Viewing the 21 cm sky: A slice of the neutral hydrogen universe

• Main Author: Spitzak, John Gerard
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1996
• Subjects: Astronomy|Astrophysics
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9639031

Among the most fundamental problems in extragalactic astronomy are understanding the processes which led to galaxy formation and establishing the existence and nature of "dark matter" in the Universe. Before any attempt can be made to solve either of these problems, an accurate and thorough knowledge of the true population of galaxies is required. Unfortunately, there is mounting evidence that our knowledge of extragalactic space may be critically incomplete. This deficiency is due primarily to our dependence on galaxy statistics that are largely derived from observations at optical wavelengths. These statistics may include only a sub-population of extragalactic objects which have properties easily detected in that narrow wavelength range, rather than the true population which may have a more diverse range of properties. To steer ourselves away from this possible optical bias, and thus to obtain a more well-rounded census of galaxy sizes, types, and locations, we have conducted a large-scale, unbiased survey for atomic hydrogen (HI) at 21cm. Because the strength of 21cm emission does not depend on the same forces which drive optical emission, this survey allows us to not only augment the compiled database of galaxies by describing the properties of previously cataloged objects we detect, but more importantly search for new types of objects which have historically remained undetected due to optical biases. Using the NAIC Arecibo Telescope, we have systematically searched a "slice" of extragalactic space known to contain 48 cataloged galaxies. Our survey has re-detected 38 by their 21cm emission, and has failed to detect 10. We have also detected an additional 41 previously unknown objects. We present atomic hydrogen and optical data for all objects and demonstrate that the newly discovered galaxies represent a population which differs distinctly from the cataloged galaxies in having lower overall luminosities and masses, larger relative HI gas contents, and bluer colors. In addition we show that several extremely low-mass, low luminosity galaxies discovered within 5 Mpc of the Milky Way imply the presence of thousands of similar objects throughout the slice search region. These low-mass objects could, in their great numbers, represent a significant fraction of the total integrated mass of all galaxies in the region. As such, they could have a profound influence on the distribution and evolution processes of all extragalactic objects.