Exploring Andromeda's Halo with The

No. 8, September 2004 THE ING NEWSLETTER

References: Hewett, P., Irwin, M., Skillman, E., Foltz, Warren, S., Hewett, P., Lewis, G., Moller, P., Iovino, A., Shaver, P., 1996, MNRAS, Bolton, A., Burles, S., Schlegel, D., Eisenstein, C., Willis, J., Warren, S., Walton, N., 278, 139. D., Brinkmann, J., 2004, AJ, 127, 1860. 2003, ApJ, 599, L37. Werner, K., Dreizler, S., Wolff, B., 1995, Hewett, P., Warren, S., Willis, J., Bland- A&A, 298, 567. Rauch, T., Kerber, F., Pauli, E.-M., 2004, Hawthorn, J., Lewis, G., 2000, ASP Conf Ser, 195, 94 (astro-ph/9905316). A&A, 417, 647. Paul Hewett ([email protected])

Exploring Andromeda’s Halo with the INT

Alan McConnachie1, Annette Ferguson2, Avon Huxor 3, Rodrigo Ibata4, Mike Irwin1, Geraint Lewis 5, Nial Tanvir 3

1: Institute of Astronomy; 2: Max-Planck-Institut für Astrophysik, Garching, Germany; 3: Physical Sciences, University of Hertfordshire; 4: Observatoire de Strasbourg, France; 5: Institute of Astronomy, University of Sydney, Australia.

he structure of the outer WFC photometry, which shows the actual length is much greater than regions of galaxies is a key inhomogeneity of this system. Metal- 100 kpc (McConnachie, 2003). The T area in which to look for fossil poor/young stars are coded blue whilst similarity of the colour of this feature remnants of the accreted masses from metal rich/older stars are coded red. with the loop of material at the north which the galaxies that we see today This spectacular image shows in of the survey suggests a connection: are thought to be built (Searle, 1978, amazing detail the wealth of deep follow-up imaging using HST/ACS White, 1978). The importance of these information that the INT is helping confirms that they possess the same regions has increased in recent years to reveal about the structure of this stellar population (Ferguson, 2004a). as cosmological theories of structure previously invisible region of galaxies. It seems likely that the northern formation become more exact in their The most obvious piece of substructure feature is an extension of the stream, predictions, and the observational visible in Figure 1 is the giant stellar after it has passed very close to the instrumentation required to conduct stream (visible in the south-east). This centre of the potential of M31 (Font, these detailed analyses becomes more extends to near the edge of our survey 2004; Ibata, 2001). sophisticated. Currently composed of —a projected distance of some 60 kpc 165 individual pointings of the Isaac (Ibata, 2001). In fact, by examining A second large stellar stream candidate Newton Telescope Wide Field Camera the systematic shift in the luminosity has also been identified with the INT (INT WFC), the M31 halo survey function of the stream as a function WFC photometry (McConnachie, consists of photometry for over 7 of galactocentric radius, we find its 2004c). The visible part of this feature million sources, on a photometric system accurate to 2% over ~40 square degrees on the sky, in some places Figure 1. A multi-colour mosaic probing the halo of Andromeda out to of the INT WFC survey of M31, 6° (~80 kpc). Observations of 800–1000 involving 165 individual seconds in the Johnson V (V′) and pointings over 40 square degrees of the sky. North is at Gunn i (i′) passbands are deep enough the top, and East is to the left to detect individual RGB stars down ′ of this image. Metal poor/young to V = 0 and Main Sequence stars stars are coloured blue, while down to V′= –1. This unique dataset metal rich/old stars are has provided, for the first time, a coloured red. The (colour- panoramic deep view of the stellar halo dependant) substructure is of a giant galaxy thought to be similar obvious, and surprising given to our own Milky Way (Irwin, 2004). the pristene nature of the Galactic disk. The dwarf The initial results of this survey could galaxies Andromeda I & III are not have been more surprising: despite visible at the bottom left of this figure; the newly discovered exhibiting a near pristene disk, M31’s dwarf spheroidal, Andromeda halo is full of substructure and points IX, is just visible at the top left to a history of accretion and disruption as a small blue dot. NGC 205 (Ferguson, 2002). Figure 1 shows an is also visible in this figure, at image of M31, constructed from the the right-hand side of the disk.

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is some 15 kpc long and Figure 2 total, we now have INT WFC shows a cartoon of its location. The photometry for all of M31’s satellites progenitor of this feature appears to visible from La Palma. A three be the satellite galaxy NGC 205, dimensional map of the Andromeda although this awaits spectroscopic subgroup, created from our confirmation. This object has long been measurements, is shown in Figure 5. known to be tidally perturbed (Choi, The homogeneous nature of our data 2002; Hodge, 1973) but it is only now has allowed accurate and internally that the full extent of its disruption self-consistent distances and is becoming clear. Considerable metallicities to be measured for each amounts of other substructure exists of these galaxies(McConnachie, 2004a, in addition to these streams. For 2004b). This allows us for the first example, near the position of the time to reliably probe the three famous M31 globular cluster, G1, lies Figure 3. An example of a new class of dimensional spatial distribution of a large blue clump of stars (bottom globular cluster around M31, much sparser these objects, revealing that far from right of Figure 1) and opposite this than typical globular clusters, being being isotropically distributed and across the minor axis, near the position discovered by the INT WFC survey. unbiased indicators of the potential of of the famous HI warp, lies a redder Fourteen new globular clusters have so Andromeda, there are strong one. The origin of both these features far been discovered, many at large indications that these objects are is currently undecided, as is the origin projected radii.Three of these objects preferentially located on the near side have morphologies similar to the of the curiously sparse cloud of stars of Andromeda, towards the Galaxy. at the far north of our survey region. above.The half- light radii of these clusters are significantly larger than normal. This result is unexpected, and Follow-up spectroscopic observations intriguing. As well as these and many other should yield important information as to obvious substructures, the INT WFC their true nature. The INT WFC survey of M31 and its is allowing the identification of environs has revealed, and continues previously unknown globular clusters to reveal, startling surprises about the in the halo of M31 (Huxor, 2004). ~80 kpc. So far 14 have been found, faint surroundings of otherwise normal These include some of the most distant including a whole new class of cluster, galaxies. Its tendency to raise more so far discovered, at projected radii of much sparser than typical globulars. questions than answers seems to be These objects, of which three continuing, and it offers a warning to candidates have currently been studies of our own Galactic halo: given identified, are far less concentrated our position inside the Galaxy, how and have larger half-light radii than would we interpret our stellar halo if normal, making their appearance it looks in the least bit like M31? ¤ fuzzy and diffuse. A colour image of one of these objects, created from our photometry, is shown in Figure 3. The identification and quantification of the globular cluster system provides yet another valuable handle on the accretion history of this giant galaxy.

The other spiral in the Local Group, the Triangulum Galaxy (M33), has also been surveyed with the INT Figure 2. Cartoon showing the path of WFC (Ferguson, 2004b). The structure the new stellar steam candidate, the progenitor of which is thought to be of this galaxy is striking in comparison NGC 205 (large red ellipse). Also to M31. Figure 4 shows the distribution highlighted are the location of several of stars in this object and the lack of fields being used to probe the kinematics substructure is immediately obvious. of the halo, as well as the dwarf elliptical It appears that not all spiral galaxy galaxy M32. The stellar arc is some 15 kpc haloes need look like M31, and it Figure 4. The spatial distribution of stellar in length and may be able to shed light raises the question: is there such a sources in the INT WFC survey of M33, on the dynamical evolution of NGC 205, thing as a ‘typical’ stellar halo? the Triangulum Galaxy. This is a small and provide a useful probe of the potential spiral, approximately one-tenth the size of M31. Although previously known to of Andromeda. The lack of substructure have been tidally perturbed, this is the There is then the question of the in this galaxy is in startling contrast to first detection of a probable significant M31 dwarf satellite galaxies, several M31 —virtually no spatial inhomogeneities extra-tidal component of NGC 205. of which are visible in Figure 1. In are present in this galaxy’s outer regions.

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Figure 5. The distribution of the satellite galaxies of M31, as derived from our INT WFC photometry of these objects. The coordinate system is an M31–centric system. The plane is the plane of the disk of M31, and each cell corresponds to 100kpc×100kpc. l is a longitude measured around the disk of M31, such that l =0 is the longitude of the Galaxy. b is a latitude, measured from the disk of M31. Solid lines indicate objects located above the plane of the disk, while dashed lines indicate objects below the plane of the disk. A clear tendency for the satellites to lie on the near side of M31 can be observed, and suggests an intriguing correlation between the M31 satellites and our own Galaxy.

References: Huxor, A., et al., 2004, in prep. McConnachie, A. W., Irwin, M. J., Ferguson, Ibata, R., Irwin, M., Lewis, G., Ferguson, A. M. N., Ibata, R. A., Lewis, G. F., Tanvir, N., 2004b, MNRAS, submitted. Choi, P. I., Guhathakurta, P., Johnston, A. M. N., Tanvir, N., 2001, Nature, 412, 49. K. V., 2002, AJ, 124, 310. Ibata, R., Chapman, S., Ferguson, A. M. McConnachie, A. W., Irwin, M. J., Lewis, Ferguson, A. M. N., Irwin, M. J., Ibata, R. N., Irwin, M. J., Lewis, G. F., McConnachie, G. F., Ibata, R. A., Chapman, S. C., A., Lewis, G. F., Tanvir, N. R., 2002, AJ, A. W., Tanvir, N., 2004, MNRAS, 351, 117. Ferguson, A. M. N., Tanvir, N. R., 2004c, MNRAS, 351, L94. 124, 1452. Irwin, M. J., et al., 2004, in prep. Searle, L., Zinn, R., 1978, ApJ, 225, 357. Ferguson, A. M. N., et al., 2004a, in prep. McConnachie, A. W., Irwin, M. J., Ibata, Ferguson, A. M. N., et al., 2004b, in prep. R. A., Ferguson, A. M. N., Lewis, G. F., White, S. D. M., Rees, M. J., 1978, MNRAS, Font, A., Johnston, K., Guhathakurta, P., Tanvir, N., 2003, MNRAS, 343, 1335. 183, 341. Majewski, S., Rich, M., 2004, AJ, McConnachie, A. W., Irwin, M. J., Ferguson, submitted. A. M. N., Ibata, R. A., Lewis, G. F., Hodge, P. W., 1973, ApJ, 182, 671. Tanvir, N., 2004a, MNRAS, 350, 243. Alan McConnachie ([email protected])

The Bull’s Eye Pattern in the Cat’s Eye and Other Planetary Nebulae

Romano L. M. Corradi (ING)

he end-point of the evolution red giant, the so-called the This newly formed dust is further of solar-type stars is Asymptotic Giant Branch (AGB) accelerated out of the gravitational T essentially determined by stage. In the last million years of the bounds of the star by the pressure of the onset of a strong stellar wind, AGB, the red giant is dynamically the radiation coming from the hot which, in a few hundred thousand unstable and pulsates with typical stellar remnant. Gas, which is coupled years completely removes the star’s periods of few hundred days: a to dust by collisions, also leaves the gaseous envelope, thereby removing prototypical star in this phase is star in this process. the fuel that has previously Mira in Cetus. The mechanical maintained the thermonuclear energy of the pulsations pushes In the last hundred thousand years energy source in its interior. This large amounts of material far away of the AGB, this mass loss process is phenomenon occur during a (second) enough from the core of the star for it so strong that the star is completely phase in which the star becomes a to cool down and condense into dust. surrounded by a thick, expanding

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