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A Hubble Source Catalog (HSC) Use Case

Example #8: Combining HSC magnitudes and HST spectra to Investigate Objects in the HSC

(Objects in the LMC Cluster R136)


GOAL: This tutorial shows you how to use the MAST Discovery Portal to make a Color-Magnitude Diagram, determine which objects have HST spectral data available, and get the details of the spectral observations.

SCIENCE CASE: The science case is investigating objects that have both HSC magnitudes and HST spectra (e.g. in the LMC Cluster R136). Finding charts for the objects can be found in Massey and Hunter 1998, ApJ, 493, 180).

NOTE: For more information on spectroscopic matching (e.g. which spectroscopic observations were matched, how was the matching performed), see the FAQ (link).

Step 1 - Search the HSC V2 for objects in R136

Go to the MAST Discovery Portal. Use the pull down menu under Select a Collection to choose the HSC (blue). Enter (you can just cut and paste if you like) the coordinates and search radius (i.e. 05:38:42.3 -69:06:03.1 r=50s) in the Search box (green). Perform the search by hitting a carriage return or by clicking on the icon (orange). The results are displayed in the List of Objects, while the AstroView window shows the objects against the DSS image. The left column is a series of Filters that can be used to refine the data selected. Note the third item has the icon which indicates there are spectroscopic observations of that object.

At this point, you may want to look directly at the HST data. This can be done by clicking on the Load Detailed Results icon (yellow) under Actions, and then clicking on the Toggle Overlay Image icon (see Use Case 1 for details).

Note that you can use the HLA Interactive Display to get a better view of the field (e.g. by changing the contrast level), which will allow you to better identify specific sources. After clicking on the Load Detailed Results icon, click on the Preview image.

Step 2 - Make a Color-Magnitude Diagram (CMD) of the Cluster.

Make a plot by clicking the icon (blue); remember to return to the Home Page (i.e., HSC: 05:38 ...) if you placed an HST image in AstroView. Under Configuration, make a V-R (A_F555W-A_F775W) column by using the Combine Columns option (green).

Next, select X = V-R and Y = A_F555W (orange). Click on the Update Series button (yellow) to plot the CMD. Reverse the Y axis to have magnitudes go from bright to faint (purple).

Step 3 - Mark objects with spectra in the AstroView

In the Filters area, look under the Spectrum Flag to see that there are 62 objects with spectra (blue). Click on "Y" to filter on those objects (green). Zoom out in Astroview to see all the objects. Note that the icon in the Actions area (orange) indicate spectral data is available (which we will examine in Step 5).

Step 4 - Show the objects with spectra in the Color-Magnitude Diagram

Click on the Edit Columns box to select all these objects in the List of Objects (purple in the previous figure). Deselect the Spectrum Flag in the Filters area (so that all objects will appear in the CMD) and hit Update Series and reset the plotting range. The objects with spectra are now highlighted.

Step 5 - Examine individual objects

Start by expanding the plot to only show the region containing stars with spectra (blue). Select an object of interest (green), deselect it and reselect it in the plot.

Go back to the search results page and see that the object is now at the bottom of the list. Use the bullseye (blue) to center the object in Astroview ((green).

To examime the spectrum of the object, click on the icon (orange in Step 3).

See the FAQ (link) for the column definitions. Note that the target name is given in the Tab name (blue).

Clicking on the icon (green) will bring up an interactive spectrum viewer.

You can change the Wavelength (blue). and Flux (green) scales under Range in the Configuration menu.

Under Options, you can smooth the spectrum (blue).

Under Fitting, you can measure the equivalent width of a feature. First, click on the Enable Fitting (blue) button (which changes to Disable Fitting), and then place your cursor on the left then the right of the line. The center of the line is marked in the plot, and the results of the fitting are shown (green). If you are measuring an extragalactic object, you can put the rest wavelength in the Rest Wavelength box (orange), and the redshift will be calculated based on the measured value. To save the measurements, you can highlight the results with your cursor and use the copy and paste function into a separate file.

For more information on using the Spectral Viewer, see MAST Discovery Portal Spectral Viewer help.