Guide to Specific Sections

A Catalog of HII Regions

This section introduces HII regions and how the SDSS finds them. The SDSS does not intentionally search for HII regions, but sometimes they are misidentified as galaxies, and therefore are sometimes selected for spectral analysis. This misidentification happens mostly with HII regions in large, extended spiral galaxies.

You may wish to discuss emission lines before students begin their search. HII regions have very strong hydrogen emission lines, but sometimes lines from other elements, notably oxygen, are also present.

The cores of active galactic nuclei (AGN) also have strong hydrogen emission lines, and have similar colors to HII regions. This is why students (and other researchers) must visually inspect possible HII regions.

Suspected HII regions are often difficult to tell from active galactic nuclei from colors or spectra. However, Photo seems to have the most difficulty with HII regions of large, extended galaxies, so it is usually easy to tell the difference visually.

Characteristics of HII Regions

Because HII regions have many protons and electrons recombining to form hydrogen atoms, they tend to give off a lot of ultraviolet light. If your students are not comfortable with this concept, pause to explain it, pointing to the animation of the electron falling into a lower energy level.

Because of the light that HII regions emit, they frequently stand out by their colors. Looking at the U-Z color can give you a lot of HII region candidates. The U-Z color of an HII region will be very low due to the large amount of ultraviolet light they emit. The U-G, U-R, and U-I colors tend to be low as well. Generally, the difference is not as large in U-G. For HII regions with particularly strong Ha lines, the U-R values are not as low, which means that students could miss some HII regions.

Writing a Query

SkyServer’s query tool returns a maximum of 1,000 entries, so it is important to write queries carefully. There are probably about 250 – 300 HII regions in the database (no one has inspected them all yet), so a well written query can retrieve them all in one shot. Students will probably take a few tries to develop an effective query.

Have students study the Object Browser to familiarize themselves with how SkyServer data are organized. To find candidate HII regions, students will need to retrieve data from more than one data table. To be sure you are looking at the same object, you have to use a join. Note the command phototag.objId = specObj.objID in the explanation. This command is called a “join”; it makes sure that the object ID (objID) is the same in each table. In other words, a join ensures that the query tool is searching for information on the same object in the two tables. If students use multiple tables, they will need to write a join between each table and every other table. For example, plateID ensures that the same object is found in the specObj table and the plate table.

Students will not initially have a much insight into where to draw their color cuts. Therefore, they may wish to write a query that looks at a smaller part of the sky using BETWEEN in the WHERE block (for example, ra BETWEEN 0 and 45). They can use this smaller query to refine which colors they will use to look for HII regions.

Once students figure out the colors of most HII regions, they can write queries to search for galaxies with certain colors that are likely to be HII regions. A sample query is shown below. It searches for galaxies with u – z < 2.


plate.plate, specObj.fiberID, photoObj.u, photoObj.g, photoObj.r, photoObj.i, photoObj.z, photoObj.ra, photoObj.dec


photoObj, specObj, plate


photoObj.objId = specObj.objID AND specObj.plateID = plate.plateID AND specObj.class = 2 and photoObj.u – photoObj.z < 2

SkyServer is very picky about syntax. Although it is not case sensitive (caps and small letters are equivalent), any small mistake will return an error. Even adding an extra space where there shouldn’t be one will cause an error message. Troubleshooting queries can sometimes be difficult. Try comparing the syntax of your students’ queries to the syntax of the sample query above. If you have too much difficulty, email us both the query and the error message, and we will try to help.

Identifying HII Regions

As students inspect HII regions to build up their catalogs, remind them that accurate and clear record keeping is extremely important. Students need to think about what information they should put in their catalogs to help other people find and identify their objects. They should also include a few paragraphs on how they selected their data (for instance, “We searched for galaxies with u – r < 2"). They also should keep notes of objects that they had difficulty classifying, and the reasons for their difficulty. Students should realize that this does not represent a failure on their parts. Frequently the most difficult objects to study turn out to be the most interesting objects from which new discoveries are made. Rather than a failure, a student's difficulty could represent a success at finding something particularly unusual or interesting!