Extragalactic Objects
Galaxies, Quasars, and the Reference Frame
Gaia will not only provide a representative census of the stars throughout the Milky Way but it will also make unique contributions to extragalactic astronomy. These include the structure, dynamics and stellar populations in the Magellanic Clouds, M31 and M33, the space motions of Local Group galaxies, and studies of supernovae, galactic nuclei and quasars.
The Magellanic Clouds
The Magellanic Clouds are substantial galaxies in their own right and provide the nearest examples of young intermediate-to-low chemical abundance stellar populations for study. The LMC and SMC will provide millions of stars for Gaia analyses. The key scientific questions for Gaia involve the dynamics of the LMC galaxy and the LMC-SMC interactions, the luminosity calibration of stellar populations, the dynamics of star forming regions, and the dynamical structure of the LMC bar. Gaia will allow kinematic mapping and membership analyses of young star-forming regions in the LMC and SMC with comparable precision to those in the Milky Way. It will then be possible to compare directly the kinematics and structure of star forming regions in a large spiral disc with those in a mid-sized irregular galaxy.
Both the LMC and SMC show significant bar-like asymmetries in their luminosity distribution. This effect is most obvious in the LMC. It is not known what the dynamical status of the bar is, or even if it is in the same plane as the main LMC disc. Gaia will provide not only three-dimensional dynamics across the whole bar and disc region, quantifying the dynamical relationship between these features, but also excellent relative distances.
Gaia proper motions will map the membership of the clouds, giving the dark halo structures of both the LMC and SMC, determining the extent of their halos, the density of the Milky Way at 50 kpc and the effects of the LMC-SMC interaction.
Dwarf Satellites of the Milky Way
There are eight known dwarf satellite galaxies beyond the Magellanic Clouds at distances up to 230 kpc. These provide key dynamical tracers of the outer mass distribution of the Milky Way, at larger distances than any other available tracer. For the nearer dwarfs, especially Ursa Minor, Gaia will allow internal dynamical studies.
An accurate two-dimensional transverse velocity map of Ursa Minor is feasible with Gaia, retaining adequate spatial resolution while attaining precision by spatial binning. Similar studies at lower spatial resolution are possible in the other satellites. The Gaia proper motions will provide excellent discrimination between field stars, and provide a clean test of the expectation that all these dwarf galaxies are parts of extended tidal tails.
Rotational Parallax Distances to the LMC, M33 and M31
Rotation curves for disc galaxies can be derived from radial velocity data, or from proper motions. Since the latter are distance dependent, whereas the former are distant independent, using both methods provides a distance measurement. This measurement can be applied to the disc galaxies within the local group, and a few other spirals (M81, NGC253, NGC55).
Gaia has a crucial contribution to make. By measuring a very large sample of stars, it will be possible to extend the analysis beyond a simple rotation curve fit to determine a map of the two-dimensional kinematic field in each galaxy. In galaxies, such as M31, where the warp is expected to dominate such information will be essential for meaningful analysis. These galaxies contain many globular clusters whose unresolved cores will be measurable astrometrically. Gaia proper motions will provide a three-dimensional determination of the halo mass in the outer parts of these galaxies.
Warps
Our own Galaxy has a measurable (or apparent) warp. Outside the Milky Way the most significant warp in terms of its observable effect is that of M31. There the systematic warp effects are comparable to the measuring precision, thus the warp must be considered explicitly when deriving the M31 rotation curve parallax with Gaia. In addition, the warp signature may be detectable statistically. In M33 (the third largest local group disc galaxy) the warp is so extreme that many points along our line of sight intersect the disc twice.
Orbits
In the Local Group it is uniquely possible to determine reliable three-dimensional orbits for a significant sample of galaxies, in a region large and massive enough to provide a fair probe of the mass density in the Universe. Analysis of such orbital information provides direct constraints on the initial spectrum of perturbations in the early Universe, on the global cosmological density parameter Omega, and on the relative distributions of mass and light on length scales up to 1 Mpc.
The required measurements are distances and transverse velocities for the relatively isolated members of the Local Group, those more distant than about 100 kpc from another large galaxy. Improved distance will be derived from the Gaia-calibrated standard distance indicators such as Cepheids and RR Lyraes. Radial velocities are known. The missing key information is the transverse motion, which will be derivable uniquely from the Gaia proper motion.
Gaia may provide the necessary measurements for more than 20 galaxies within 1.5 Mpc and for a few large spirals out to 2.5 Mpc.
Galaxies
Ongoing studies, using redshift and imaging surveys of galaxies and the microwave background experiments require very wide area imaging surveys with high spatial resolution to provide high-reliability catalogues of galaxies and quasars extending to low Galactic latitudes. It will detect and provide multicolour photometry with about 0.35 arcsec spatial resolution for all sufficiently high surface brightness galaxies crossing the focal plane. This provides a valuable and unique data set at two levels: for statistical analysis of the photometric structure of the central regions of many tens of thousands of galaxies; and for study of the large-scale structure of the local Universe.
One of Gaia's scientific goals concerns the amplitude, shape and length of structures in the Local Universe. If we are to understand local large-scale structure, a reliable near all-sky galaxy survey is essential. The Gaia Galaxy Survey will be an all-sky, complete magnitude-limited survey and will fulfill this role. The combination of Gaia's spatial resolution and multi-colour photometry will allow substantially improved analysis which will also provide multi-colour information for individual galaxies, allowing detailed multi-coloured photometric studies of their central regions. This will include those galaxies for which redshifts are being obtained directly linking morphology and spectra.
Gaia will produce photometrically useful multi-colour image data for at least one million galaxies.
Supernovae
Gaia will detect all objects brighter than I=20, so that in principle supernovae can be detected to a modulus of m-M=39 mag corresponding to a distance of 500 Mpc or z ~0.10. Simulations show that in 4 years, Gaia will detect about 100 000 supernovae of all types. This will allow determination of the supernovae rate in galaxies as a function of galaxy type, as well as discovery of supernovae in low surface-brightness galaxies, which generally are excluded from present surveys.
Jets of AGN
Gaia astrometry will provide a completely new way of studying relativistic jets of AGN. Ground-based radio interferometry shows these are compact down to scales of 100 microarcsec and contain much structure on small scales. VLBI observations trace plasma knots which are moving in these jets and find apparent velocities in excess of the speed of light (c), indicating relativistic speeds and small inclinations from the line-of-sight. The knots emit synchrotron emission. In a few jets of nearby radio galaxies direct optical imaging is possible with the Hubble Space Telescope. In these jets a close similarity between radio knots and optical emission is found. This suggests that the relativistic particles which are accelerated in the jets reach sufficiently high energy to emit optical synchrotron radiation. The motion of plasma knots can hence be studied at optical frequencies as well.
The amplitude of jet photocentric motions will be resolvable for Gaia only in nearby nuclei, where the effect will exceed a milliarcsec within a few months. Such studies will allow unprecedented investigations of the trajectories of plasma knots in general (with a temporal sampling clearly exceeding those achieved in VLBI experiments). It will allow detailed studies of the synchrotron cooling of the highly relativistic particles responsible for the optical emission.
Quasars
The astrometric programme to V=20 mag will provide a census of about 500000 quasars at intermediate-to-high Galactic latitudes. The surface density of approximately 25 per square degree will allow a direct link between the Gaia astrometric reference system and an inertial frame. It will also provide a substantial, uniformly selected quasar sample for scientific analysis.
The Gaia broadband filter system, with coverage over the full optical waveband 300-900 nm is well-suited to the identification of quasars via established multi-colour selection techniques. The vast majority of the quasars will have redshifts between 0.2 and 3, and absolute magnitudes -28 < M<sub>v</sub> < -23. Redshifts to a precision of 0.01 or better can be obtained by cross-correlation of the Gaia spectra with a small library of existing reference quasar spectra. The Gaia catalogue will include a factor of approximately 50 more objects than the quasar catalogue planned following the completion of the SLOAN survey.
Existing ground-based studies of gravitational (macro)-lensing among the quasar population are restricted to resolutions of about 1 arcsec. Gaia will provide sensitivity to multiply imaged systems with separations as small as 0.2 arcsec.
The Gaia survey will provide new constraints on lensing by the bulk of the galaxy population, including spiral galaxies, rather than the high-mass tail of ellipticals to which existing surveys are predominantly sensitive.
Galactocentric Acceleration
The Sun's absolute velocity with respect to a cosmological reference frame causes the dipole anisotropy of the cosmic microwave background. The Sun's absolute acceleration can be measured astrometrically: it will result in proper motions of quasars.
Radio/Optical Reference Frame
The major improvement in the precision of proper motions by Gaia implies that they should be referred to a non-rotating reference system. The Gaia reference frame materializing the reference system must be such that the biases introduced by its inaccuracy should be significantly smaller than the random errors of the phenomena that are referred to it.
The International Celestial Reference System (ICRS) is primarily materialized by the International Celestial Reference Frame (ICRF) which is based on extragalactic radio source positions. The extension of the ICRF to visible light is via the Hipparcos catalogue which gives sub-milliarcsec rms uncertainties. The Gaia catalogue will permit a materialization of the ICRS more accurate by one or two orders of magnitude than the present realizations.