Introduction to Science in the Galactic Plane
Lesson 2: Object Identification
This exercise will allow the instructor/teacher to present to his/her
students a lesson on measuring the intensity of the sources obtained
from the First Galactic Plane Survey; hereafter known as GPA. A
measurement of intensity obtained from this lesson is necessary to
ultimately measure the temperature of each source.
The First Galactic Plane Survey is an observation of the plane of the
Milky Way Galaxy taken simultaneously at two frequencies in the
microwave part of the radio spectrum. The frequencies used are 8.35
GHz and 14.35 GHz in the X- and Ku-bands respectively. Radio
intensity maps of the plane of the galaxy were made from the observing
data; one for each of the two frequencies observed.
The intensity of the various sources recorded by the survey show
variations at the two frequencies used. This variation in intensity
of the same object at different frequencies is a tool that can be used
to identify the type of object. A graph of an object that is a
supernova remnants will have a decrease in intensity as the frequency
increases; whereas, a graph of an object that has a relatively uniform
intensity at the two different frequencies is indicative of a thermal
object. A graph of intensity versus frequency for a thermal object
follows the classical Gaussian curve.
Thermal objects are regions of interstellar hydrogen gas that has been
ionized by the ultraviolet radiation from nearby stars. A cloud of
ionized hydrogen is called an H II region.
In this project, you will use SKY IMAGE PROCESSOR (SIP) to load and
analyze sets of images taken during the GPA survey. The analysis is
based on a careful measurement of the intensity of the same object at
the two survey frequencies.
Those objects that yield a reasonably constant intensity will be
classified as thermal objects. Those objects that yield a decreasing
intensity as frequency increases will be classified as supernova
remnants. Positional data for each identified object will then be
obtained. Comparison between your measured position and the true
position from astronomical tables will confirm your hypothesis.
DESCRIPTION OF THE DATA
This CDROM contains calibrated radio wavelength images of the Milky
Way (our galaxy). These images are obtained by the Green Bank Earth
Station, a 45 foot diameter parabolic reflector and radio receiver
operating in the microwave part of the radio spectrum. The receiver
frequencies are 8.35 GHz and 14.35 GHz. The CDROM contains FITS
images as well as software to analyze the images. FITS stands for
Flexible Image Transport System.
It is the standard format for recording and transmitting astronomical
images and data. The images contain information on what part of the
sky has been observed and what is the intensity of the radio signal
from that part of the sky.
To get the most out of this activity, students should have a basic
understanding of the following concepts:
loading image processing software
loading required FITS images
galactic coordinate system
Intensity vs frequency relationship
IMAGE PROCESSING INSTRUCTIONS
Recommendation: students work in groups of NO MORE than 3/group;
therefore, students are more individualy involved in the research.
The region of interest for the first data analysis is the Cygnus
region of the Milky Way. The Cygnus region is one of the spiral arms
of our galaxy and is rich in its variety of radio sources.
Review the instructions in Introduction to Image Processing.
Load Sky Image Processor (SIP) program.
Load FILE cyg-xcal.fit.
To access the Sky Image Processing program, connect to:
In your data table, record the following
Galactic coordinates of source;
Sum of pixels per box at each of the two observed frequencies;
DATA TABLE FOR GPA SURVEY OBJECT IDENTIFICATION EXERCISE
Construct a data table that contains the following headings:
GALACTIC COORDINATES: [LONGITUDE] [LATITUDE]
SUM OF PIXELS/BOX for both [8.35 GHz image] and [14.35 GHz image]
RATIO OF SUM OF PIXELS/BOX: [8.35 GHz/14.35 GHz]
MAX/BOX: [14.35 GHz]
INTENSITY: [14.35 GHz]
ANALYSIS TO BE PERFORMED BY STUDENTS USING SKY IMAGE PROCESSOR
The equation shown above will calculate the object flux of each source.
Variables used from [SIP] are:
ObFl = object flux; related to source intensity
SumPixBox [freq] = sum of pixels/box at a given frequency
MaxBox [freq] = maximum in box at current frequency;
related to source brightness
ObFl=((SumPixBox @ 8.35 GHz)/(SumPixBox @ 14.35 GHz))*(MaxBox)
From your data of the Cygnus X3 region, what is the number of thermal
From your data of the Cygnus region, what is the number of supernova
In an area of 10 by 10 degrees centered on Cygnus X3, use your data
of (1) and (2) above to calculate the number of thermal and
non-thermal sources per unit area.
Do this calculation for both frequencies.
This calculation shows the distribution of sources in the Cygnus arm.
Create a graph with INTENSITY on the y-axis and FREQUENCY on the
x-axis. Use this graph to plot your intensity vs frequency data for
each source. Use a scheme of different colors or different shapes to
identify each source.
Print a map (hard-copy) of the 10 by 10 degree region of Cygnus X3.
On your map, use your data table to locate and record the type of
object: thermal or supernova remnant. You have now identified certain
characteristics of the Cygnus arm of the Milky Way.
Does your graph of INTENSITY vs FREQUENCY agree with theory?
Explain your answer.
In your next exercise, you will calculate the angular size of
Galactic Plane Science