A short course in the Galactic diffuse ISM

S. R. Kulkarni

Purpose. The purpose of this "mini" course (3 lectures) is to lay the foundations for students interested in studying diffuse gas. We will review the basic thermal and ionization balance of the diffuse warm neutral and warm ionized medium (WNM, WIM). In the near term progress can be undertaken with narrow band observations in the optical/UV, low frequency free-free and recombination lines. In the longer run, the course will prepare students prepare for far-infrared missions focused on diffuse fine structure line emission.

Background. The ISM in our Galaxy, apart from the molecular medium, consists of the Hot Ionized Medium (HIM), the Warm Neutral/Ionized Medium (WNM/WIM) and the cold neutral medium (CNM). A rough pressure equilibrium is expected to exist between the three phases. In this short pedagogical course we will investigate the essential thermal equilibrium of the Warm and Cold phases. We will build a simple model with heating by dust particles, ionization from star light and cosmic rays and cooling by forbidden lines (primarily fine structure lines). This numerical model will allow the student to explore the thermal equilibrium of diffuse clouds in different locations in the Galaxy and a variety of electron densities (including those relevant for extreme scattering events). Separately, we will study the ionization of diffuse clouds by cosmic rays.

Preparation, The primary book for this course is Draine (2011). Students who wish to take the course should review and have passing familiarity with Chapters 1-3 and 12-14 of Draine well ahead of this course. For the cosmic ray lecture students should read up Chapter 5.1-5.6 and 15.1-15.8 of Longair (2011).

Who Should Take the Course? Students who are interested in ISM research using radio techniques. However, the lecturer expects that the student is sincerely interested in spending some time to learn the topic discussed above. Those who wish to participate passively (merely listen) will not find their time to be well spent.

Textbooks

• Draine (2011) "Physics of the Interstellar & Intergalactic Medium"
• Spitzer (1978) "Physical Processes in the Interstellar Medium"
• Tielens (2005) "The Physics & Chemistry of the Interstellar Medium"
• Longair (2011) "High Energy Astrophysics"
• Rybicki & Lightman "Radiative Processes in Astrophysics"

I will start the course with a 30-minute introductory words and thoughts on "Essentials for an Observer".

I have two requests for the students. (1) No texting or emailing (or equivalent) during the classes. If you are that busy then you should not be taking this course. (2) No social media regarding the course.

• Lecture 1: 1400-1530
The Galactic Ionization Rate due to Cosmic Rays. SRK Notes
Gamma-rays from ISM  |  Gas-ion Chemistry  |  Cosmic-ray Ionization Rate from Chemistry
Open problems: is cosmic ray rate reduced inside clouds? are there several major sources of low energy cosmic rays? is there significant variation in the intensity of cosmic rays?
• Lecture 2: 1400-1530
The Thermal & Ionization Balance of Diffuse Gas SRK Notes
A Review of Fine Structure Lines  |  Motley slides on PAH
• Lecture 3: 1400-1530
The Two-phase model of the ISM (same notes as for lecture 2)
Open problems: what fraction of ISM could be in intermediate phases? plausible locatin of extreme scattering events require high pressure regions. investigate what can be done with current generation of IFU narrow band imagers, by LOFAR (radio recomb and free-free absorption).
• Concluding Session: 1330-1500
• Open discussion of "Future areas of research in diffuse ISM" (with emphasis on low frequency radio astronomy and deep IFU spectroscopy with large optical telescopes). Participation is not merely for the students but also others at the Institute (including faculty) who are interested in the general field of diffuse ionized gas