|1.3||About galaxies: a brief introduction|
This catalog is a collection of digital images of 113 galaxies. These galaxies were 'photographed' using CCD devices and each galaxy was taken using two or three color filters. Altogether we have 257 pictures, which are available in three different file formats. Including the header files the catalog consists of more than 1000 files. Images of 31 galaxies were taken with the 1.5 meter telescope of the Palomar Observatory in 1991 in 3 passbands of the Thuan-Gunn system: g, r, and i. Images of the other 82 galaxies were taken with the 1.1 meter telescope of the Lowell Observatory in 1989 in 2 passbands (J and R) of the photometric system developed by Gullixon et al. At Palomar we used a camera with an 800x800 TI CCD, at Lowell the camera had an RCA 512x320 CCD.
This User Guide will give you all the information on how the catalog was constructed, how to use these pages, and how to get the best image quality on your screen. Below are a few remarks about the HTML files. We also have a very short introduction to galaxies and galaxy classification. Please take a look at the acknowledgment where we have listed the many people who contributed their efforts to the construction of this catalog during the past years. In section 2 we describe the various file formats selected for this catalog and list image display programs for different computer platforms. Also described is the algorithm we used to convert the images to the various file formats.
In section 3 of this User Guide we summarize the observations and image processing steps. This chapter is a shorter version of the paper we published about this catalog in the Astronomical Journal. The full paper is also available in HTML, PDF, and PostScript formats. Many properties (like the Hubble type, size, brightness, position on the sky) of the catalog galaxies are listed there. The last section is a description of how we constructed the color images based on the images taken with different color filters.
Starting from the first page of the catalog you have two options to select the galaxy you want to view. You can either look at a set of thumbnail-size images and select the galaxy by clicking on the image, or you can select the desired galaxy based on its New Galaxy Catalog (NGC) number. After you make this selection you will be presented with a screen that describes the galaxy and allows you to select the desired file format and to pick the passband of your choice. Clicking on the small image corresponding to the file format and filter you chose will bring up the full-size image of the galaxy.
Due to the wide-spread use of web browsers, there is no need for extensive description of how to handle these pages. We want to make only three important points, specific to the data presented in this catalog.
Most important, the catalog is a collection of high-quality images, both in browser-supported JPEG and GIF formats, and also in FITS, which are not viewable within web-browsers. Depending on how the browser allocates the color-cube when it is started (this usually depends on the color depth of your setup and the number of colors still available for allocation), you may have more or fewer colors available to render JPEG and GIF images within the browser. If the number of colors is low, some of the image display programs we listed in section 2 of this User Guide will give you better results with JPEG and GIF than your browser does.
After you select a galaxy - either by browsing through the thumbnail images or the NGC numbers - you will be presented with a page that describes the galaxy and gives you options of file formats and filters before actually viewing the full-size image. Obviously, JPEG, GIF, and FITS images will have different file sizes (JPEGs being the smallest and FITSs being the largest). To help you select the format you find optimal for your needs and network connection speed, the latest versions of the most popular browsers will display the file sizes if you move your cursor over the postage-stamp-sized preview images.
The CD-ROM version of the catalog holds all the data that is part of the catalog itself (HTML files, galaxy pictures, etc). You do not have to be connected to the web to take advantage of our data. However, throughout these pages, especially in the User Guide, we linked many pages found on the World Wide Web, which provide supplementary information. You do not need these to use the catalog, but they can provide useful background information. Obviously, by using the CD-ROM version of this catalog you will be able to follow the links to the web only if you are connected to the Internet.
The existence of galaxies is clearly evident today, but it was not evident at all at the beginning of this century. Even a couple of hundred years ago the only celestial objects known to us were point-like sources: planets, satellites, comets (as well as the Sun and the Moon) in the Solar System, and relatively steady, distant stars.
While working at an Observatory in Paris in 1758, looking for Halley's comet, Charles Messier first discovered extended, faint objects in the sky. He called these nebulae, and started to catalog them to aid observations of comets, his main interest at that time. The first entry in his catalog, M1, turned out to be what we call today the Crab nebula, the remnant of supernova 1054. When he published his famous catalog in 1781, he listed 103 nebulae. Seven more objects were added to his catalog later.
The nature of the nebulae was constantly questioned, primarily because telescope resolution was not good enough during the last century to see detail in these objects. In 1920, the National Academy of Sciences in Washington organized a debate for Harlow Shapley and Heber Curtis concerning the nature of the nebulae, or, more precisely, about "The Scale of the Universe." The two opposing theories were that (a) the Universe is rather small, containing nebulae that are nearby gas-clouds, and (b) the nebulae are very distant "island Universes." The debate was centered around three key issues: (1) how to measure the distance to these objects, (2) how to determine the material composition (gas or many-many stars), and (3) why is there a "zone of avoidance," a region of the sky where nebulae are not found.
A few years later, Edwin Hubble settled the issue. He isolated Cepheid variable stars in the Andromeda galaxy (which is M31) and measured the distance to these stars. The distance turned out to be much larger than the possible size of the Milky Way; therefore M31 must be a distant "island Universe," a spiral galaxy, similar to our own. Hubble started to take pictures of many galaxies and soon realised that they come in many shapes and sizes. In order to make sense of his findings he implemented a subjective method of morphological classification, based on the optical appearance of these systems.
The two distinct types of galaxies he discovered were ellipticals and spirals. Spiral galaxies are divided into two groups. Some are normal spirals, with a bulge in the center and spiral arms extending out from the central regions, and barred spirals where spiral arms originate from the tips of a straight bar, going through the center of the galaxy. He also introduced the classes of lenticular and irregular galaxies. Lenticulars are "between" ellipticals and spirals. They consist of a large bulge with a small flattened disk around them, without visible spiral structure. Irregular galaxies, which account for about 5 percent of the galaxies we observe nearby, are forming, colliding, dynamically disturbed systems that do not fit in any of the other three categories.
Hubble constructed a "tuning-fork" diagram to illustrate his classification system. He put elliptical (E) galaxies on the stem of the fork. Indices 0 to 7 subdivide this class. An E0 galaxy is a round-looking elliptical, while E7 is very elongated in appearance. Lenticulars are denoted S0 (or SB0, with a bar), and subdivided to classes 1, 2, and 3, based on dust absorption within their disks. Spirals are denoted S (normal) or SB (barred). To distinguish among spirals, he introduced a, b, and c classes. An Sa (or the barred version, SBa) has a large bulge and relatively weak, tightly wound spiral arms; on the other end, an Sc (or SBc) has a small bulge and strong, open spiral arms. Below is a simple illustration of the tuning-fork diagram using representative galaxies found in our catalog:
Since the first half of the twentieth century, tens of thousands of galaxies have been imaged with various kinds of instruments and telescopes. We saw many galaxies not fitting the simple classification system of Hubble, which led to a proliferation of extensions to his system (for example, that of De Vaucouleurs, who made the tuning fork three-dimensional with the distinction between s-shapes and rings, and added classes Sd and Im to the sequence), and, more recently, efforts to automate the classification task. You will find an immense amount of information on this subject in popular science books, introductory- to graduate-level textbooks, and in several well-constructed pages on the web.
Since we imaged the nearest, largest systems, 22 out of the 40 galaxies found in the Messier catalog are included in our collection (the rest of the nebulae in the Messier catalog turned out to be nearby gas and dust clouds, supernovae remnants, globular or open clusters of stars, or simply a not-well-resolved double star). Among the 22 galaxies we have 16 spirals (M58, M61, M63, M65, M81, M88, M90, M91, M95, M96, M98, M99, M100, M104, M106 and M108), 4 ellipticals (M49, M59, M87 and M105), and 2 lenticulars (M84 and M86).
It is our pleasure to acknowledge the great number of people who participated in the observations and data reductions. Their work made it possible to construct the catalog presented here. Students at the Department of Astrophysical Sciences, Princeton University, did observations at Lowell as part of a student project. We thank Jill Knapp and Anthony Tyson for organizing and leading the project. We are indebted to several people at Lowell: Jay Gallagher, who made it possible to use the observatory for the project, Craig Gullixson, who helped with the telescope, Neal Hartsough and Rick Wenk who built the software system, Anthony Tyson again, who built the camera, and Robert Lee for the camera-software.
We are greatly indebted to the former students who took part in the data acquisition and reduction: Arif Babul, Renyue Cen, Charles Gammie, Bin Gao, Lynne Hillenbrand, Neil Katz, Man Hoi Lee, Joanna Lees, Kevin Long, Shude Mao, Jordi Miralda-Escude, Changbom Park, Duncan Walsh, and David Weinberg. We acknowledge very useful discussions with Robert Lupton and Neil Tyson. The patience of the Palomar time allocation committee, who continued to give time for this project through five completely clouded-out years, is also much appreciated. We are indebted to Peter Teuben for his help to conform with the FITS standard.
Copyright © 1999 by
Princeton University Press.|
Created by Zsolt Frei and James E. Gunn. Email remarks to email@example.com
This page was last updated on June 16, 1999.