Absorption Lines and Galaxies Toward PKS0312-330
Advisor: Chris Churchill (New Mexico State U.)
Collaborators: Chip Kobulnicky (U. of Wyoming)
Jason Prochaska (U. of California, Santa Cruz)
Nicolas Lehner (U. of Wisconsin)
Gerard Williger (Johns Hopkins U.)
James Lowenthal (Smith College)
Glenn Kacprzak (New Mexico State U.)
| Quasars are valuable tools
in studying the evolution of galaxies. They
were discovered when astronomers began looking at sources of radio
waves through telescopes. They found bright stellar-looking objects and
called them "Quasi-Stellar Radio Sources" or "quasars" for short.
Quasars are found at the center of galaxies and are powered by black
holes. The quasar PKS0312-770 is a radio loud quasar that can be seen if we look towards the space called the Magellanic Bridge, which is the gaseous extension between the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). PKS0312-770 has a redshift of z=0.223 which means it is about 3 billion light years away from us. We used this quasar as a probe of the objects between us and the quasar. I have written a brief introduction to astronomy that you might want to visit before I get into the details. Astronomy 101 |
 |
This project began with taking a
spectrum of the quasar using the Hubble Space Telescope’s Space
Telescope Imaging Spectrograph (STIS). A quasar’s spectrum is
what astronomers use to learn details about the quasar itself and the
objects between us and the quasar. A typical quasar spectrum
looks like the following:
z = (Observed wavelength - Rest wavelength)
(Rest wavelength)
We also found a HST (Wide Field Planetary Camera) WFPC-2 image of PKS0312-770 and a galaxy next to it, which we have named G216. We used this image to model this galaxy.
Wavelength
is along the horizontal axis given in units of the Angstrom, which is
1/10000000000th of a meter. Flux, or the amount of energy output
each little unit of space on the quasar gives out per wavelength, is on
the vertical axis. The “peaks” in the line are emission lines and
the “dips” are absorption lines. Each one of the absorption and
emission lines corresponds to a particular element. Each quasar
spectrum has a strong hydrogen emission line (labeled Lya emission in
the picture above). Since each element has its own
characteristic wavelength, we know that that particular line should
show up at 1215Å. But if you look at the picture above, the
line is shift to the right, i.e. the center of the line falls at about
2850Å. Astronomers call this shift a “redshift” and label
it with the letter z. Everything in the universe is expanding and
the wavelengths are getting longer, which is redder. It is this
redshift that can tell us how far away the object is from us and how
old it is because we know how fast light travels. We
calculate the redshift using the equation below:
z = (Observed wavelength - Rest wavelength)
(Rest wavelength)
All
of the absorption lines to the left of that big Lya peak are from gas
clouds between us and the quasar absorbing different elements.
Looking at the redshift of these absorption lines can tell us how far
away these clouds are as well.
The HST STIS spectrum of PKS0312-770 was taken using two different gratings (the E140 grating over range 1150Å-1700Å and the E230M grating over range 2300Å-3100Å) from March to October 2001. The resolution was very good at 6.5 km/s. The spectrum is given below:
The HST STIS spectrum of PKS0312-770 was taken using two different gratings (the E140 grating over range 1150Å-1700Å and the E230M grating over range 2300Å-3100Å) from March to October 2001. The resolution was very good at 6.5 km/s. The spectrum is given below:
For this spectrum, we fit the
continuum (thin red line), identified 5 sigma significant features
(marked with red ticks), measured the equivalent widths of these
features, and identified which lines these features were.
Jason Prochaska went to Las Campanas Observatory down in Chile to take a picture of the field surrounding our quasar (picture of the star field at the top of this page) to use to select which objects we wanted to get spectra from. He selected these targets within a certain radius of the quasar (10 arcminutes (1 degree = 60 arcminutes)) and within a certain brightness level (magnitude in the r band greater than about 19.5), which was determined by how faint the camera could go. He then made 5 metal slit masks to put on the telescope. Each slit on the mask was located at the position of the desired object. He then used the Wide Field Reimaging CCD (WFCCD) camera on the du Pont 2.5 m telescope at the same observatory to take spectra of all of these objects. Spectra were taken of 132 objects in the optical (visible) wavelengths of 3600Å to 7600Å with a resolution of 10Å (not too great).
I reduced these spectra using the astronomy software package entitled Image Reduction and Analysis Facility or IRAF. I extracted the data, subtracted the background and camera noise, calibrated it using He/Ne lamps, and ended up with spectra of each object. Three of the objects were too faint or too close to the edge of the slit to see, so we ended up with a total of 129 spectra.
Jason Prochaska went to Las Campanas Observatory down in Chile to take a picture of the field surrounding our quasar (picture of the star field at the top of this page) to use to select which objects we wanted to get spectra from. He selected these targets within a certain radius of the quasar (10 arcminutes (1 degree = 60 arcminutes)) and within a certain brightness level (magnitude in the r band greater than about 19.5), which was determined by how faint the camera could go. He then made 5 metal slit masks to put on the telescope. Each slit on the mask was located at the position of the desired object. He then used the Wide Field Reimaging CCD (WFCCD) camera on the du Pont 2.5 m telescope at the same observatory to take spectra of all of these objects. Spectra were taken of 132 objects in the optical (visible) wavelengths of 3600Å to 7600Å with a resolution of 10Å (not too great).
I reduced these spectra using the astronomy software package entitled Image Reduction and Analysis Facility or IRAF. I extracted the data, subtracted the background and camera noise, calibrated it using He/Ne lamps, and ended up with spectra of each object. Three of the objects were too faint or too close to the edge of the slit to see, so we ended up with a total of 129 spectra.
| Jason Prochaska used a Sloane Digital Sky Survey (SDSS) automated computer code called zfind to calculate the redshifts of each of the objects. I also independently calculated the redshifts using a program written in IDL (Interactive Data Language) by Chip Kobulnicky. His program plotted up the spectra and then plotted lines on top of them located at particular wavelengths. For example, he plotted lines that were located at the wavelengths for the H & K lines of Calcium, Hydrogen lines, and Oxygen lines. I could shift these lines over to particular redshifts until I found that they matched up with the emission and absorption lines seen. An example galaxy with the over plotted lines looked like (this particular galaxy had a redshift of z = 0.2026: |  |
We also found a HST (Wide Field Planetary Camera) WFPC-2 image of PKS0312-770 and a galaxy next to it, which we have named G216. We used this image to model this galaxy.
We
used these four data sets to look at the absorbing gas and galaxies
between us and the quasar. We also look at G216 in particular.
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last updated 11/5/05