Seeing between the lines: Images in detail

Everything we can see is made up of molecules and atoms of one sort or another, and astronomical objects such as comets, stars, galaxies and clouds of dust are no exception.  A particular molecule will emit more light at some wavelengths than others, known as emission lines, but will also absorb light more at particular wavelength, and these are absorption lines.  Each type of molecule has its own set of emission and absorption lines, and looking for these lines tells astronomers what the composition of the object is.  A spectrometer splits the light into many different wavelength slices, and allows us to see at which exact wavelengths the object we are looking at is emiting or absorbing the most. The images here, carried out during the "performance verification" phase of the Herschel mission, illustrate how spectrometers on all three of Herschel's instruments can be used to detect the signature of a huge range of atoms and molecules in a wide range of environments, from giant stars to icy comets.

SPIRE spectrum of the giant star VY Canis Majoris
SPIRE spectrum of the giant star VY Canis Majoris, with the inset showing the star as seen by the SPIRE camera.  Image credits: Herschel/SPIRE "MESS" consortium.


spectra of VY CMa as measured by PACS (red) and ISO (grey)
PACS spectrum of VY CMa (red line) compared to the measurements of a previous mission, ISO (grey line). Image credits: ESA/Herschel/PACS "MESS" consortium and NASA/ESA/STSci (background image)

Giant star VY Canis Majoris

On the right is the SPIRE spectrum of VY Canis Majoris (VY CMa), a giant star near the end of its life, which is ejecting huge amounts of gas and dust into interstellar space, including elements such as carbon, oxygen and nitrogen (which form the raw material for future planets, and eventually life). The inset is a SPIRE camera image of VY CMa, in which it appears as a bright point-source near the edge of a large extended cloud. The spectrum is amazingly rich, with prominent features from carbon monoxide (CO) and water (H2O). More than 200 other spectral features have also been identified, many due to water, showing that the star is surrounded by large quantities of hot steam. Observations like these will help to establish a detailed picture of the mass loss from stars and the complex chemistry occurring in their extended envelopes.

In the PACS spectrum of VY CMa (bottom image, red curve) between 50 and 220 microns, thanks to its excellent signal-to-noise ratio, even the weakest spectral features can be identified (see inset). A comparison with data obtained by the Long Wavelength Spectrometer (LWS, grey line) which was on board the ESA Infrared Space Observatory (ISO, 1995-1998) demonstrates the great advances made possible with Herschel and PACS in studying the complex chemistry in this highly evolved phase which is at the end of its "life" as a star.  In the PACS wavelength range, more than 400 emission lines, of which more than 270 are due to water, have been detected. In that sense, VY CMa resembles nuclear power plants on Earth, where water is used to cool the environment of the central engine. Through stellar winds, these inorganic and organic compounds are injected into the interstellar medium, from which new stars orbited by new planets may form. Most of the carbon which supports life on planet Earth was forged by this kind of evolved star!

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SPIRE spectrum of the Orion Bar region in the Orion Nebula
SPIRE spectrum of part of the Orion Bar in the Orion Nebula, showing the methylidynium emission line in amongst the CO lines.  Image credits: ESA/Herschel/SPIRE "Evolution of Interstellar Dust" consortium and NASA/Spitzer (inset).


HIFI spectrum of DR21 star-forming region
HIFI spectrum of the DR21 star-formation region, showing emission from a number of molecules.  Image credits: ESA/Herschel/HIFI and NASA/Spitzer (background image)

Star formation regions: the Orion Bar and DR21

SPIRE also measured one position on the Orion Bar, part of the Orion nebula in which the gas on the edge of the nebula is partly ionised by intense radiation from nearby hot young stars. The inset shows a near infrared picture from NASA’s Spitzer Space Telescope. The SPIRE spectrum has many features from carbon monoxide (CO), appearing as the dominating narrow lines, seen here for the first time together in a single spectrum. These mean that the entire spectrum is observed at the same time and calibrated together. The brightness of the spectral features will allow astronomers to estimate the temperature and density of interstellar gas. The spectrum also shows the first detection of an emission feature from the molecular ion methylidynium (CH+), a key building block for larger carbon-bearing molecules. This and similar regions are large, and the SPIRE spectrometer’s will be extremely powerful in characterising how the gas properties vary within such sources.

The HIFI spectrum on DR21 reveals the fingerprints of a number of key organic molecules: formaldehyde (H2CO), carbon sulfide (CS), the formyl ion (HCO+), the ethynyl radical (C2H), the methyladyne radical (CH).  HIFI can for the first time observe the submillimetre spectral regime unhampered by atmospheric absorption and hence will provide an unbiased inventory of the molecular composition of such regions and new insights in the formation and early evolution of stars.

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SPIRE spectrum of Arp220
SPIRE spectrum of the galaxy Arp 220, formed by the collision of two large galaxies with inset from the Hubble Space Telescope.  Image credits: ESA/Herschel/SPIRE "Nearby Galaxies" consortium and NASA/ESA/STSci (inset).


PACS spectrum of Markarian 231
PACS spectrum of the galaxy Mrk 231, focusing on an Oxygen emission line. The background image is from the Hubble Space Telescope. Image credit: ESA/Herschel/PACS and NASA/ESA/STSci (background image).

Galaxy mergers: Arp 220 and Markarian 231

The most favoured models of how galaxies like our own Milky Way involve the merging of smaller galaxies.  When to galaxies collide, the stars in the galaxy are extremely unlikely to collide with each other, but the gas and dust in the galaxy are compressed and heater by the collision.  This compression and heating causes more star formation, which can be detected by looking form emission by particular molecules and elements. 

Much further from home, Arp 220 is a galaxy 250 million light years away from Earth with very active star formation triggered when two large spiral galaxies collided to produce the complex object we see today. Arp 220 is an important template for understanding even more distant galaxies and galaxy formation in the early universe. This SPIRE spectrum shows many emission features of carbon monoxide (CO), and water (H2O) features are seen both in emission and absorption. The inset is an optical image of Arp 220 made with the Hubble Space Telescope.

In addition to intense broadband emission and strong molecular line emission, "starburst galaxies", which are forming stars at a high rate, also emit weak far-infrared spectral lines. Using PACS on Herschel for the first time these weak emission line signatures from such an object have been measured in the extraordinary galaxy Markarian 231. This line emission from different atoms and ions is a signature from the neutral and ionized interstellar medium in galaxies and is important to diagnose the dust-enshrouded power sources in bright infrared galaxies. The weak emission line, never before detected in a galaxy of this type, originates from neutral oxygen located in the interstellar medium of the galaxy. Its ratio with a stronger line of ionized carbon is a good diagnostic of the power source and the conditions in the interstellar medium of the galaxy. Markarian 231 has long tidal tails and a disturbed shape, a typical signature of colliding galaxies, and is located about 600 million light-years away from Earth in the constellation of Ursa Major.

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SPIRE spectrum of M82
SPIRE spectrum of the nearby galaxy Messier M8. Image credits: ESA/Herschel/SPIRE "Nearby Galaxies" consortium


Results of PACS imaging-spectroscopy of M82
Hubble Space telescope image of M82, with PACS measurements carbon and oxygen. Image credits: ESA/Herschel/PACS and NASA/ESA/STSci (background image)

Neaby star-forming galaxy: M82

On the left is the SPIRE spectrum of Messier 82 (M82), a nearby galaxy (only 12 million light years away) with very active star formation. It is part of an interacting group of galaxies including the large spiral M81. The accompanying image (inset) is a spectacular three-colour composite picture of the two galaxies made with the SPIRE camera, showing material being stripped from M81 by the gravitational interaction with M82. The SPIRE spectrum of M82 shows strong emission lines from CO over the whole wavelength range, as well as emission lines from atomic carbon and ionized nitrogen.

PACS provides for the first time the necessary sensitivity and imaging capability with sufficient spatial resolution to study these variations in detail. Spectral images from two important tracers of the interstellar medium in the far infrared are shown: line emission from doubly ionized oxygen ([O III], bottom right inset) and from singly ionized carbon ([C II], top right inset). The ratio [O III]/[C II] (bottom left inset) of these two lines, a diagnostic of ionized gas vs. neutral gas drops rapidly going outwards from the galaxy centre along the disk. In contrast to this the ratio drops less when going outward in direction of the "super-wind" which is driving material out, as seen in the background image from the Hubble Space Telescope.

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HIFI spectrum of Comet Garradd
HIFI spectrum of Comet Garradd. Image credits: ESA/Herschel/HIFI and Bradford Robotic Telescope (backround image).

Icy comets in the Solar System

In the everlasting cold and darkness of the outskirts of Solar system, comets move unseen in their orbits. Comets are “dirty snowballs” – collections of small dust grains held together by ice – that were formed some 4.6 billion years ago in cold, outer regions of the Solar system. They represent left-over material from the formation of the Solar system. For most of their lives, comets dwell far from the Sun and hence they represent a rather pristine record of the early conditions from when the Solar system and the planets were formed. Small changes in their orbit can propel comets inwards towards the Sun. When they approach the Sun, the increased heat releases water and other molecules from the frozen nucleus, allowing us to measure the composition and characteristics of the comet. Comets may have contributed greatly to the inventory of reactive compounds such as water on the Earth and other terrestrial planets.

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