Astr212 Galaxy Projects, Spring 2005

Dwarf Elliptical Galaxy NGC4436, Anthony Bouvette

NGC 4436 (center) is located in the Virgo cluster, approximately 18 Megaparces (Mpc) away, or about 58.7 million lightyears, and is classified as a dwarf elliptical. Despite their name, dwarf ellipticals are not fainter versions of true elliptical galaxies, but are structurally distinct. A typical dE has mass of about one billion of our suns, which is about one thousandth that of a typical elliptical galaxy. They contain very little or no gas, making them different from dwarf irregular galaxies which typically contain lots of gas. Hundreds of similar galaxies to NGC 4436 can be found in the Virgo cluster and even more in the large Coma cluster, making them the most common type of galaxy in the universe. Dwarf ellipticals are of special interest because they pose a striking similarity to the galaxy fragments from which larger galaxies are thought to have formed.

Contents of the Image:

In this image, we see three galaxies. From left to right; the barred spiral NGC 4440, the dwarf elliptical NGC 4436, and the SO galaxy NGC 4431. Of primary interest is the dwarf elliptical NGC 4436.
By definition, an elliptical galaxy has no disk, but is instead an enormous mass of stars, each with its own particular path, locked in orbit around the central mass of the galaxy. Dwarf ellipticals consist of old population 2 stars which are more red in color than younger , hotter stars, which appear more blue in colour. This is because Dwarf ellipticals contain little or no interstellar medium (ISM) from which new stars are formed. This can been seen towards the center of NGC 4436, characterized by the galaxy's reddish hue.

Multi-Wavelength Comparison:

In this image, courtesy of the NASA/IPAC Extragalactic Database (NED), we see the dwarf elliptical NGC 4436 up close and in the near-infrared spectrum (1.11 - 2.32 microns). The image we see is produced from the light given off by the F, K, and M class stars that populate the galaxy. As there is little or no ISM or dust present in dwarf ellipticals like NGC 4436, the light produced by these cooler stars (around 2000 Kelvin) is not obscured or extinguished on its way to us here on earth.

Further Investigation:

Further investigation of NGC 4436 and other dwarf ellipticals in the Virgo constellation conducted by van Zee shows a large but smooth gradient in plots of many of the galaxies' rotation curves. Such evidence supports theories that dwarf ellipticals are evolved from dwarf irregulars since similar rotation curves are often found in galaxies with recent and continuing star formation due to high dust and gas concentrations. Because dwarf ellipticals and dwarf irregulars are so similar in many of there other traits such as surface brightness and size, it seems possible that after star formation has stopped and the all the hot, high-mass stars have burnt out, the blotchy appearance of a dwarf irregular would change to something more along the lines of what we observe as dwarf ellipticals.

Light Profile:

The light profile of NGC 4436 is shown below. A light profile is a plot of the brightness starting at the center and moving outward along a given radius. This is important to characterize because for an elliptical galaxy, the light profile can be used to determine the mass enclosed in an effective radius. The effective radius (re) is of special interest because inside that radius, half the total brightness given off by the galaxy is emitted. The equation below shows a relationship for the brightness and radius.

Two characteristics of a dwarf elliptical galaxy light profile are:

• The brightness decreases with increasing radius, as can be seen in the plot below.

• The log of the brightness is directly proportional to the radius to the 1/4th power, seen below.

By performing similar calculations on the minor axis and then comparing the results to those of the major axis, the ellipticity number of the galaxy can be found. Ellipticity is a measure of how elongated an elliptical galaxy is. The standard scale used by astronomers ranges from E0 to E7, with the higher numbers representing a larger elongation. NGC 4436 is classified as an dE6 galaxy.

References:

Elmegreen, D.M. 1998, Galaxies and Galactic Structure (New Jersey: Prentice Hall)

Jarrett, T. H; Chester, T.; Cutri, R.; Schneider, S. E.; Huchra, J. P."The 2MASS Large Galaxy Atlas", 2003, Astronomical Journal, 125, 525

Kutner, M. L. 2003, Astronomy: A Physical Perspective, 2nd ed. (Cambridge: Cambridge University Press)

van Zee, L.; Skillman, E.; Haynes, M., " Rotationally Supported Virgo Cluster Dwarf Elliptical Galaxies: Stripped Dwarf Irregular Galaxies?", 2004, The Astronomical Journal, 128, 121

This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.