Spiral Galaxies

Contents

Introduction

Spiral galaxies are a subset of galaxies, tracing the late stages of the Hubble classification. Spiral Galaxies are defined by their thin disks, thick central bulges, the presence of spiral arms, and blue colors.

Characteristics of Spiral Galaxies:

• Spiral galaxies constitute ¾ of the total population of galaxies in the field. This fraction changes both with redshift and the local environment density. (see Morphology-Density Relation)
• Typical lengths: ~ 1-50 kpc.
• Masses: 10⁹ – 10¹² solar masses.
• Star formation rates: ~1-5 solar masses/year.
• M_V ~ -16 to -23.
• Peak rotational velocities: ~ 150-300 km/s.
• They exhibit both old and young stellar populations.
• They are composed of both dynamically cold and hot stars. The former follow elliptical or near-circular orbits in the disk with little random motions, while the latter follow more chaotic orbits with high dispersions in the bulge.
• Most spiral arms trail the galaxy's direction of rotation.
• More luminous galaxies have higher rotational velocities; this is the basis of the Tully-Fischer relation.

Basic Components of Spiral Galaxies:

Disk: The disk contains metal-rich stars and ample interstellar medium (ISM); disk components are generally dynamically cold. Disks are also sites of spiral arms and their triggered star formation regions. Consequently, spiral arms are especially prominent when viewed in blue and ultraviolet light, which characteristic of the arms' younger, luminous stellar populations.

Bulge: The central bulge is composed of metal-poor to extremely metal-rich, dynamically hot stars.

Bar: These flat, linear structures of stars and ISM are present in about 50% of spiral galaxies. They tend to be as thick as the disk, and have length/width ratios on the order of 5:1. Additionally, bar endpoints can be low-shear environments where triggered star formation may also occur.

Nucleus: Nuclei are the innermost, most dense regions of spiral galaxies; they are thought to contain supermassive black holes or intense starbursting regions at their very centers.

Halo: The low density environment surrounding the galactic disk, halos contain metal-poor stars, globular clusters, and hot, low density ISM.

Dark Matter Halo: The dark halo extends far beyond the visible extent of the galaxy, providing most of the galaxy's mass and controlling its dynamics. We have yet to understand the fundamental composition of this material.

Trends in Spiral Galaxies

While there are really no arms to speak of in the S0 systems, we notice that spiral arms tend to wrap themselves tighter as the Hubble classification proceeds from Sa to Sm. This is no surprise; Hubble based his classification scheme on this phenomenon. Furthermore, systems with an abundance of gas are more likely to develop the flocculent structures (as in NGC 2841) resulting from previous generations of supernovae pushing bubbles of star-forming material outward into the ISM for the next cycle of star birth. Otherwise, the trend compliments a generally increasing supply of gas and dust from Sa to Sm.

Moreover, galaxies tend to get bluer and fainter as they progress from early type to late type spiral. This has much to do with mass and stellar composition; early types are the most massive, showing absorption lines indicative of cooler K and M stars, while the late types are smaller, and display H and K lines of ionized Calcium, among other spectral and photometric indicators of young, blue stars, especially in the active spiral regions.

Rotation Curves and Dark Matter

Thanks to the Virial Theorem, if we can measure an object’s rotational velocity while knowing its radius, we have a measure of the mass internal to that particular radius. In the case of spiral galaxies, we find that this velocity is constant (past a certain distance) as the radius of the galaxy increases, so the mass internal to that radius must also increase. This inferred mass exceeds what we directly observe by as much as an order of magnitude in some galaxies. This unobserved, yet necessary, mass may be provided by dark matter.

In practice, astronomers obtain a rotation curve by either observing the motions of HI (neutral hydrogen) or CO (carbon monoxide) gas at their characteristic radio emission lines out to a galaxy's edge. We can model a so-called spider diagram, whose velocity contours are close to those measured in HI or CO and thereby obtain a galaxy's velocity as a function of radius as well as its inclination. This provides a measure of the total mass of the system. 

References

• "Galaxies - 2: Spirals" by Gronwall & Caryl. Penn State Astronomy and Astrophysics, State College, PA. 2008.
• "Galaxies in the Universe" by Sparke, Linda, S., Gallagher & John S. Cambridge University Press, New York, NY, 379 pp. 2005.
• "Normal Galaxies - The Large-Scale Structure of the Universe,"  James Brau, Knight Professor of Natural Science, Ph.D., Department of Physics, University of Oregon.
• "The Hubble Tuning Fork — Classification of Galaxies" from "Starry Bulges Yield Secrets to Galaxy Growth," HubbleSite News Release Number: STScI-1999-34, October 6, 1999.

Related EoC Articles

• Hubble Classification - Hubble Tuning Fork Diagram
• Spiral Arm Structure

External Links

• Edwin Powell Hubble (1889-1953) – Hubble Space Telescope, NASA.
• "Galactic Astronomy" by Binney, James, and Merrifield, Michael, 1998. Princeton University Press, Princeton, NJ, 796 pp.
• Hubblesite. 2008

Preview Image

"Barred Spiral Galaxy NGC 1672" -  Many spiral galaxies have bars across their centers. Even our own Milky Way Galaxy is thought to have a modest central bar. Prominently barred spiral galaxy NGC 1672, pictured above, was captured in spectacular detail in this recently released image taken by the orbiting Hubble Space Telescope. Visible are dark filamentary dust lanes, young clusters of bright blue stars, red emission nebulas of glowing hydrogen gas, a long bright bar of stars across the center, and a bright active nucleus that likely houses a supermassive black hole. Light takes about 60 million years to reach us from NGC 1672, which spans about 75,000 light years across. NGC 1672, which appears toward the constellation of the Swordfish (Dorado), is being studied to find out how a spiral bar contributes to star formation in a galaxy's central regions. View full-size image.  (Source: NASA/ESA/Hubble Heritage (STScI/AURA). Image Credit: L. Jenkins (GSFC/U. Leicester).)