO Press Photo eso0142c) is located within a complex reflection nebula, completely unseen at visible wavelengths. From the near-infrared brightness of the star, the astronomers judge it to be more massive than our own sun and very young (in astronomical terms), perhaps only 100,000 years old. ESO Press Video eso0142 allows a direct comparison between the Hubble and VLT views of this region.
Right at the tip of Column 2 (ESO Press Photo eso0142d), another young star also illuminates a small reflection nebula, again undetected in the Hubble image. And to the south-east, the head of Column 4 (ESO Press Photo eso0142e) shows complex red nebulosity which the astronomers take to be the signpost of very young objects, so deeply embedded that they are not directly detected in the VLT images. The present team of astronomers has recently investigated this object  and believe it is hiding the driving source of a so-called "Herbig-Haro jet", a speedy outflow of gas that can be seen where it ends in a shock, the bright purple arc at the lower edge of ESO Press Photo eso0142b .
Turning to smaller scales, the astronomers made a very accurate alignment of the Hubble and VLT images, and then examined the location of each EGG, searching for stars within them. This search had to be carried out very carefully, given the small sizes of the EGGs, and also because, once in a while, a perfectly ordinary background star might seem to be aligned with an EGG purely by chance.
After completing their search, they found that 11 of the 73 EGGs clearly have stars associated with them. Only one of these had been previously been seen in the Hubble images, and another five EGGs were noted as possibly containing stars.
Judging from their near-infrared brightness, most of these stars seem to be less massive than our Sun. Interestingly, most of the EGGs with stars are located on Column 1, and roughly half of them right at the tip of the head, not far from the more massive star that illuminates the reflection nebula. This may be evidence for a small cluster of young stars associated with Column 1 which will soon be revealed as the column is eaten away.
Even though the remaining 57 EGGs appear to be empty, it is important to note that there may nevertheless be more young stars in the M16 pillars. After all, neither of the bright young stars at the tips of Columns 1 and 2 are related to any of the Hubble EGGs. Also, it is clear from the VLT image that parts of the pillars and a few of the EGGs are so dense that they remain completely opaque even at near-infrared wavelengths, and may still be harbouring other new stars. An interesting example is the apparently empty EGG number 23, from which another high-speed Herbig-Haro jet seems to be emerging (ESO Press Photo eso0142c).
The new VLT infrared image shows that there is now firm evidence for the recent birth of stars in the Eagle Nebula and that at least some of the Eagle's EGGs are fertile, not sterile!
A deeper look at even longer wavelengths will be needed to make a complete census of all the star formation in the Eagle Nebula, perhaps using the VLT thermal-infrared camera, VISIR, when it becomes available or, ultimately, less than a decade from now, the infrared-optimised Next Generation Space Telescope (NGST), the NASA/ESA/CSA successor to the HST. At longer wavelengths, observations with the planned Atacama Large Millimeter Array (ALMA) will also be most useful.
From images alone, however, it is not possible to tell which came first: the stars or the EGGs? Were those young stars already forming inside dark clouds before the intense ultraviolet radiation of the nearby massive hot stars swept over the pillars? Or did that radiation compress empty clumps in those clouds and trigger the birth of the stars?
In either case, those young stars will soon be exposed to the full fury of the ionisation storm as the columns are evaporated. How will their fate have been affected? Ripped prematurely from the cloud, they will be cut off from the reservoir of material from which they grew, and thus may end up smaller than would otherwise be expected. Also, the dense disks of gas and dust known to girdle young stars will suddenly be heated and boiled away by the ultraviolet radiation, as has been seen happening in the Orion Nebula, perhaps preventing the formation of planets around those stars. Theoreticians studying these problems now have some new data to work with.
Nevertheless, to keep things in perspective, it is important to remember that the towering pillars cover only a small fraction of the Eagle Nebula. While a few tens of new stars may be forming in the pillars today, at least a thousand young stars were born in the adjacent NGC 6611 cluster within the last few million years, including the massive stars themselves.
The story of the formation of that cluster may be something else altogether, but perhaps just as spectacular.
 The Hubble Space Telescope team consisted of Jeff Hester and Paul Scowen (Arizona State University, USA) and 21 collaborators. Their M16 image was made at visible wavelengths using the Wide-Field Planetary Camera 2 (WFPC-2) instrument of the HST, selecting the emission lines of double ionised oxygen [OIII], the hydrogen line H-alpha, and single ionised sulphur [SII] in the visible wavelength interval (from 500 to 671 nm). The image was released by the Space Telescope Science Institute (PR95-44) in 1995 and the scientific data analysis was published by Jeff Hester et al. in the Astronomical Journal in 1996 (Vol. 111, p. 2349).
 A research paper discussing the embedded object in the head of Column 4 and its role in driving the Herbig-Haro jet ending in HH 216 ("Molecular cloud structure and star formation near HH216 in M16", by Morten Andersen, Jens Knude, Bo Reipurth, Alain Castets, Lars-Åke Nyman, Mark McCaughrean and Steve Heathcote) has been submitted for publication in the European research journal "Astronomy & Astrophysics".
 Mark McCaughrean would like to dedicate these VLT images of the Eagle Nebula to his own new baby star, Finn, born in Berlin on December 1st, 2001, when his father was working on them and also to Sybille and Catriona, the other stars in his family cluster!
The research described in this press release is presented in more detail in a research paper ("The Eagle's EGGs: fertile or sterile?"), to be submitted to the European research journal "Astronomy & Astrophysics Letters". The work has been carried under the auspices of the European Commission Research Training Network "The Formation and Evolution of Young Stellar Clusters" (HPRN-CT-2000-00155) .
Astrophysikalisches Institut Potsdam