A short course in the Galactic diffuse ISM

S. R. Kulkarni

This website is under construction and will updated now and then.

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.


I expect to hold three lectures, described below. Students should read the notes in advance. I will merely undertake a rapid review of the notes and then open up the floor for discussion. At the end of each lecture, I will review reading material and also suggest future (smallish) research topics.

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.