Supporting Figures for Stephan's Quintet HST Image

Collaborators: Sarah Gallagher, Jane Charlton, Sally Hunsberger, Dennis Zaritsky, & Jayanne English

R-band image of Stephan's Quintet taken with the 1.5-m telescope at Palomar Observatory. The regions of interest have been labeled; north is up and east is to the left. The members of Hickson's Compact Group 92 (a.k.a. Stephan's Quintet) have been labeled in addition to NGC 7320C and specific regions of interest. NGC 7320C possibly passed through Stephan's Quintet approximately 1 billion years ago, pulling out the long tidal tail from NGC 7319 in the process. NGC 7318B has a velocity almost 1000 km/s less than the rest of the group. The high speed collision of this galaxy with the rest of the group probably triggered the active star formation in the Northern Starburst Region and along the Eastern Spiral Arm. With the high spatial resolution of the Hubble Space Telescope, young star clusters (less than 10 million years old) can be found all along the impact region which spans approximately 40 kiloparsecs (approximately 130,000 light years). Note that 7320 is a foreground galaxy which is not physically related to the rest of Stephan's Quintet. The 3.7 arcmin by 2.5 arcmin region imaged with WFPC2 in two separate pointings is outlined by the white box.

Image adapted from Hunsberger, Charlton & Zaritsky 1996. Higher Resolution:gif jpg

Composite WFPC2 V-band image of Stephan's Quintet with the point sources indicated with yellow circles. The display scale is logarithmic, and the pixels have been binned by a factor of two in x and y. These point sources were found in all B, V, and I images and have color errors less than 0.2 mag. The regions corresponding to the symbols in the color-color plot have been labeled. Note that the immediate vicinity of the foreground galaxy NGC 7320 was excluded from the source searching.

The coincidence of many of the point sources with the giant galaxies and their tidal features strongly suggests that these are star clusters - groups of hundreds of thousands to millions of stars all born at the same time and gravitationally bound together.
As a star cluster ages, the hottest, bluest stars die, and the color of the entire cluster becomes redder. Therefore, the redder a cluster is, the older it is, and the amount of light emitted by a star cluster in different color bands can indicate its age. The youngest, bluest star clusters are in the upper left of the plot. The black line in this figure represents the evolutionary path in "color space" of a star cluster as it ages. Labels along the track indicate the logarithm of the age in years. The symbols, as indicated in the legend, represent the physical regions as defined in the previous figure. The red arrow indicates the effect of dust on the colors of the clusters - a cluster embedded in a dusty cloud will appear redder (and therefore older) than it actually is. The objects in the lower right of the figure are likely to be stars in our Galaxy.

Stephan's Quintet has hosted active star formation for billions of years. This formation appears to occur in bursts triggered by dynamical interactions between giant galaxies, some of which may no longer be part of the current group of galaxies. There is a population of very young (approximately 2 million year old) star clusters found in both the Northern Starburst Region and along the Eastern Spiral Arm of NGC 7318B. Star clusters of ages from less than 100 million up to 1 billion years old are found in the Tidal Tail region of NGC 7319. Since the Tail itself is thought to be about than 200 million years old, this suggests that the star clusters were born 30 kiloparsecs (100,000 light years) from the galaxy that originally hosted the gas from which they are formed.
A spectrum of several clumps in the Eastern Spiral Arm of NGC7318B obtained with the Marcario Low-Resolution Spectrograph on the Hobby-Eberly Telescope (HET). The HET is a 9 meter telescope, owned and operated by an international collaboration between The Pennsylvania State University, the University of Texas at Austin, Stanford University, Ludwig-Maximilians-Universität, and Georg-August-Universität. The parallel white lines in the lower left panel shows the region of the Eastern Spiral Arm for which the spectrum was obtained. The largest concentration in the middle of this "slit" produces the strong continuum running horizontally along the full spectrum at the top of this figure, and also gives emission lines due to specific chemical transitions. From left to right the prominent emission lines are H-beta, the [OIII] doublet, H-alpha, [NII], and the [SII] doublet.The different tidal debris clumps along the slit each produce a different spectrum as noted by the white arrows connecting the HST image on the left to the "zoom-in" on the region of H-alpha in the HET spectrum to the right. The fact that the emission line patterns are at different horizontal positions signifies that the different regions are moving at different velocities relative to one another. The velocities (redshifts) are noted on the spectrum. The debris at the lower part of the slit is at a velocity that matches most of the galaxies in the group (NGC7319, NGC7318A, and NGC7317), while that at the top of the slit is at the velocity of the high velocity perturber, NGC7318B. This is clearly a highly disturbed and complex region which even by itself provides a record of the history of the group.