Elementos de la Terapia

With the help of optical observations, astronomers study the stellar and globular cluster population of M 31. An optical image of M 31 is shown in Fig. 3.1. As part of their “Local Group Galaxies Survey” (LGS)

Massey et al. (2006) obtained optical photometry down to at least 21 (23) mag3_{for the stars in the field of}

M 31. While colour magnitude diagrams are dominated by foreground dwarfs and giants at intermediate colours, at extreme colours the diagrams are populated with blue and red supergiants (Massey et al. 2009) belonging mostly to M 31. Drout et al. (2009) used the large negative systematic velocity of M 31 to sepa- rate the population of yellow supergiants from foreground stars. The Andromeda galaxy was also observed

in the framework of all-sky surveys in the optical (e. g. Monet et al. 2003, USNO-B1) and infra red (e. g.

Skrutskie et al. 2006, 2MASS).

Accurate distance measurements to the Local Group galaxies are crucial to calibrate the cosmic distance scale. The distance to M 31 has been estimated using a variety of methods. Stanek & Garnavich (1998) used

red clump stars observed with theHipparcossatellite in the Milky Way and with theHubble Space Telescope

Figure 3.1: The Andromeda galaxy in the optical, with two of their companion galaxies. M 32 is the bright yellow spot in the lower part of the image, while NGC 205 is located in the upper right part. North is at the top and east to the left hand side of the image. The image was taken with the Schmitt telescope at the Tautenburg observatory. Source: Th¨uringer Landessternwarte Tautenburg;

(HST)in M 31. Red clump stars are the metal-rich equivalent of the horizontal-branch stars. Theoretical models predict that their absolute luminosity depends only weakly on their age and chemical composition.

The determined distance modulus is(m−M)o = 24.471±0.035±0.045(=ˆ784±13±17 kpc). The first

uncertainty is statistical and the second uncertainty is systematic. Based on fitting theoretical isochrones to the observed red giant branches of 14 globular clusters in M 31, Holland (1998) derived a distance modulus

((m−M)o = 24.47±0.07) that is in excellent agreement with the value reported by Stanek & Garnavich

(1998). Ribas et al. (2005) used an eclipsing binary to determine the distance to M 31. From their spectral

analysis, they obtained a distance modulus of(m−M)o = 24.44±0.12(=ˆ772±44 kpc). Other distance

estimates are based for instance on the location of the tip of the red giant branch, which acts as a standard

candle in old, metal-poor stellar populations (McConnachie et al. 2005,(m−M)o = 24.47±0.07), or on

fitted period-luminosity relations of Cepheids (Macri et al. 2001).

Williams (2003) investigated the star formation history of M 31, using six fields of the LGS observa-

tions. For the total disc of M 31 he estimated a star formation rate of∼1 M¯ yr−1. In addition the analysis

routine used also gives distances to the examined parts of M 31. The distance examination provides a hint that the disc southeast of the major axis is more distant. This finding is in agreement with the spatial orien- tation determined from the location of the absorbing dust lanes (Simien et al. 1978) and from the differential reddening among globular clusters (Iye & Ozawa 1999). In summary, the northwestern side of the disc of M 31 is nearer to us than its southeastern side.

Studies of the globular clusters of M 31 revealed two subpopulations, associated with the halo and bulge of the galaxy. Perhaps the main difference between the globular cluster systems of M 31 and the Milky Way

is that the former are more populous, with an estimated total of∼450members and may contain a significant

population of intermediate age (3–6 Gyr) globular clusters (Alves-Brito et al. 2009, and references therein). Recent discoveries are that of a metal-poor stellar halo in M 31 (Kalirai et al. 2006; Chapman et al. 2006), and that of outer halo globular clusters beyond a projected radius of 70 kpc from the centre of M 31 (Alves- Brito et al. 2009).

The globular cluster population of M 31 was also studied in the near and far ultraviolet (NUV/FUV)

using theGalaxy Evolution Explorer (GALEX). Rey et al. (2007) analysed the UV properties of the globular

cluster population of M 31 and compared them with the Milky Way globular cluster population. Figure 3.2 presents a UV image of M 31, which clearly shows the star forming regions in the spiral arms of the galaxy

(cf. Kang et al. 2009).

Surveys of hydrogen lines (Brinks & Shane 1984; Pellet et al. 1978), CO (Dame et al. 1993), molecular gas (Nieten et al. 2006) and of the 20 cm radio continuum emission (Beck et al. 1998) detected and con- firmed an outer ring of star formation at a radius of ten kiloparsecs, whose centre is offset from the M 31 nucleus. In addition, the outer galaxy disc is warped, as seen in both optical (Ibata et al. 2001) and radio (Braun 1991) wavelengths. A second, inner dust ring, whose centre is offset by about half a kpc from the

centre of M 31, was detected in an 8µm image – after subtraction of a scaled 3.6µm image – taken with

the Infrared Array Camera (IRAC) on-board theSpitzer Space Telescope(Block et al. 2006, Fig. 3.3). Both

rings appear to be density waves propagating in the disc. Based on numerical simulations Block et al. (2006) propose that the rings result from a companion galaxy that plunged through the centre of the disc of M 31, about 210 million years ago. The most likely candidate for that interloper is M 32.

Using HST spectroscopy of the centre of M 31 Bender et al. (2005) showed that the core of M 31 is

a triple. In addition to the previously known double brightness peaks P1 and P2 they found that the blue nucleus embedded in P2 consists of a hot star population (P3). The kinematics of P3 are consistent with a circular stellar disc in Keplerian rotation around a supermassive black hole. The derived properties of the

Figure 3.2:A UV image of M 31. The mosaic is composed of NUV (red) and FUV (blue) observations taken withGALEX. The spiral arms, with their star forming regions, are clearly visible in blue. Taken from Thilker et al. (2005).

Figure 3.3:Infrared view of M 31, obtained with the IRAC on-boardSpitzer. The image clearly shows the inner and outer dust ring of M 31. North is on the left hand side and east at the bottom side. From Block et al. (2006).

P3 disc and the central black hole mass are in agreement with previous results based on studies of P1 and P2. Therefore astrophysical arguments strongly favour the conclusion that the dynamically detected central dark object in M 31 is a black hole.