The following will briefly describe the main telescopes and instruments used to obtain the images presented in the atlas or those used in observations discussed in the text. Detectors are not described. Since there exists significant overlap between telescopes used for submillimeter and radio observations these are considered together.

A1 Gamma ray

kT > 500 keV

The Compton Gamma Ray Observatory (CGRO; Figure A.1) was launched into Earth orbit at 450 km altitude on April 5th, 1991 and re-entered on June 4th, 2000. CGRO contained instruments that could detect radiation with energies from 15 keV to 30 GeV and it was the second of NASA's “Great Observatories”.

These instruments included the Energetic Gamma Ray Experiment Telescope (EGRET) that detected events between 20 MeV and 30 GeV with a positional accuracy of ∼1°. The Imaging Compton Telescope (COMPTEL) covered 1–30 MeV with a positional accuracy of ∼2°. Currently, gamma-ray imaging observations of nearby galaxies are restricted due to a small number of recorded events at poor positional accuracy.

The Fermi Gamma Ray Space Telescope (hereafter Fermi), formerly called the Gamma Ray Large Area Space Telescope or GLAST, was launched on June 11th, 2008 into a 560 km altitude orbit. It is detecting radiation between 8 keV and 300 GeV using the primary instrument, the Large Area Telescope (LAT), and the complementary GLAST Burst Monitor (GBM). The LAT has a large field of view, over 2 steradians (one-fifth of the entire sky), can measure the locations of bright sources to within 1 arcminute and is sensitive to photons from 30 MeV to greater than 300 GeV.

In the early 1930s Karl Jansky detected the center of our Galaxy in radio waves at a wavelength of 14.6 m or a frequency of 20.5 MHz.

Until this time all observations of galaxies had been made in the optical region, which has radiation with wavelengths between 330 and 800 nanometers. Galaxies that can be seen by eye, appearing as faint, cloud-like objects, are the two Magellanic Clouds (named after the Portuguese explorer Ferdinand Magellan) in the southern hemisphere and the Andromeda Galaxy (NGC 224 or Messier 31) in the northern hemisphere. Observers with very good eyesight may also detect NGC 598/M 33, in the constellation Triangulum. We also get a myopic view of nearby stars (about 6000 within a few thousand light-years) and a wide-field view of diffuse light, “the Milky Way”, from distant stars, near the plane of our own Galaxy.

Since the early seventeenth century, telescopes have provided higher resolution observations to fainter brightness levels than possible with the eye. In 2009 the world celebrated the International Year of Astronomy (IYA), as sanctioned by the United Nations and directed by the International Astronomical Union partly to celebrate 400 years of telescopic observations since the initial endeavors of Thomas Harriot and Galileo Galilei.

In terms of effective galaxy research tools, we had to wait until the mid-nineteenth century for large telescopes to be built. Observations by eye, photography and by spectroscopy all played vital roles.

In this part, we discuss the properties of the Galaxy center, and finish with a selection of multiwavelength all-sky images of the Galaxy.

The Galaxy is an Sbc/SBbc spiral galaxy and in the atlas it is classified in the normal (N) category of our sample, yet additional categories interacting (I), and active? (A?) (see Table 1, page 18) are also assigned. The Galaxy (along with the LMC and SMC) is classified I due to the existence of the H I Magellanic Stream (Wannier and Wrixon 1972; Murai and Fujimoto 1986) as shown in Figure 1.22. An A? classification is also given because the optical spectrum integrated over the central parsec of the Galaxy resembles a Seyfert galaxy (Mezger et al. 1996). This is consistent with results that suggest the presence of a several million M⊙ black hole in the nucleus.

Quiet monster – Sagittarius A*

Our Galaxy's nucleus, some 8.5 kpc distant in the constellation of Sagittarius, has long been a target for multiwavelength observations. Because of the large amounts of obscuring dust towards the nucleus and in the disk of the Galaxy, infrared observations have proved invaluable in showing the content and structure of this region. Figure 3.1 shows a 48° by 33° IRAS IR image of the central region of the Galaxy. The disk of the Galaxy is the bright band running diagonally across the image.

The bright central region contains the nucleus. The concentrated blobs of yellow are giant clouds of interstellar gas and dust heated by recently formed stars and nearby massive, hot stars.

Earth's atmosphere and extraneous radiation

Not all photons emitted by astronomical objects are detected by ground-based telescopes. A major barrier to the photons path is the Earth's ionosphere and upper atmosphere that absorbs or scatters most incoming radiation except for the optical (wavelengths of 3300–8000 Å where 1 Å = 10-10 m), parts of the near-IR (0.8–7 μm) and radio (greater than 1 mm) regions. Absorption greatly affects radiation with the shortest wavelengths. In general, gamma rays are absorbed by atomic nuclei, X-rays by individual atoms and UV radiation by molecules. Incoming IR and submillimeter radiation are strongly absorbed by molecules in the upper atmosphere (e.g. H2O and carbon monoxide, CO). Observations in these regions greatly benefit by locating telescopes at high altitude. Mountain-top sites like Mauna Kea (altitude 4200 m) in Hawaii, Cerro Pachon (2700 m), Las Campanas (2500 m) and Paranal (2600 m) in Chile, and La Palma (2300 m) in the Canary Islands are used to decrease the blocking effect of the atmosphere. The Antarctic, in particular the South Pole, provides an atmosphere with low water vapor content. Most of the continent is at high altitude, with the South Pole 2835 m above sea level, again helping to reduce the amount of obscuring atmosphere. The Antarctic has therefore also become a very useful IR and submillimeter site.

The transmission properties of the Earth's atmosphere (Figure 2.1) has prompted the exploration of the gamma ray, X-ray, UV, mid- and far-IR regions of the electromagnetic spectrum via satellite and high-altitude balloon observations.

NGC 253

NGC 253:

Itoh et al. (2002) detected TeV gamma-ray emission greater than 0.5 TeV with the CANGAROO-II 10 m imaging atmospheric Cerenkov telescope.

Pietsch (1994) and Vogler and Pietsch (1999) show ROSAT PSPC images. The (0.1–2.4 keV) X-ray image shows discrete sources (probably low- and high-mass X-ray binary systems), with a high concentration near the nucleus, as well as faint diffuse emission. The soft (0.1–0.4 keV) image shows spectacular diffuse emission extending above the plane of the galaxy. This emission is 105–6 K gas being energized and ejected from the plane by SN explosions. Weaver et al. (2002) present CXO ACIS data and detect a heavily absorbed source (>1039 ergs s-1) of hard X-rays embedded within the nuclear starburst region.

OVRO CO observations (Canzian, Mundy and Scoville 1988) detect a large amount of molecular gas in the form of a central bar. The mass is ~5 × 108 M⊙ and displays rigid body rotation suggesting the existence of a large mass in the center of the galaxy. High-resolution VLA radio continuum images (Ulvestad and Antonucci 1997) show numerous compact sources in the inner 200 pc.

Multiwavelength images of normal, interacting, merging, starburst and active galaxies are presented. Galaxies are ordered by right ascension in each category sample as listed in Table 1 (page 18). The images come from a variety of sources – some single-wavelength images are presented in false color, whilst multiwavelength images are color coded.

A brief description of each galaxy including its major multiwavelength properties (literature references start on page 243) is given. The description is ordered via wavelength, usually starting with X-ray properties and ending with radio properties. However, papers that present multiwavelength properties have their findings summarized concurrently. Each galaxy description is provided to highlight some of the more recent and interesting scientific findings. The description is, however, not intended to be a comprehensive description of the galaxy. Readers interested in comprehensive literature searches on individual galaxies should use a nearby mirror site of the NASA Astrophysics Data System listed in, e.g.

http://ads.nao.ac.jp/mirrors.html

or use the NASA/IPAC Extragalactic Database (NED)

http://nedwww.ipac.caltech.edu/

Technical information about the galaxy images – telescope/instrument, observer(s), λ/v/energy/filter, exposure, resolution and image source – is listed in Appendix C (page 224). For each image north is up and east is to the left unless otherwise indicated. Inclinations of galaxies are referred to in the sense that 0° is face-on and 90° is edge-on to our line of sight. The spatial scale of each galaxy image is not always the same although the field of view is listed when it is known.

Discovery and nature of neutron stars

Until the discovery of radio pulsars by Jocelyn Bell and Antony Hewish in 1967 (Hewish et al., 1968), neutron stars had existed only in the minds of theoretical physicists. First proposed as an end state of stellar evolution by Robert Oppenheimer and George Volkoff (1939), they are now accepted as the only explanation for radio pulsars. The discovery was serendipitous. No one had conjectured or even dreamed that this sort of signal might be generated.

Bell, then a student, had just spent months wiring antennas for a new radio telescope. In the course of testing, she noticed ‘a bit of scruff’ in the recorded signal. This ‘scruff’ was found to repeat, not every 24 hours (the solar day), but every 23 hours and 56 minutes (the sidereal day), showing that the source was anchored in the sky, not to the rotating Earth. Furthermore, when present, the signal was periodic with a remarkably stable period of 1.337 s. Thus CP 1919, the first of the radio pulsars, was discovered. The stable period could only be explained by rotation, and only a small object with strong gravity could rotate this fast without breaking up. A neutron star was the only reasonable explanation (Gold, 1968).

The discovery of X-rays

On the second story of the building at Röntenring 8 in Würzburg, Germany, there is a plaque: ‘In diesem Hause entdekte W. C. Röntgen im Jahre 1895 die nach ihm benannten Strahlen’ – In this building, in the year 1895, W. C. Röntgen discovered the radiation named for him. Here was the laboratory of Wilhelm C. Röntgen, a 50-year-old professor of physics, who was studying phenomena associated with electrical discharge in gasses. On the afternoon of 8 November, working alone in his laboratory, he noticed a curious phenomenon. When high voltage was applied to the electrodes in the partially evacuated glass discharge tube, he noticed a faint glow from a fluorescent screen placed at the other end of the laboratory table. The room was dark and he had previously covered the tube with black cardboard so no light would escape. Why was the screen glowing?

That evening he verified that the discharge tube was indeed the source of the energy that caused the screen to glow, and that no visible radiation was escaping from the shrouded tube. He quickly found that the unknown radiation would pass through paper, wood, and aluminum but was stopped by heavy metals. Then, when holding a lead disc in front of the screen to observe its shadow, Röntgen also saw the shadow of bones in his hand! In a week he had measured the basic characteristics of this new form of radiation.

Introduction

Cataclysmic variables are remarkably similar to the low-mass X-ray binaries (LMXBs) described in Chapter 11 but are significantly less luminous in X-rays, particularly at the higher X-ray energies of the first X-ray satellites. Indeed, only one previously known CV, EX Hya, was found in the Uhuru survey of the X-ray sky. But these objects have a long history that goes back well before the era of X-ray astronomy, as they include among their number both dwarf novae and novae. As we shall see in this chapter, dwarf novae and novae are powered by fundamentally different processes than occur in supernova events. Supernovae represent the final, and irreversible, moments in the lives of massive stars, when they collapse rapidly under gravity and then explode catastrophically. For any given object it happens once, whereas nova and dwarf nova eruptions can and do recur. Indeed, it is hypothesised that all novae recur, but the typical recurrence time is long: at least hundreds, perhaps thousands, of years.

Cataclysmic variables are interacting binaries similar to LMXBs, except that the compact object is a white dwarf, accreting material from its (usually) cool, late-type companion star in a short period (approximately hours) binary system. They are one of the few classes of object in this book that were known and observed prior to the twentieth century. Novae have been known to mankind throughout history, and dwarf novae were first recorded in the mid-nineteenth century.