Megamaser

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A new type of extra-galactic maser was discovered with the detection of OH emission from Arp 220 (IC 4553) with a then unequalled luminosity of ~103LO. The most luminous galactic OH maser (in HII regions) has a luminosity of ~10-3LO, so this maser was called a megamaser[1] . Now more than 50 OH megamaser sources have been observed, ranging in Redshift up to z = 0.2[2].

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[edit] Occurrence

Galaxies that exhibit megamaser activity are all active galaxies (AGN) with enormous IR luminosities (LIR > 1011 LO), and are very rich in molecular gas compared to ordinary spiral galaxies[3]. Indeed the chance of finding OH megamasing increases with LIR and reaches 50% for galaxies with LIR > 1012 LO[4]. The profile of the spectra of theses galaxies are steep (have large gradient) in the mid-IR and sloping (have small gradient) in the far-IR, compared to most galaxies[5]. Dust absorbing high-energy radiation and re-emitting it in the infrared is the source of the high LIR values.

The galaxies have been characterised as Seyfert2 or LINERS in 20 cases, Starburst nuclei in 13 cases and Seyfert1 in two cases[6]. A large number of these galaxies are in the process of mergers, collisions or gravitational interactions. There is a relationship between the type of activity the galaxies show (high luminosity and extensive star-formation) and the fact that they are interacting with other galaxies - most ultra-luminous IR galaxies are experiencing gravitational interactions[7]. Starburst activity is most probably caused by tidal forces acting on molecular clouds in the galaxies, causing rapid star formation.

Megamaser galaxies form a class of low redshift objects in a state of rapid evolution - probably the megamaser phase is the initial phase of galactic collisions[8].

[edit] Known Megamaser Species

The following species have been observed in stimulated emission from astronomical environments:

  • OH
  • H2CO
  • H2O

[edit] Characteristics

Megamaser emission can be contrasted to that of ordinary astronomical masers:

  • Megamaser amplification factors are very low: often the radiation is not even doubled in intensity on passing through a megamaser, and so the line narrowing effect is very small
  • Megamasers emission does not have narrow lines: megamasers have huge velocity ranges of hundreds of km/s in them, and so Doppler shifts stretch out the lines so that they are very broad
  • Megamasers are not saturated
  • Megamasers are not bright relative to galactic masers (though of course are still much brighter than any black body of similar temperature)
  • Megamaser emission is not polarised.

[edit] OH Megamaser Characteristics

A useful and easily measured characteristic of OH masers is the 18cm mainline emission ratio: R1665 = T1667 / T1665. This is 9:5 for optically thin emission in local thermodynamic equilibrium. OH masers in star-forming regions usually emit strongest in the 1665 MHz line (i.e. R1665 < 1), while late-type stars show dominance in the 1612 MHz line with a wide range of R1665 values. In OH megamasers R1665 typically exceeds the local thermodynamic equilibrium ratio, and has a value of ~2 or higher.

OH megamaser spectra typically show broad linewidths, with typical velocity distributions of ~300-500km/s. This in part arises from the rotation of the molecular clouds about the nucleus, and from possible infall into the nucleus. OH megamaser emission has been recently shown to display Zeeman splitting, caused by magnetic fields in the maser region[9].

[edit] Causes of Megamaser Activity

With the strong relationship between megamasing and the far-IR output of their parent galaxies a logical conclusion is that far-IR is important for the functioning of megamasers.

[edit] OH Megamaser Causes

The interaction of OH molecules with the far-IR dust radiation is the dominant pumping mechanism in OH masers. The megamaser clouds are emitting by low gain amplification of the background radio continuum source (Synchrotron emission from the nucleus). The amplification process is unsaturated in the case of OH megamasers, but in some cases it is possible that the saturation limit is being approached. Only a small factor of the continuum source (usually the nucleus) is covered by the masing cloud, which makes the gain of megamasers difficult to calculate.

Larger molecular clouds tend to show weak 18cm emission or 18cm absorption for the same OH column densities. It is possible that OH megamasers are on an evolutionary sequence whereby megamasing clouds merge and become depleted in OH, reducing their maser emission and eventually turning them into OH absorbers when the dust is no longer warm enough to excite the masers.

An important discovery in this field is the detection of extended unusual OH emission in galactic star-forming regions[10]. This emission is similar to, but much weaker than, OH megamaser emission. This may show that OH megamaser conditions are present in some parts of our galaxy, but not to the extremes of active galaxies.

[edit] References

  • ^  [1] Baan W.A., Wood P.A.D., Haschick A.D. 1982 Astrophys. J. 260 L49-52
  • ^  [2] Elitzur M. Annu. Rev. Astron. Astrophys. 1992 30 75-112
  • ^  [3] Cohen R.J. Rep. Prog. Phys. 1989 52 881-943
  • ^  [4] Henkel C., Wilson T.L., 1990 Astron. Astrophys. 229 p431-440
  • ^  [5] Burdyuzha V.V., Vikulov K.A. 1990 Mon. Net. R. Astr. Soc. 244 p86-92
  • ^  [6] Randell J., Field D., Jones K.M., Yates J.A., Gray M.D., Aston. Ast
  • ^  [7] Andreasian N., Alloin D. 1994 Astron. Astrophys. supplement series 107 No.1 p23
  • ^  [8] Sinclair M.W., Carrad G.J., Caswell J.L., Norris R.P., Whiteoak J.B. Monthly Notices of the Royal Astronomical Society 1992 256 No.2p33-34
  • ^  [9] Robishaw T., Quataert E., Heiles C. (2008) The Astrophysical Journal, 680.
  • ^  [10] Harvey-Smith L., Cohen R.J. (2006) Monthly Notices of the Royal Astronomical Society, 371, p1550.