Radiative Transfer in Spiral Galaxies

1. Radiative Transfer in Spiral Galaxies Nick Kylafis University of Crete and Foundation for Research and Technology - Hellas

2. Introduction • If spiral galaxies were simple objects, e.g., consisting only of stars, then radiative transfer would not be necessary.

3. Introduction • If spiral galaxies were simple objects, e.g., consisting only of stars, then radiative transfer would not be necessary. • We would simply add up all the emitted light along a line of sight and that would be the surface brightness at that point.

4. Introduction • If spiral galaxies were simple objects, e.g., consisting only of stars, then radiative transfer would not be necessary. • We would simply add up all the emitted light along a line of sight and that would be the surface brightness at that point. • However, the presence of dust (that absorbs and scatters radiation) in spiral galaxies, makes radiative transfer absolutely necessary.

5. A question: • Can we get by with simple models ( “sandwich”-type) where the radiative transfer can be done more or less analytically?

6. A question: • Can we get by with simple models ( “sandwich”-type) where the radiative transfer can be done more or less analytically? • The answer is emphatically NO!

7. A question: • Can we get by with simple models ( “sandwich”-type) where the radiative transfer can be done more or less analytically? • The answer is emphatically NO! • The reason is that spiral galaxies are complex systems (bulge, spiral structure, clumpiness).

8. A question: • Can we get by with simple models ( “sandwich”-type) where the radiative transfer can be done more or less analytically? • The answer is emphatically NO! • The reason is that spiral galaxies are complex systems (bulge, spiral structure, clumpiness). • To zeroth order, galactic disks are described by exponential distributions of stars and dust in R and z, which are far from constant functions.

9. “It is more important to be stimulating than right” (M. Rees) • In my opinion, two papers were extremely stimulating in the study of spiral galaxies: • Disney, Davies, & Phillips 1989, MNRAS • Valentijn 1990, Nature

10. “It is more important to be stimulating than right” (M. Rees) • In my opinion, two papers were extremely stimulating in the study of spiral galaxies: • Disney, Davies, & Phillips 1989, MNRAS • Valentijn 1990, Nature • The conference in Cardiff in 1994 rendered the subject of the opacity of spiral galaxies as an important problem for people to work on. • At the time, it was not clear at all whether spiral disks are transparent or opaque.

11. Better late than never • An earlier paper on radiative transfer in spiral disks (Kylafis & Bahcall 1987) went largely unnoticed. In my opinion for the following reasons:

12. Better late than never • An earlier paper on radiative transfer in spiral disks (Kylafis & Bahcall 1987) went largely unnoticed. In my opinion for the following reasons: • The authors had not worked on this subject before. (This was actually a minus and a plus.)

13. Better late than never • An earlier paper on radiative transfer in spiral disks (Kylafis & Bahcall 1987) went largely unnoticed. In my opinion for the following reasons: • The authors had not worked on this subject before. (This was actually a minus and a plus.) • They also stopped working on the subject afterwards. (The work was not presented at any conference.)

14. Better late than never • An earlier paper on radiative transfer in spiral disks (Kylafis & Bahcall 1987) went largely unnoticed. In my opinion for the following reasons: • The authors had not worked on this subject before. (This was actually a minus and a plus.) • They also stopped working on the subject afterwards. (The work was not presented at any conference.) • People thought that it was not necessary to do detailed radiative transfer calculations.

15. It was the conference in Cardiff in 1994 that convinced me that I should return to the subject of the opacity in spiral galaxies. • Having experience with radiative transfer calculations and a 1.2 m telescope at home (Skinakas Observatory), we were able to start a systematic study of the opacity of spiral galaxies.

16. Radiation transfer methods • Two methods have been used extensively to do radiative transfer in spiral galaxies: • Method 1: Scattered intensities • Method 2: Monte Carlo

17. Method of scattered intensities (van de Hulst & de Jong, 1969, Physica) • The observed intensity along a line of sight can be written as • where is the n-times scattered intensity.

18. Method of scattered intensities (van de Hulst & de Jong, 1969, Physica) • The observed intensity along a line of sight can be written as • where is the n-times scattered intensity. • The series converges, but the calculation of the terms with n>1 is very computer intensive.

19. Method of scattered intensities (van de Hulst & de Jong, 1969, Physica) • The observed intensity along a line of sight can be written as • where is the n-times scattered intensity. • The series converges, but the calculation of the terms with n>1 is very computer intensive. • Therefore an approximation is needed.

20. Approximation • We made the reasonable assumption that

21. Approximation • We made the reasonable assumption that • Back in the 80’s, it was close to impossible to verify this approximation.

22. Approximation • We made the reasonable assumption that • Back in the 80’s, it was close to impossible to verify this approximation. • Baes & Dejonghe (2001) have verified that it is a good approximation at the ~1% level. Not bad for astrophysics! 

23. Monte Carlo method(Cashwell & Everett 1959, book) • The Monte Carlo method is simple and elegant, but only in the last decade it became possible to use it for radiative transfer problems in spiral galaxies.

24. Monte Carlo method(Cashwell & Everett 1959, book) • The Monte Carlo method is simple and elegant, but only in the last decade it became possible to use it for radiative transfer problems in spiral galaxies. • An incomplete list of papers: • Bianchi et al. 1996, ApJ • De Jong 1996, A&A • Wood & Jones 1997, AJ • Baes & Dejonghe 2001, MNRAS • Baes et al. 2003, MNRAS • Pierini et al. 2004, ApJ • Bianchi 2007, A&A

25. Monte Carlo method • All physical processes that are quantified, can be simulated by the Monte Carlo method.

26. Monte Carlo method • All physical processes that are quantified, can be simulated by the Monte Carlo method. • In essence, the Monte Carlo method does exactly what Nature does!

27. Monte Carlo method • All physical processes that are quantified, can be simulated by the Monte Carlo method. • In essence, the Monte Carlo method does exactly what Nature does! • A photon in Nature propagates and it does not know whether it will be absorbed or scattered, but something does happen.

28. Monte Carlo method • All physical processes that are quantified, can be simulated by the Monte Carlo method. • In essence, the Monte Carlo method does exactly what Nature does! • A photon in Nature propagates and it does not know whether it will be absorbed or scattered, but something does happen. • Similarly, in a Monte Carlo code the decision about what will happen is made with the use of a properly selected random number.

29. Some model results • Here I will show some results obtained with the method of scattered intensities.

30. Some model results • Here I will show some results obtained with the method of scattered intensities. • Results obtained with the Monte Carlo method will be presented mainly by other participants.

31. Some model results • Here I will show some results obtained with the method of scattered intensities. • Results obtained with the Monte Carlo method will be presented mainly by other participants. • The fresh, off-the-oven results of Simone Bianchi demonstrate that the two methods agree extremely well.

32. Some model results • Here I will show some results obtained with the method of scattered intensities. • Results obtained with the Monte Carlo method will be presented mainly by other participants. • The fresh, off-the-oven results of Simone Bianchi demonstrate that the two methods agree extremely well. • Relief !!! 

33. Model for late-type spiral galaxies • Old stars: Disk with exponential distribution in z and R + a de Vaucouleurs spheroid.

34. Model for late-type spiral galaxies • Old stars: Disk with exponential distribution in z and R + a de Vaucouleurs spheroid. • Dust: Disk with exponential distribution in z and R (with different length-scales).

35. Model for late-type spiral galaxies • Old stars: Disk with exponential distribution in z and R + a de Vaucouleurs spheroid. • Dust: Disk with exponential distribution in z and R (with different length-scales). • Young stars: Disk with exponential distribution in z and R (also with different length-scales).

36. It has been shown that: • The effects of spiral structure are negligible (Misiriotis et al. 2000, A&A).

37. It has been shown that: • The effects of spiral structure are negligible (Misiriotis et al. 2000, A&A). • Similarly for the effects of clumpiness (Misiriotis & Bianchi 2002, A&A). Watch for Bianchi’s talk though.

38. It has been shown that: • The effects of spiral structure are negligible (Misiriotis et al. 2000, A&A). • Similarly for the effects of clumpiness (Misiriotis & Bianchi 2002, A&A). Watch for Bianchi’s talk though. • 3-D radiative transfer calculations have been performed to produce model images that best fit the observations (Xilouris et al. 1997, 1998, 1999, A&A).

39. Example: NGC 891 • From the optical and NIR images, we have inferred with our model the total amount of dust (warm and cold) and its spatial distribution.

42. Model fits • Qualitatively, the fits are good. • Quantitatively, how well does the model fit the observations?

44. Main results from the study of 10 edge-on late-type spiral galaxies: • Central, face-on optical depth in the V band < 1. Thus, IF all the dust has been accounted for, late-type spiral galaxies are transparent. • identical to the galactic one. • Similar conclusions have been reached by Alton et al. 1998, A&A, and Davies et al. 1999, MNRAS.

45. Can the IR spectrum be computed? • YES. This is because:

46. Can the IR spectrum be computed? • YES. This is because: • The energy absorbed at every point in the galaxy has been calculated.

47. Can the IR spectrum be computed? • YES. This is because: • The energy absorbed at every point in the galaxy has been calculated. • Then, we make an assumption about the IR dust emissivity (e.g., what is thought appropriate for our Galaxy).

48. Can the IR spectrum be computed? • YES. This is because: • The energy absorbed at every point in the galaxy has been calculated. • Then, we make an assumption about the IR dust emissivity (e.g., what is thought appropriate for our Galaxy). • We equate the two to determine the temperature of the dust at every point in the galaxy.

49. And then we integrate over all the points in the galaxy to compute the IR spectrum.

50. And then we integrate over all the points in the galaxy to compute the IR spectrum. • This was done by Popescu et al. 2000, A&A, under the assumption that the FIR emissivity of the dust is the same as the one thought appropriate for our Galaxy.