Continuing the ALFALFA Survey:
Finding Masses of Neutral Hydrogen Sources with HIMassCalc
Tristan Wolfe '08

Abstract:

   I have contributed to the ongoing Arecibo Legacy Fast ALFA Survey (ALFALFA). This is a blind survey using the Arecibo Radio Telescope’s L-Band Feed Array (ALFA, a new receiver) seeking clouds of neutral hydrogen (HI) over a 7000 square degree portion of sky. HI is detected with ALFA by its emission of energy at a wavelength of 21 cm in the L-band of radio frequencies at 1420 MHz. I worked with a cube of data from ALFALFA; a 2.4 square degree section of sky centered about 11^h48^m +13°, in the constellation Leo across a frequency space spanning 1335 to 1435 MHz in 4096 5-MHz channels. The frequencies correspond to a Doppler-shifted velocity range of –1,600 km/s to 18,000 km/s. The negative range includes approaching (blue-shifted) sources such as clouds of HI within our galaxy. Taking the velocities of the receding (red-shifted) sources as due to the expansion of the universe gives a corresponding distance range out to 850 million light years. My project was the construction of a computer programs called HIMassCalc, used to calculate the total flux density of these sources, or their rates of energy flow through given areas of space, from graphs of their spectral flux densities (flux density per channel of frequency space). The total flux was given by the area under the curve of the spectral flux graphs, a summation of the spectral flux values multiplied by the number of frequency channels within the appropriate range. The program then calculated the masses from the total flux density. The resulting masses were compared to expected values of HI masses in certain sources, as well as stellar masses given by optical counterparts of some of the sources.

   For more information, contact Dr. Aileen O'Donoghue:

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