Diffuse UV Background
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[edit] INTRODUCTION
We can get diffuse background radiation field at all measurable wavelengths starting from radio to gamma rays and they carry a lot of astrophysical information, e.g.-The 2.7K black body microwave background radiation.
The diffuse cosmic ultraviolet background radiation is one among them that occurs at various frequencies and it is important because of a number of astrophysical topics:
- Physical characteristics and spatial distribution of gas and grains (Jura, 1979).
- Galaxy evolution (Tinsley, 1973)
- The search for intergalactic matter and massive neutrinos (Sherman and Silk, 1979)
The enhanced capabilities of UV instrumentation, a better understanding of basic emission processes and the rapid progress in the associated field have contributed a lot to the growth of the UV astronomy.
But there are certain difficulties in studying the diffuse UV background emissions:
a) Inherent instrument background
b) Zodiacal light: results from solar radiation scattered by interplanetary dust and is one of the contributors to the sky brightness.
c) Airglow (line emission originating in the geocorona)
Therefore the observations in the far UV band (912-2000A) is best suited wavelength range to explore the galactic and extragalactic diffuse UV background because
i) Zodiacal light intensity drops zero below 2000A ii) The interstellar radiation field in the far UV is dominated by the contribution from O,B stars with V magnitude less than 10 that are easily identifiable or removable from the database (Henry,1977) iii) Low latitude bright airglow emissions are supressed by very high optical depth of far UV radiation in the atmosphere.
A number of galactic and extragalactic sources have been suggested either by observation or model calculations as possible component to the diffuse UV background. The most discussed components are as follows:
[edit] SOURCES
[edit] Dark Count
The dark count is instrumental background arising generally through fast particles hitting the detector. The typical rate for low Earth orbit is on the order of 5 counts cm-2 s-1. The astronomical rate corresponding to this will depend on the calibration and the field of view. For GALEX this corresponds to 6(4) counts cm-2 s-1 sr-1 A-1 in FUV(NUV) bands (Martin et al. 2005, ApJL).
[edit] Airglow
Airglow arises in the Earth's atmosphere and is a strong function of time of observation and height. It can consist of either line or band emission. The strongest lines are the Lyman lines where solar radiation is resonantly scattered by atoms in the upper atmosphere at altitudes of greater than 1000 km. As such, these geocoronal lines are always strong. The night intensity is on the order of 3 kR.
Other lines are typically at altitudes of less than 400 km. Upward looking observations at night at 600 km or higher will not see much emission.
Here is a list of airglow lines with FUSE.
There are several publications which should have more information: Feldman et al. (1992) Meier (1991) Also P. D. Feldman's airglow papers
[edit] Zodiacal Light
Zodiacal light consists of Sunlight scattered by interplanetary dust grains. It is generally assumed to be the same color as the Sun and so the distribution of the zodiacal light in the UV follows that in the visible. Zodiacal light does not contribute below 2000 A.
[edit] Galactic Sources
[edit] Dust-Scattered Galactic radiation
An new observational result which points to galactic component of diffuse ultraviolet radiation field is the correlation between observed background intensity and line of sight neutral hydrogen column density Paresce (1980) and Maucherat-Joubert (1980). As discussed by these authors such a correlation can be understood qualitatively if this ultraviolet diffuse radiation is due to galactic plane starlight scattered off high latitude interstellar dust (Sandage,1976; Jura,1979).
[edit] Fluorescence of molecular hydrogen
Duley and Williams (1980) has suggested that emission due to molecular hydrogen fluorescence also an important source of ultraviolet diffuse galactic radiation. The ultraviolet photons can be absorbed by molecular hydrogen in in its Lyman and Werner bands at wavelengths from about 900-1100A. The initial absorption is followed by fluorescence to the vibrational continuum of the ground state with a probability of 0.01.The remaining 90% of the fluorescent transitions populate various bound excited vibrational rotational levels of the ground state(J=0 to J=14). Then molecules in these excited states decay to unbound J>14 levels of the ground state which leads to dissociation of the molecules(Black,1987). Both these emissions are in UV.expected correlation between diffuse UV background and line of sight neutral hydrogen column density is due to the combined effect of both the emission (dust scattered light & emission due to molecular fluorescence) and it is in good agreement with the observations.
[edit] Unresolved Stars
Unresolved stars influence the result of the UV background observations because some of these sources will be in the field of view of an instrument attempting to measure the diffuse background. There are several methods explained by Paresce (1980) to eliminate the stellar contribution from the diffuse UV. The best way to correct the stellar contamination is to use the TD1 catalogue of stellar UV fluxes (Thompson, 1978) covering the range 1500-2500A down to mv=8-9. After taking a lots of precautions, because of the unknown distribution of the remaining residual faint (mv>10) stars, the possible contribution of the unresolved faint stars. Crawford (1979) estimated this contribution using standard main sequence colours, absolute visual magnitudes,scale heights and luminosity functions and it is not exceeding 50 photons taking the contribution due to sum of all main sequence stars (30 photons) and non main sequence stars (20 photons) of mv>10 in the galaxy at 1500A in the direction of galactic poles.
[edit] Atomic Emission Lines
Rocket observations at low resolutions of the spectrum of far-ultraviolet background near the north galactic pole suggest that atomic emission lines have contribution to the diffuse far ultraviolet background radiation. Jakobsen and Paresce (1981) calculated the intensity of the diffuse background emission in the far ultraviolet from the coronal gas present in the plane and halo of the galaxy and the emission lines they got are CIII (1909A), CIV (1549A), NIII (1749A), NIV (1487A), SiIII (1207A) and SiIV (1398A) and predicted that these emission lines are from the collisional excitation of atoms in a hot galactic corona. At temperature in the range T=10*4-10*6K coronal gas cooling occurs which in turn gives rise to collisionally-excited line radiations that contribute to the background radiation. But Sciama and Melott (1981) have proposed that the highly ionized carbon (CIV) and silicon (SiIV) within several kiloparsecs from the plane of galaxy,attributed by a hot coronal (T=10*5) of the galaxy is due to photo ionization of much colder gas by the photons resulting from the decay of heavy (m=100ev) neutrinos.Removal of the line emission leaves a residual uniform cosmic ultraviolet background radiation of only 150+/-50 photons,about half that that derived by Anderson (1979).
[edit] Extragalactic Radiation
[edit] Galaxies and QSOs
The line-of-sight integrated light of galaxies and quasars will give an average background I(lambda) at wavelength =(one mathematical formula).... To estimate the expected UV background due to galaxies and QSOs, detail information on the UV spectra and evolution of these objects required. The expected galaxy background contribution, neglecting evolution is calculated for omega=0 and is plotted as the curve 'Galaxies/1'in the figure (Paresce and Jackobsen,1980). If the color distribution of galaxies in line of sight is such that it predominantly favours the spiral galaxies, if the mean flux can be extrapolated to the Lyman limit with a power law and if we neglect any evolution effects, the predicted contribution comes very close to minimum observations and this is marked in curve 'Galaxy/2' in the figure. It is more likely that the true value lies somewhere in between the two.
A probable better way to get the extragalactic contribution due to galaxies is to look for small scale angular correlations because the stars are randomly distributed in angular separation in the sky while the galaxies are known to cluster. The result obtained by Martin and Bowyer (1990) on the spatial distribution of weak far ultraviolet sources. They plotted the radial power spectrum of the sources with angular frequency. The stars contributed a non correlated, white noise component to that data. The expected radial power spectrum as a function of angular frequency is a power law with index -1.2 which is consistent with the data and is what expected from the integrated light of galaxies.
[edit] Radiative Recombination in a Lukewarm Photoionized IGM
The curve Ly alpha in the figure shows the expected background contribution due to red shifted recombination Ly alpha from a dense fully ionized IGM of temperature 10^4 K, having a closure density appropriate for a Hubble constant of 50 km/s/Mpc^2. The background intensity increase at longer wavelengths corresponding to emission from earlier, more dense, epochs as as the recombination emissivity is proportional to square of the density. Same is the case for the HeII304A line.
[edit] Collisional Excitation in a hot,dense IGM
The IGM may have been reheated by some dissipative processes some time in the past.In this case,collisional excitation of the HI Ly alpha and HeII , lines at the appropriate temperature are the dominat processes.Beacause of the higher emissivity of the collisionally excited plasma ,this line emission smeared by the redshift,could be significantly brighter that that due to recombination.
[edit] The sources of IGM Ionization
The high transparency of IGM, implied by Gunn Peterson test, must be maintained by a high level of ionizing flux, a flux that should turn up in the UV flux. The curve Photoionized IGM in the figure is the estimate of the background level produced by redshifted H Ly alpha continuum radiation necessary to photoionize a dense IGM to the required level.
[edit] Radiative Decay of Massive Particles
Stecker (1980) and Kimble, Bowyer & Jackobsen used the intensity of far ultraviolet background to derive constraints on on the radiative lifetime of massive neutrinos assuming these particles form a cosmic sea of material. Stecker interpreted a possible step in the spectrum of the far ultraviolet background as the product of neutrino decay and derived a lifetime for the decay. One mathematical formula will be introduced here.
[edit] Component of the far UV background with aproximate intensities
Total intensity--------------------300-1500 photons cm*-2 s-1ster*-1 A*-1
(from Bowyer,2001)
Galactic components
- Scattering by dust------------ 200-1500 photons cm*-2 s-1ster*-1 A*-1
- HII two photon emission----------50 photons cm*-2 s-1ster*-1 A*-1
- H2 fluorescence ----------------100 photons cm*-2 s-1ster*-1 A*-1(in molecular clouds)
- Hot gas line emission-----------10 photons cm*-2 s-1ster*-1 A*-1
Components of the uniform high latitude background(Extragalactic ??)
- Galaxies----------------------50-150 photons cm*-2 s-1ster*-1 A*-1
- QSOs/AGNs--------------------- <10 photons cm*-2 s-1ster*-1 A*-1
- Intergalactic medium-----------<10 photons cm*-2 s-1ster*-1 A*-1
- Unexplained(dust,...?)---------none to <200 photons cm*-2 s-1ster*-1 A*-1
[edit] Observations
In 1980s many experiments were carried out to investigate the correlation between the intensity of far ultraviolet background and any parameters associated with the Milky Way galaxy. Any such parameter that will correlate with the far UV background will signify that the flux is galactic. The most common parameter is Galactic neutral hydrogen column density as derived from the 21 cm radio studies.
[edit] Apollo-Soyuz Mission
Paresce, McKee and Bowyer (1980) used the data from the telescope flown on the Apollo-Soyuz orbital mission which carried out a photometric survey at 1400A in ~300 separate view directions and analysed the data for the galactic latitude range |b|>30. The instrument had both a high sensitivity to the diffuse radiation and small field of view to identify the point sources that may be contributing to the diffuse background. During their measurement of the diffuse UV background, they taken well precaution for the airglow emission, zodiacal light and stellar contamination. The diffuse UV flux they have measured through their observation is from 300-2000 photons cm*-2 A*-1 S*-1 ster*-1 putting a strict upper limit to the extragalactic contribution of 300 photons cm*-2 A*-1 S*-1 ster*-1 at 1400A. The resulting diffuse background showed a nice correlation with the galactic neutral hydrogen column density determined by the 21-cm radio data which provided evidence that the majority of UV background is galactic in origin. They have mentioned few sources that contribute to the extragalactic flux at high galactic latitude.
[edit] Analysis of D2B Satellite data
The high galactic latitude (b>30) whole sky survey obtained with the D2B -Aura satellite at 1690A & 2200A is analysed by Joubert, Mansou (1983) and they confirmed the existence of a correlation between the 1690 and 2200A fluxes at high galactic latitude and the column density of atomic hydrogen or the interstellar extinction as derived from the galaxy counts. This shows that the stellar light scattered off dust grains mixed with the gases is a dominant galactic component to the UV background radiation. From their calculation they expect a marginal contribution due to fluorescence of molecular hydrogen at 1690A. Using Jura's (1979a,b) model they derived values of asymmetry factor of the scattering phase function g=0.6 to 0.8 which implies that dust is forward scattered in the far UV. They also calculated the intensity of diffuse UV background due to extragalactic component which is in the range 300-690 CU at 1690A and 160-360 CU at 2200A, which is in good agreement with other determinations.
[edit] Berkeley UVX Shuttle Experiment
part of UVX
[edit] Summary of Observations
| Paper | Wavelength | Cosecant Law | UV/IR | EGL | a | g | remarks |
|---|---|---|---|---|---|---|---|
| Perault et al. 1991 | 1690 | 821 - 1172 | 60 - 122 | - | - | - | D2B |
| Sasseen & Deharveng 1996 | 1400 - 1800 | - | 255 | - | individual fields vary more. |
[edit] References
(1) Jura, M. 1979, ApJ, 227, 798.
(2) Tinsley, B. M. 1973, A&A, 24, 89.
(3) Jakobsen, P. & Paresce, F. 1981, A&A, 96, 23.
(4) Paresce, F. & Jakobsen, P. 1980, Nature, 288, 119.
(5) Davidsen, A., Bowyer, S. & Lampton, M. 1974, Nature, 247, 513.
(6) Feldman, P. D., Brune, W. H. & Henry, R. C. 1981, ApJ, 249, 51.
(7) Sciama, D. W. & Melott, A. L. 1981, PRL, 46, 1369M.
(8) Anderson, R. C., Brune, W. H., Henry, R. C. & Feldman, P. D., & Fastie, W. G. 1979, ApJ, 234, 415.
(9) Thompson, G. I. et al. 1978, Catalogue of Stellar Ultraviolet Fluxes (Science Research Council).
(10) Crawford, R., Morgan, B., Paresce, F., Lampton, M., & Bowyer, S. 1979 A&A, 36, 371.
(11)Bowyer,S.,IAU symposium,Vol.204,2001
(12) Paresce,F.,McKee,C.,&Bowyer,S.1980,ApJ,240,387
(13) Black,J.H.,& van Dishoeck,E.F.1987,ApJ 322,412
