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Extinction (astronomy)

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Extinction, sometimes called reddening, is variously known as galactic extinction, intersellar extinction, and atmospheric extinction when it happens within Earth's atmosphere. It usually happens when dust - either in space or in the earth's atmosphere - dims light[1], mainly by absorbing and scattering (short) blue light waves, allowing (longer) red light waves through, making stars appear redder than they are, for example when dust particles in the atmosphere of earth contribute the red effect of sunset.[2]

Atmospheric extinction

Atmospheric extinction is the reduction in brightness of steller objects as their light passes through Earth's atmosphere. The longer the path length through the atmosphere, the more the starlight is dimmed, hence a star will appear brighter when high in the sky than when it is close to the horizon. The path length through the atmosphere is known as 'air mass'. If you look straight up, that is the shortest air mass. There are mathematical calculations that can be made with regard to atmospheric extinction.[3][4] There are three main factors that cause atmospheric extinction. Molecular absorption, mainly due to atmospheric ozone and water, accounts for about 0.02 magnitudes per air mass. Molecular scattering, known as Rayleigh scattering, by air molecules, accounts for up to 0.14 magnitudes per air mass. Rayleigh scattering is the cause of the blue sky - the average colour of scattered sunlight is the blue of the sky.[5] Aerosol scattering (by dust, water and manmade pollutants) adds about 0.12 magnitudes per air mass. There is less extinction in winter than in summer, believed to be due to less water in the atmosphere. Extinction scatters blue light more than red, and so causes a reddening effect with stellar objects, the lower in the sky a stellar object is, the more extinction, causing the light to appear redder.

Galactic extinction

The concept of galactic extinction is believed to have originated from Åke Anders Edvard Wallenquist.[6] Starlight is dimmed and reddened by galactic dust (absorbing clouds).[7]) The Galactic extinction causes a non uniform selection effect across the sky.[8]

Absorbing clouds

The composition of absorbing clouds is deduced using dust spectroscopy.[9] The absorbing clouds and their extinction effect are subject to continuing study.[10]

Effect of extinction on redshift

Various claims are made regarding the effect of extinction on redshift. Extinction was shown to have an effect on redshift in The Optical Redshift Survey II.[11] Redshift is believed to be intrinsic[12] and distance related. A fringe theory by Ling Jun Wang suggests that extinction is largely responsible for redshift.[13] Mainstream science holds that the effect of extinction on redshift is small.[14]

Technical Ecstacy

Extinction is a term used in astronomy to describe the absorption and scattering of electromagnetic radiation by matter (dust and gas) between an emitting astronomical object and the observer. Interstellar extinction—also called Galactic extinction, when it occurs in the Milky Way—was first recognized as such in 1930 by Robert Julius Trumpler.[15][16] However, its effects had been noted in 1847 by Friedrich Georg Wilhelm von Struve,[17] and its impact on the colors of stars had been observed by a number of individuals who did not connect it with the general presence of Galactic dust.

For Earth-bound observers, extinction arises both from the interstellar medium (ISM) and the Earth's atmosphere; it may also arise from circumstellar dust around an observed object. The strong atmospheric extinction in some wavelength regions (such as X-ray, ultraviolet, and infrared) requires the use of space-based observatories. Since blue light is much more strongly attenuated than red light, resulting in an object which is redder than expected, interstellar extinction is often referred to as interstellar reddening. This is not to be confused with the quite separate phenomenon of red shift.[18]

Galactic extinction is often measured in units of apparent magnitude decrease per kiloparsec of distance (mag/kpc).

General characteristics

Broadly speaking, interstellar extinction is strongest at short wavelengths. This results in a change in the shape of an observed spectrum. Superimposed on this general shape are absorption features (wavelength bands where the intensity is lowered) that have a variety of origins and can give clues as to the chemical composition of the interstellar material, e.g. dust grains. Some known absorptions features include the 2175 Å bump, the diffuse interstellar bands, the 3.1 μm water ice feature, and the 10 and 18 μm silicate features.

Usually, the rate of interstellar extinction in the Johnson-Cousins V-band is taken to be 0.7-1.0 mag / kpc in the Solar Neighborhood.[citation needed] This means that a star will have its brightness reduced by about a factor of 2 in the V-band for every kiloparsec it is further away from us.

The amount of extinction can be significantly higher than this in specific directions. For example, some regions of the Galactic center have more than 30 magnitudes of extinction in the optical, meaning that less than 1 optical photon in 1012 passes through.[19] This results in the so-called zone of avoidance, where our view of the extra-galactic sky is severely hampered, and background galaxies, such as Dwingeloo 1, were only discovered recently through observations in radio and infrared.

The general shape of the ultraviolet through near-infrared (0.125 to 3.5 μm) extinction curve in our own Galaxy, the Milky Way, is fairly well characterized by the single parameter R(V) (which is different along different lines of sight through the Galaxy),[20][21] but there are known deviations from this single parameter characterization.[22] R(V) is defined to be A(V)/E(B-V), and measures the total, A(V), to selective, E(B-V) = A(B)-A(V), extinction in set bands. A(B) and A(V) are the total extinction at the B and V filter bands. Another measure used in the literature is the absolute extinction A(λ)/A(V) at wavelength λ, comparing the total extinction at that wavelength to that at the V band.

R(V) is known to be correlated with the average size of the dust grains causing the extinction. For our own Galaxy, the Milky Way, the typical value for R(V) is 3.1,[23] but is found to be between 2.5 and 6 for different lines of sight. The relationship between the total extinction, A(V), and the number of neutral hydrogen atoms in a 1 cm2 column, NH, shows how the gas and dust in the interstellar medium are related. From studies using ultraviolet spectroscopy of reddened stars and X-ray scattering halos in the Milky Way, Predehl and Schmitt (1995) [24] found the relationship between NH and A(V) to be approximately,

(see also: [25][26]).

Astronomers have determined the three-dimensional distribution of extinction in the solar circle of our Galaxy, using visible and near-infrared stellar observations and a model of the distribution of stars in the Galaxy.[27] [28] The dust giving rise to the extinction lies along the spiral arms, as observed in other spiral galaxies.

Measuring extinction towards an object

To measure the extinction curve for a star, the star's spectrum is compared to the observed spectrum of a similar star known not to be affected by extinction (unreddened).[29] It is also possible to use a theoretical spectrum instead of the observed spectrum for the comparison, but this is less common. In the case of emission nebulae, it is common to look at the ratio of two emission lines which should not be affected by the temperature and density in the nebula. For example, the ratio of hydrogen alpha to hydrogen beta emission is always around 2.85 under a wide range of conditions prevailing in nebulae. A ratio other than 2.85 must therefore be due to extinction, and the amount of extinction can thus be calculated.

The 2175-angstrom feature

One prominent feature in measured extinction curves of many objects within the Milky Way is a broad 'bump' at about 2175 Å, well into the ultraviolet region of the electromagnetic spectrum. This feature was first observed in the 1960s[30][31] but its origin is still not well understood. Several models have been presented to account for this bump which include graphitic grains with a mixture of PAH molecules. Investigations of interstellar grains embedded in interplanetary dust particles (IDP) observed this feature and identified the carrier with organic carbon and amorphous silicates present in the grains.[32]

Extinction curves of other galaxies

Plot showing the average extinction curves for the MW, LMC2, LMC, and SMC Bar.[33] The curves are plotted versus 1/wavelength to emphasize the UV.

The form of the standard extinction curve depends on the composition of the ISM, which varies from galaxy to galaxy. In the Local Group, the best-determined extinction curves are those of the Milky Way, the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC). In the LMC, there is significant variation in the characertistics of the ultraviolet extinction with a weaker 2175 Å bump and stronger far-UV extinction in the region associated with the LMC2 supershell (near the 30 Doradus starbursting region) than seen elsewhere in the LMC and in the Milky Way.[34][35] In the SMC, more extreme variation is seen with no 2175 Å and very strong far-UV extinction in the star forming Bar and fairly normal ultraviolet extinction seen in the more quiescent Wing.[36][37][38] This gives clues as to the composition of the ISM in the various galaxies. Previously, the different average extinction curves in the Milky Way, LMC, and SMC were thought to be the result of the different metallicities of the three galaxies: the LMC's metallicity is about 40% of that of the Milky Way, while the SMC's is about 10%. Finding extinction curves in both the LMC and SMC which are similar to those found in the Milky Way[33] and finding extinction curves in the Milky Way that look more like those found in the LMC2 supershell of the LMC[39] and in the SMC Bar[40] has given rise to a new interpretation. The variations in the curves seen in the Magellanic Clouds and Milky Way may instead be caused by processing of the dust grains by nearby star formation. This interpretation is supported by work in starburst galaxies (which are undergoing intense star formation episodes) that their dust lacks the 2175 Å bump.[41][42]

Atmospheric extinction

Atmospheric extinction varies with location and altitude. Astronomical observatories generally are able to characterise the local extinction curve very accurately, to allow observations to be corrected for the effect. Nevertheless, the atmosphere is completely opaque to many wavelengths requiring the use of satellites to make observations.

Atmospheric extinction has three main components: Rayleigh scattering by air molecules, scattering by aerosols, and molecular absorption. Molecular absorption is often referred to as 'telluric absorption', as it is caused by the Earth ("telluric" is a synonym of "terrestrial"). The most important sources of telluric absorption are molecular oxygen and ozone, which absorb strongly in the near-ultraviolet, and water, which absorbs strongly in the infrared.

The amount of atmospheric extinction depends on the altitude of an object, being lowest at the zenith and at a maximum near the horizon. It is calculated by multiplying the standard atmospheric extinction curve by the mean airmass calculated over the duration of the observation.

Interstellar reddening

In astronomy, interstellar reddening is a phenomenon associated with interstellar extinction where the spectrum of electromagnetic radiation from a radiation source changes characteristics from that which the object originally emitted. Reddening occurs due to the light scattering off dust and other matter in the interstellar medium. Interstellar reddening should not be confused with the redshift, which is the proportional frequency shifts of spectra without distortion. Reddening preferentially removes shorter wavelength photons from a radiated spectrum while leaving behind the longer wavelength photons (in the optical, light that is redder), leaving the spectroscopic lines unchanged.

In any photometric system interstellar reddening can be described by color excess, defined as the difference between an objects observed color index and its intrinsic color index (sometimes referred to as its normal color index). An objects intrinsic color index is the theoretical color index which it would have if unaffected by extinction. In the UBV photometric system the color excess is related to the B-V colour by:

References

  1. ^ http://www.asterism.org/tutorials/tut28-1.htm Atmospheric Extinction and Refraction
  2. ^ http://www.astro.virginia.edu/class/skrutskie/astr121/notes/redden.html Interstellar Reddening, Extinction, and Red Sunsets
  3. ^ http://www.asterism.org/tutorials/tut28-1.htm Atomospheric Extinction and Refraction
  4. ^ http://star-www.rl.ac.uk/star/docs/sc6.htx/node15.html Atmospheric Extinction and Air Mass
  5. ^ http://mintaka.sdsu.edu/GF/explain/optics/scatter.html Atmospheric Scattering
  6. ^ Attribution to Wallenquist on Page 165 http://adsabs.harvard.edu/cgi-bin/bib_query?1930LicOB.420..154T PRELIMINARY RESULTS ON THE DISTANCE, DIMENSIONS AND SPACE DISTRIBUTION OF OPEN STAR CLUSTERS. Authors: Trumpler, Robert Julius
  7. ^ http://adsabs.harvard.edu/full/1937AnHar.105..359G THE EFFECT OF ABSORBING CLOUDS ON THE GENERAL ABSORBTION COEFFICIENT. Author: Jesse L Greenstein
  8. ^ Page 18. http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.53.80 The Optical Redshift Survey II. Authors: Basílio X. Santiago , Bas'ilio X. Santiago , Michael A. Strauss , Ofer Lahav , Marc Davis , Alan Dressler , John P. Huchra
  9. ^ http://ned.ipac.caltech.edu/level5/Sept07/Li2/Li3.html DUST SPECTROSCOPY - DIAGNOSIS OF DUST COMPOSITION In "The Central Engine of Active Galactic Nuclei", ASP Conference Series, Vol. 373, Xi'an, China, 16-21 October, 2007, eds. L. C. Ho and J.-M. Wang
  10. ^ (recommend Page 99) http://iopscience.iop.org/1538-3881/125/1/98/pdf/1538-3881_125_1_98.pdf THE PHYSICAL CONDITIONS OF INTERMEDIATE-REDSHIFT Mg II ABSORBING CLOUDS FROM VOIGT PROFILE ANALYSIS. Authors: Christopher W, Churchill, Stephen S. Vogt, Jane C. Charlton
  11. ^ http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.53.80 The Optical Redshift Survey II. Authors: Basílio X. Santiago , Bas'ilio X. Santiago , Michael A. Strauss , Ofer Lahav , Marc Davis , Alan Dressler , John P. Huchra
  12. ^ Page 104 http://iopscience.iop.org/0004-637X/518/1/103/pdf/0004-637X_518_1_103.pdf THE DISTRIBUTION OF HIGH-REDSHIFT GALAXY COLORS: LINE-OF-SIGHT VARIATIONS IN NEUTRAL HYDROGEN ABSORPTION. Authors: Matthew A. Bershady, Jane C. Charlton, Janet M. Geoffroy
  13. ^ http://www.utc.edu/Faculty/LingJun-Wang/RedshiftEssay.pdf Dispersive Extinction Theory of Redshift. Author: Ling Jun Wang
  14. ^ http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2010.16464.x/abstract On the impact of intergalactic dust on cosmology with Type Ia supernovae. Authors Brice Ménard, Martin Kilbinger, Ryan Scranton
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  21. ^ Valencic, Lynne A. (2004). "Ultraviolet Extinction Properties in the Milky Way". Astrophysical Journal. 616 (2): 912–924. arXiv:astro-ph/0408409. Bibcode:2004ApJ...616..912V. doi:10.1086/424922. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
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  33. ^ a b Gordon, Karl D. (2003). "A Quantitative Comparison of the Small Magellanic Cloud, Large Magellanic Cloud, and Milky Way Ultraviolet to Near-Infrared Extinction Curves". Astrophysical Journal. 594 (1): 279–293. arXiv:astro-ph/0305257. Bibcode:2003ApJ...594..279G. doi:10.1086/376774. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  34. ^ Fitzpatrick, Edward L. (1986). "An average interstellar extinction curve for the Large Magellanic Cloud". Astronomical Journal. 92: 1068–1073. Bibcode:1986AJ.....92.1068F. doi:10.1086/114237.
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  39. ^ Clayton, Geoffrey C. (2000). "Magellanic Cloud-Type Interstellar Dust along Low-Density Sight Lines in the Galaxy". Astrophysical Journal Supplements Series. 129 (1): 147–157. arXiv:astro-ph/0003285. Bibcode:2000ApJS..129..147C. doi:10.1086/313419. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  40. ^ Valencic, Lynne A. (2003). "Small Magellanic Cloud-Type Interstellar Dust in the Milky Way". Astrophysical Journal. 598 (1): 369–374. arXiv:astro-ph/0308060. Bibcode:2003ApJ...598..369V. doi:10.1086/378802. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  41. ^ Calzetti, Daniela (1994). "Dust extinction of the stellar continua in starburst galaxies: The ultraviolet and optical extinction law". Astrophysical Journal. 429: 582–601. Bibcode:1994ApJ...429..582C. doi:10.1086/174346. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
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Further reading

  • Binney, J.; Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 0691004021. {{cite book}}: Unknown parameter |lastauthoramp= ignored (|name-list-style= suggested) (help)
  • Howarth, I. D. (1983). "LMC and galactic extinction". Royal Astronomical Society Monthly Notices. 203: 301–304. Bibcode:1983MNRAS.203..301H.
  • King, D. L. (1985). "Atmospheric Extinction at the Roque de los Muchachos Observatory, La Palma". RGO/La Palma technical note. 31.
  • Rouleau, F.; Henning, T.; Stognienko, R. (1997). "Constraints on the properties of the 2175Å interstellar feature carrier". Astronomy and Astrophysics. 322: 633–645. arXiv:astro-ph/9611203. Bibcode:1997A&A...322..633R.
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