1. INCIDENCE OF ABSORPTION AT THE
INTERFACE OF GALAXIES AND IGM
Ramiz Ahmad
Indian Institute of Space Science and Technology
May, 2012

2. Plan of the talk
1. Introduction.
2. Details about data.
3. Data analysis.
4. Method of measurement.
5. Results and observations.
6. Conclusion.

3. What is out there?

4. What is out there?
What is it made up of?

5. What is out there?
What is it made up of?
Composition of the universe (present day)

6. Distribution of the baryonic matter:
~ 6-10 % are in the form of galaxies.

7. Distribution of the baryonic matter:
~ 6-10 % are in the form of galaxies.
=> 90-93 % of the baryonic matter are in the form of IGM

9. Owing to the high temperature of the IGM, the UV region of the
electromagnetic spectra is the best region to study it.

10. Owing to the high temperature of the IGM, the UV region of the
electromagnetic spectra is the best region to study it.
Cosmic Origin Spectrograph- Aboard the Hubble Space Telescope
COS is capable of producing high sensitivity medium and low
resolution far-UV and near-UV spectra

11. The data was obtained from the public archive of the COS (Cosmic Origin
Spectrograph) available at Multi-mission Archive (MAST) at Space Telescope
Science Institute (STScI) webpage : http://archive.stsci.edu/

12. The data was obtained from the public archive of the COS (Cosmic Origin
Spectrograph) available at Multi-mission Archive (MAST) at Space Telescope
Science Institute (STScI) webpage : http://archive.stsci.edu/
COS Grating Parameters

13. Data Analysis

14. Data Analysis
Sample table

15. Data Analysis
Step1: The data is converted into either ASCII or the .sav format depending upon the
requirement.
Step2: The data is re-binned.
Step3: Then various rebinned files are co-added with the exposure time as the
weightage factor.
Step4: The spectra is then plotted using the co-added files and from the spectra the
continuum normalized spectra is obtained on which all the final analysis is done.

16. Continuum Normalized Spectra of HE0153-4520

17. Line Measurements Using Apparent Optical
Depth Method
The direct integration of the observed optical path difference is done

18. Line Measurements Using Apparent Optical
Depth Method
The direct integration of the observed optical path difference is done

19. Line Measurements Using Apparent Optical
Depth Method
The first moment of the optical depth gives the central velocity of the absorption line

20. Line Measurements Using Apparent Optical
Depth Method
The first moment of the optical depth gives the central velocity of the absorption line
And we get the Doppler width from the second moment of the apparent optical depth

21. Line Measurements Using Apparent Optical
Depth Method
The first moment of the optical depth gives the central velocity of the absorption line
And we get the Doppler width from the second moment of the apparent optical depth
Where,

22. Line Measurements Using Apparent Optical
Depth Method
The general expression for the calculation of equivalent width is
Where f(v) is the continuum normalised flux of the spectra in the velocity space.

23. Observations and Results

24. Observations and Results
1. HE0153-4520
2. HE0226-4110
3. HE0238-1904
4. MS0117.2-2837

25. Observations and Results
System plot for HE0153-4520

26. Observations and Results
System plot for HE0226-4110

27. Observations and Results
System plot for HE0238-1904

28. Observations and Results
System plot for MS0117.2-2837

29. Observations and Results
If the line is clearly resolved, a value of the parameter can be obtained. In case of
the absence of the absorption line or unresolved line, only an upper limit could be
calculated.

30. Observations and Results
If the line is clearly resolved, a value of the parameter can be obtained. In case of
the absence of the absorption line or unresolved line, only an upper limit could be
calculated.
The table shows the measured values and the upper limit of the various parameters
(Wr (mA)= Equivalent width in milli-angstrom, ba= Doppler parameter)

31. Observations and Results
The table shows a list of the line of the sights, associated absorbers, impact parameters
and the equivalent width.

32. Observations and Results
Variation of the equivalent depth with impact parameter.

33. Conclusion
1. There is no obvious relation between the impact parameters of the galaxies and the
equivalent width of the absorption lines.

34. Conclusion
1. There is no obvious relation between the impact parameters of the galaxies and the
equivalent width of the absorption lines.
2. Based on the measurement of the Doppler parameter, the average value of the
temperature was found to be 5 x 10^4 K, which is consistent with the temperature of the
photoionized component of the IGM.

35. Conclusion
1. There is no obvious relation between the impact parameters of the galaxies and the
equivalent width of the absorption lines.
2. Based on the measurement of the Doppler parameter, the average value of the
temperature was found to be 5 x 10^4 K, which is consistent with the temperature of the
photoionized component of the IGM.
3. Although in none of the case any other absorption line by any metal ion is detected,
from the measurement an upper limit was calculated for the ions in all the cases
and is in agreement with the model of cosmic abundance percentage. The absence of
metal absorption is consistent with low metallicity (of ~1/10 solar) typical of low-z
IGM.

36. Conclusion
1. There is no obvious relation between the impact parameters of the galaxies and the
equivalent width of the absorption lines.
2. Based on the measurement of the Doppler parameter, the average value of the
temperature was found to be 5 x 10^4 K, which is consistent with the temperature of the
photoionized component of the IGM.
3. Although in none of the case any other absorption line by any metal ion is detected,
from the measurement an upper limit was calculated for the ions in all the cases
and is in agreement with the model of cosmic abundance percentage. The absence of
metal absorption is consistent with low metallicity (of ~1/10 solar) typical of low-z
IGM.
4. Though as of now, nothing can be said about the relation between the impact parameter
and the equivalent width because of the lack of the number of data points, one
conclusion that can be drawn based on the above graphs is that there is a possibility that
the IGM surrounding the galaxies is patchy.

37. References:
1. Penton, Steven V.; Stocke, John T.; Shull, J. Michael, “The Local Lyα Forest. IV. Space
Telescope Imaging Spectrograph G140M Spectra and Results on the Distribution and
baryon Content of H I Absorbers”, The Astrophysical Journal Supplement Series , 152:29-62,
2004 May.
2. Lehner, N.; Savage, B. D.; Wakker, B. P.; Sembach, K. R.; Tripp, T. M. “Low-
Redshift Intergalactic Absorption Lines in the Spectrum of HE 0226-4110”, The
Astrophysical Journal Supplement Series , 164:1 – 37, 2006 May
3. Blair D. Savage, Kenneth R. Sembach, “The Analysis of Apparent Optical Depth Profiles for
Interstellar Absorption Lines”, The Astrophysical Journal, 379: 245-259,1991 September
20
4. Joel N. Bregman, "The Search for the Missing Baryons at Low Redshift", Annu.
Rev. Astron. Astrophys. 2007. 45:221–59
5. Anand Narayanan, Bart P. Wakker, Blair D. Savage, Brian A. Keeney, J. Michael Shull,
John T. Stocke, and Kenneth R. Sembach, "Cosmic Origins Spectrograph And Fuse
Observations of T ∼ 10^5 K Gas In A Nearby Galaxy Filament", The Astrophysical
Journal, 721:960–974, 2010 October

38. Thank You!