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HI Line Profiles of Galaxies Claremont Colleges Scholarship @ Claremont Pomona Senior Theses Pomona Student Scholarship 2008 H I Line Profiles of Galaxies: Tilted Ring Models Erica Nelson Pomona College Recommended Citation Nelson, Erica, "H I Line Profiles of Galaxies: Tilted Ring Models" (2008). Pomona Senior Theses. Paper 27. http://scholarship.claremont.edu/pomona_theses/27 This Open Access Senior Thesis is brought to you for free and open access by the Pomona Student Scholarship at Scholarship @ Claremont. It has been accepted for inclusion in Pomona Senior Theses by an authorized administrator of Scholarship @ Claremont. For more information, please contact [email protected]. H I Line Profiles of Galaxies: Tilted Ring Models Erica Nelson May 8, 2008 Abstract Two-dimensional information on the kinematics and spatial distribution of gas in spiral galaxies is encoded in radio observations of their one-dimensional 21-cm neutral hydrogen (HI) line profiles. More than ten thousand HI pro- files have been published and are publicly available. In order to explore the parameter space mapped out by the 21-cm neutral hydrogen line profile, we have modified and run a FORTRAN-based computer simulation code. We have identified 7 control parameters that define the morphology of the mod- elled galaxy: they describe the neutral hydrogen gas distribution (density and spatial location of the gas), characteristics of its rotation curve, warps, asym- metries, and finally, the viewing angle. All except the last of these parameters tell us significant physical information about the galaxy but a determination of them is not immediately apparent from the two-dimensional 21-cm line profile. Hence, the goal of this exploration is to find meaningful correlations between the observed 21-cm line profile features and the underlying physical parameters. Contents 1 Introduction 4 1.1 Introduction . 4 1.2 Neutral Hydrogen . 7 1.3 Velocity Fields . 12 1.4 21-cm Line Observations . 13 1.5 Modeling and Classification . 16 2 The Model 20 2.1 Viewing Angle . 22 2.2 Density Profile . 24 2.3 Rotation Curve . 27 3 Classification 32 3.1 Introduction . 32 3.2 Visual Classification: HPST . 33 3.3 Quantitative Classification: Kurtosis . 38 3.4 Classifying with Kurtosis . 41 3.5 Results of Kurtosis Classification . 47 3.5.1 Kurtosis . 49 3.5.2 SNR Effects . 49 4 Data 56 4.1 Modeling Individual Parameters . 57 4.2 The Parameter Cube . 61 5 Analysis 64 5.1 Viewing Angle . 65 5.2 Maximum Rotational Velocity . 67 2 5.3 HI Density . 69 5.4 Adding up the Annuli . 71 5.5 Velocity Turnover Radius . 71 5.6 Density Profile Width . 76 5.7 A Central Gas Depression . 77 5.7.1 Secondary Density Gaussian Height . 77 5.7.2 Secondary Density Gaussian Width . 80 5.8 The Changing Parameter Cube . 91 6 Conclusion 98 6.1 Findings . 98 6.2 Future Work . 101 3 Chapter 1 Introduction 1.1 Introduction From an indicator of potential star forming regions to a tracer of galactic interaction history for use in redshift surveys, extragalactic neutral hydrogen as measured by the 21-cm emission line has a rich history of scientific use in astronomy. It has been studied extensively using single-dish radio telescopes, which yield an integrated line flux profile as a function of heliocentric velocity of an unresolved galaxy in the beam. For the wealth of published profiles, we introduce a classification scheme based HI line profile morphology and a model to translate between profile shape and physical properties of the contributing galaxy. Until the early 20th century, the word galaxy was synonymous with the word universe (Kauffman, 2002). The extent of the known universe encom- passed 1 billion stars and extended light sources then thought to be stars in early stages of formation. However, in 1923 Edwin Hubble established the ex- 4 istence of an external galaxy by finding the distance to M31 (Hubble, 1929), launching the study of the universe through an important structural build- ing block galaxies. While stellar structure and evolution and cosmological structure and evolution are somewhat well understood, our understanding of the structural properties and evolution of galaxies lags behind, creating a strange gap in knowledge in the midrange of astrophysical sizes. Galaxies are comprised of stars, dust, gas, and dark matter and are classi- fied by their optical morphologies roughly in two types: elliptical galaxies and spiral galaxies. Elliptical galaxies are old, gas and dust-free, star-dominated, and elliptically-shaped. Spiral galaxies morphologically take the form of a disk-like structure with a centrally located bulge. The highly flattened spiral galaxy is an organized structure in which stars, gas, and dust move in circular orbit around the center of the galaxy (Kauffman, 2002). Characteristically younger, the disks of these galaxies are rich in gas and dust and have active star formation processes (figure 1.1). The composition of spiral galaxies varies, but we can say a few things about the relative abundances of the various components from observation. The overall baryonic mass is dominated by stars with the ratio of gas to total baryonic mass < Mgas=Mtotal > ranging from 0.04 to 0.25 as shown by anal- yses of neutral hydrogen HI, ionized hydrogen (HII), and carbon monoxide (CO) emission (which is a good tracer of molecular hydrogen H2) (Carol & Ostilie, 1996). Gas constitutes a greater fraction of the total mass in galax- ies with low bulge-to-disk ratios, while stellar mass dominates to a greater 5 Figure 1.1: Sample Spiral Gemini and Elliptical galaxies. (Spiral galaxy M74 - courtesy of the Gemini project, Elliptical galaxy M87 - courtesy of the Anglo-Austraian Telescope. image credit:www.galex.caltech.edu/ SCI- ENCE/galaxies.html) extent in galaxies with high bulge-to-disk ratios. Spiral galaxies have a rich variety of spiral structure in terms of the number of spiral arms and how tightly these arms are wound. Structurally they range from grand design spirals with two arms, to multi-armed spirals, to flocculent spirals with indistinguishable arms. These arms are dominated by O- and B- type main sequence stars and HII regions, with dust and HI regions more prevalent on the inner edges of the arms. Spiral structure is due to density waves in which a spiral arm has a mass density that is greater than the average mass density of the galaxy by 10-20% (Carol and Ostilie, 1996). 6 1.2 Neutral Hydrogen In the following introductory remarks, I have relied heavily on the com- prehensive review by Riccardo Giovanelli and Martha Haynes. Readers are referred to that article for further details and references therein. Galaxies can be observed in a vast range of different wavelengths corresponding to different galactic components with different associated energetic processes. One of these is 21-cm line emission from the hyperfine energy transition in neutral hydrogen gas (HI). This is the magnetic dipole transition between the two ground state energy levels of the hydrogen atom. The hyperfine splitting of energy levels in the ground state comes from the electronic and nuclear spin vectors being parallel or anti-parallel. If they are parallel, the atom is in a slightly higher energy triplet state; if they are anti-parallel, the atom is in a slightly lower energy singlet state. When an atom makes the very small (or rather hyperfine) energy transition between these two energy levels, it emits a low-energy photon that has a wavelength of 21-cm (see Figure 1.2). The 21-cm line conveys a great deal of information about a galaxy as tracer of composition and evolutionary history. Neutral hydrogen clouds play an important role in stellar evolution as the starting point for collapse of matter into stars. Neutral hydrogen is the first phase of star formation before the gas congeals into denser molecular hydrogen and collapses to start stellar nuclear burning. As an indicator, abundant HI then signals poten- tially active star formation processes, while a lack of HI is a guarantee of a 7 Figure 1.2: (image credit: www.ts.infn.it/physics/ experiments/qcl/) barren galaxy with an old stellar population. (Giovanelli & Haynes, 1988) Therefore, neutral hydrogen is of primary importance in understanding stel- lar population evolution of a galaxy. HI is also useful as an indicator of galactic evolutionary history in terms of interactions. HI is vulnerable to environmentally driven gas removal mech- anisms and can therefore be used to probe the effect of the environment surrounding a galaxy on its development. The fragile outer layers of HI are easily disturbed by close encounters with other galaxies. This means HI can be used to probe the nature of galactic interactions, which are essential to a complete picture of galaxy formation and evolution (Giovanelli & Haynes, 1988). While HI is an important part of the evolutionary history of a galaxy, its dynamical role is somewhat less important as a result of its small mass 8 Figure 1.3: Radio and optical composite image of M51 interacting with its companion. The tidally disrupted HI is shown in blue. Image courtesy of NRAO/AUI fraction in most galaxies. The optical surface brightness of the disk in a spiral galaxy falls off exponentially with distance from the center of the galaxy, resulting in half the spatially integrated luminosity being contained in a 4-5 kpc radius. Similarly, most of the interstellar gas lies in the plane of the disk and its density falls off exponentially with a similar effective radius. Within 11 this optical radius, HI constitutes 10% of the total mass of 1:5x10 Msolar. Considering galactic composition out to 100 kpc, the dynamic mass increases to 1012, making the HI mass fraction only 1% of the total.
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