IC 1613

Introduction

The dwarf irregular galaxy IC 1613 was discovered by Wolf (1906) and is described as “F,eeL” (i.e., faint and most extremely large) in the Second Index Catalogue of Nebulae (Dreyer 1908). The true nature of IC 1613 was first recognized by Baade (1935), who determined its distance by using the period–luminosity relation for Cepheids having periods ranging from 14 days to 42 days. Baade concluded that “it is without doubt a system of low luminosity.” Subsequently it was included as a bona fide member of the Local Group by Hubble (1936, p. 145). IC 1613 is a nonbarred irregular that serves as the prototype for DDO type Ir V. The fact that IC 1613 was already known almost a century ago, even though it is quite faint (MV = –15.3), suggests that the inventory of all but the dimmest Local Group members is (excepting objects at low Galactic latitude) probably reasonably complete. A blue image taken with the Palomar 1.2-m Schmidt telescope is shown in Figure 11.1. A higher resolution photograph of IC 1613, which was obtained with the Palomar 5-m reflector, is shown in Volders & Högbom (1961). A beautiful photograph inthe light of Hα is reproduced in Sandage (1971).

Distance and reddening

The discovery of Cepheids in IC 1613 by Baade (1935) was followed up by a more detailed investigation (Baade 1963, pp. 218–226) that resulted in the identification of 25 Cepheids with periods ranging from 2.4 days to 146 days.

Introduction

While inspecting a mosaic of nine deep IIIaJ plates of M31 and its environment taken with the Palomar 1.2-m Schmidt telescope, van den Bergh (1972a) discovered three faint nebulous patches that were immediately suspected of being dwarf spheroidal companions to the Andromeda galaxy. These objects were dubbed Andromeda I, Andromeda II, and Andromeda III. Subsequently van den Bergh (1972b) used the Palomar 5-m Hale reflector to resolve And III into stars that had approximately the same V magnitude as the brightest stars in NGC 185 and NGC 205. Later van den Bergh (1974b) showed that And I and And II resolve at about the same magnitude as And III. These observations indicated that And I, And II, and And III are all located at about the same distance as M31. The suspicion that they are physically associated with M31 was strengthened by a Palomar Schmidt survey of an area of ∼700 square degrees surrounding the Andromeda galaxy, which showed that And I, II, and III are concentrated toward M31. Of a fourth candidate van den Bergh (1972a) wrote: “The object And IV is probably not a dwarf spheroidal galaxy. It is smaller and bluer than the other objects in Table 1 [i.e., And I, II & III]. Furthermore, And IV has a much higher surface brightness than do the other galaxies in this table. This suggests that And IV, which is located very close to M31, might be a relatively old star cloud in the outer Disk of M31.”

The present section will deal with the three most luminous Local Group spheroidal galaxies: NGC 205, NGC 185, and NGC 147, all of which are companions to the Andromeda galaxy. These galaxies, which are too luminous to be called dwarf spheroidals (dSph), will be designated spheroidals (Sph). In the past such objects have been classified as being of types dE, Ep, S1, or dE/dSph. It should, however, be emphasized that Sph and dSph galaxies are, respectively, the bright and faint representatives of the same morphological class of galaxies.

NGC 147 and NGC 185 are located at only 0.22 Mpc from our adopted center of the Local Group. This places them closer to the center of mass of the Local Group than any other presently known Group members.

The spheroidal galaxy NGC 205

Introduction

The luminosity of NGC 205 is similar to that of M32. However, it has a much more extended structure. Furthermore M32 rotates, but NGC 205 does not (Bender, Paquet & Nieto 1991). A good photograph of NGC 205 is shown in the Hubble Atlas of Galaxies (Sandage 1961). Images taken in the ultraviolet (Baade 1951) show it to contain a small number of young blue stars. Deep exposures (see Figure 12.1) reveal some extended patches of dust absorption. Moreover, inspection of the image of NGC 205 on the prints of the Palomar Sky Survey, or deeper plates (see, for example, Fig. 7 of Kormendy 1982), clearly show that the outer isophotes of this galaxy are distorted – presumably due to the tides exerted by M31, from which it is separated by only 37′(8.2 kpc).

Introduction

A detailed discussion of galaxies that may be located along the outer fringes of the Local Group has been given by van den Bergh (1994b). In general three criteria can be used to assess the probability that a galaxy might be associated with the Local Group: (1) The distance to that galaxy should be ≲1.5 Mpc (see Section 18.1), (2) it should lie close to the relation between radial velocity and distance from the solar apex (Vr versus cos θ diagram) for well-established Local Group members, and (3) it should not appear to be associated with a group of galaxies that is known to be located well beyond the limits of the Local Group. On the basis of these criteria van den Bergh (1994b) concluded that it was safe to exclude the following galaxies from membership in the Local Group: (1) Sculptor irregular = UKS 2323–326, (2) Maffei 1 and its companions, (3) UGC-A86 = A0355 + 66, (4) NGC 1560, (5) NGC 1569, (6) NGC 5237, and (7) DDO 187 (Hoessel, Saha & Danielson 1998). A particularly strong concentration of Local Group suspects, which includes (2), (3), (4), and (5) listed above, occurs in the IC 342/Maffei group (van den Bergh 1971, Krismer, Tully & Gioia 1995). Krismer et al. place this group at a distance of (3.6 ± 0.5) Mpc. Cassiopeia 1, which was once regarded as a Local Group suspect (Tikhonov 1996), also appears to be a member of the IC 342/Maffei group.

Introduction

Gaseous material is expected to flow into galaxies by the capture of high-velocity clouds (Oort 1966, 1970). Oort estimated that the mass of the Milky Way system might be increasing by ∼1% per Gyr as a result of such inflow. However, tidal interactions between galaxies will result in the return of some interstellar gas in galaxies to intergalactic space (Morris & van den Bergh 1994). In addition, some gas may be ejected from galaxies by supernova-induced fountains (Heiles 1987). Perhaps the best example of inflow into the Galactic Disk is provided by “Complex C” (Wakker & van Woerden 1997). From the nondetection of absorption features in stars of known distance, these authors conclude that D > 2.4 kpc and that Complex C is falling toward us with a velocity > 100 km s-1. From spectroscopic observations of S II absorption lines (which are not affected by depletion onto dust) with the Hubble Space Telescope, Wakker et al. (1999) find that Complex C has a metallicity of 0.094 ± 0.020 times solar. This very low metallicity rules out the possibility that the gas in Complex C was ejected in a Galactic fountain. Furthermore, the data on the age–metallicity relation of the Small Magellanic Cloud (Mighell, Sarajedini & French 1998) show that the gas in Complex C is so metal poor that it could only have been stripped from the SMC ≳5 Gyr ago (i.e., long before the Magellanic Stream was formed).

Introduction

The great spiral in Andromeda is the most luminous member of the Local Group. The first known record of this object is by al-Sufi (903–986). M31 was first viewed through a telescope by Marius, who described it as looking “like a candle seen through a horn,” in 1612. The spiral nature of the Andromeda galaxy was first clearly shown in photographs obtained by Roberts (1887) with a 0.5-m reflector. Ritchey (1917) referred to these arms as “great streams of nebulous stars.” It is listed as object number 31 in Messier's catalog of nebulae. An atlas of finding charts for clusters, associations, variables, etc. in M31 has been published by Hodge (1981). Van den Bergh (1991b) has written a long review paper on this object. The monograph The Andromeda Galaxy by Hodge (1992) provides a detailed historical discussion of research on this object and an annotated bibliography for the period 1885–1950. A monograph entitled Tumannost Andromedy (The Andromeda Nebula) has been published by Sharov (1982).On the sky M31 covers an area of 92′ × 197′ (Holmberg 1958). This large angular size makes the Andromeda galaxy particularly suitable for detailed studies of its structure and stellar population content. De Vaucouleurs (1959b) has shown that the distribution of surface brightness I in spiral galaxies can often be decomposed into a spheroidal component in which I ∼ exp(–R1/4) and an exponential disk in which I ∼ exp(–αR), where R is radial distance and α is a constant.

Introduction

Inspection of Table 2.1, which lists the known members of the Local Group, shows that dwarf spheroidals are the most common type of galaxy in the Local Group. Observations of rich clusters (Trentham 1998a,b) appear to show that the faint ends of their luminosity functions are steep. Moreover, the colors of these faint galaxies suggest that they are mostly dwarf spheroidals. Since dwarf spheroidals are ubiquitous in poor clusters, and even more frequent in rich clusters, it appears safe to conclude that such dwarfs are the most common type of galaxy in the Universe. The fact that they were not discovered until 1938 is entirely due to their low luminosity. Their discoverer (Shapley 1943, p. 142) writes

The suggestion that dwarf spheroidals might be distributed uniformly throughout the Local Group (Ambartsumian 1962) appears to conflict with the observation that strong clusterings of such objects are now known around the Milky Way system and the Andromeda galaxy.

In April of 1968 I gave a series of lectures on the structure, evolution, and stellar content of nearby galaxies at the University of California in Berkeley. An outline of these talks was printed as a slender volume entitled “The Galaxies of the Local Group” (van den Bergh 1968a). Since the publication of this booklet the number of known members of the Local Group has doubled. Furthermore both the quantity, and the quality, of the data that are available on the previously known Local Group members have increased enormously.

Particularly exciting developments since 1968 have been (1) the discovery of the Sagittarius dwarf, which is the nearest external galaxy, (2) the discovery of six dwarf spheroidal companions to the Andromeda nebula, (3) the application of CCD detectors to studies of stellar populations in various Local Group systems, and (4) deep high-resolution observations of various objects in the Local Group with the Hubble Space Telescope. With the presently available enlarged sample, and the improved quality of data on individual objects, we are now in a much better position to start exploring the evolutionary history of the Local Group and its constituent galaxies. Finally (5) it has become clear during the past quarter century that the masses of dark matter halos are typically an order of magnitude greater than the masses of the baryonic galaxies that are embedded within them.

Introduction

The Small Cloud is an irregular dwarf of DDO type Ir IV–V that has a low mean metallicity and a high mass fraction remaining in gaseous form. These characteristics suggest that the SMC is, from an evolutionary point of view, a more primitive and less evolved galaxy than the Large Cloud. The metallicity difference between the Galaxy and the LMC was discovered by Arp (1962), who wrote: “Taken together with the marked differences in the evolved giant branches and [C]epheid gaps in the SMC and [G]alactic clusters, there exists the inescapable implication that the chemical composition of the SMC stars is different from the chemical composition of the solar neighborhood.”

The fact that the SMC contains only a single true globular cluster may indicate that star formation started off later, or more gradually, than it did in the Large Cloud.

At the present time the SMC is forming stars less actively than is the LMC. Prima facie evidence for this is that the Small Cloud contains much smaller, and less spectacular, H II regions than does the Large Cloud. Furthermore, the LMC presently contains 110 massive Wolf–Rayet stars, whereas there are only 9 WR stars in the Small Cloud. Finally, CCD observations by Bothun & Thompson (1988) show that the SMC is redder than the LMC. They find B – V = 0.52±0.03 for the Large Cloud, versus B – V =0.61±0.03 for the integrated color of the Small Cloud.

Introduction

The compact E2 galaxy M32 is the closest companion to the Andromeda galaxy. The projected separation of these two objects on the sky is only 24′ (5.3 kpc). It was first suggested by Schwarzschild (1954) that the tides induced by M32 were responsible for the distortion of the spiral structure of M31 and the warping of its disk. Later Faber (1973) noted that CN and Mg absorption in M32 was stronger than might have been expected from its luminosity (i.e., it has the metallicity usually seen in significantly more luminous objects). She therefore proposed that M32 was initially a much more luminous galaxy had suffered severe tidal truncation by M31. The very compact object NGC 4486B, which is a companion to M87, appears to be another example of a similar type of galaxy that has suffered tidal truncation. The absence of globular clusters in M32 probably also results from its outer swarm of globulars being stripped off by tidal forces. [The innermost M32 globulars might have been sucked into its massive semi-stellar nucleus by tidal friction (Tremaine, Ostriker & Spitzer 1975).] It would be interesting (M. Mateo 1999, private communication) to study the distribution of globular clusters in NGC 4486B, which, like M32, is thought to have suffered severe tidal truncation.

Kormendy (1985) and Ziegler & Bender (1998) have pointed out that M32 is quite a unique object and that there are very few other ellipticals like it. Compared to other dwarf galaxies having similar values of MV, its central surface brightness is four orders of magnitude higher, and its core radiusrc is three orders of magnitude smaller.

In this book, the term solar dynamo refers to the complex of mechanisms that cause the magnetic phenomena in the solar atmosphere. Usually, however, that complex is broken down into three components: (1) the generation of strong, large-scale fields of periodically reversing polarity, (2) the rise of these fields to the photosphere, and (3) the processing in, spreading across, and removal from the photosphere of magnetic flux. Components (2) and (3) are discussed in Chapters 4–6; in this chapter, we concentrate on aspect (1). Even on this limited topic, there is a stream of papers, but, as Rüdiger (1994) remarked, “it is much easier to find an excellent… review about the solar dynamo… than a working model of it.”

In dynamo theory, the mean, large-scale solar magnetic field is usually taken to be the axially symmetric component of the magnetic field that can be written, without loss of generality, as the sum of a toroidal (i.e., azimuthal) component Bφ ≡ (0, Bφ, 0) and a poloidal component, which is restricted to meridional planes: Bp ≡ (Br, 0, Bθ′), where θ′ is the colatitude. The poloidal component is usually pictured as if a dipole field aligned with the rotation axis were its major component, which is a severe restriction.

All solar-cycle dynamo models rely on the differential rotation v0(r, θ′) to pull out the magnetic field into the toroidal direction, as sketched in Fig. 6.10a; about this mechanism there is no controversy.