Active Galactic Nuclei
There are three principal ways in which Active Galactic Nuclei (AGN) will be identified in LSST data. AGN can be identified by matching LSST objects with data at other wavelengths (e.g. radio and infrared). Quasars can be selected directly from colours in the LSST six-band filter system in specific redshift ranges. Finally, AGN can be identified from their optical variability in LSST images. The latter method will be a powerful technique to routinely detect both obscured and unobscured AGN in the LSST era.
Colour selection
Unobscured AGN with a broad range of redshifts can be isolated in well-defined regions of optical– near-IR multicolor space (e.g. Richards et al. 2001). At low redshift (z < 2.5), quasars are blue in u − g and g − r (these are the ultraviolet excess sources of Sandage 1965 and Schmidt & Green 1983), and are well-separated from stars in colour-colour space. Fig 1 shows colours of known quasars and stars in the SDSS convolved with the LSST filters. The u-band data are crucial for selection of low-redshift (z < 2) AGN. Observations in this filter allow one to distinguish between AGN and stars (in particular white dwarfs and A and B stars). High-redshift AGN can be easily distinguished - the y-band filter will allow quasars with redshifts of ~7.5 to be identified. In comparison, SDSS, whose filter set ends with the z band, is able to identify quasars only up to z~ 6.4; e.g. Fan et al. 2006). As with SDSS, most of the sample contamination is in the range 2.5 < z < 3, where quasar colours overlap the stellar locus in most projections. It is also difficult to select quasars at z ∼ 3.5, where Lyman-limit systems cause quasars to be invisible in the u and g-bands but quasars have similar colors to hot stars at longer wavelengths. Nevertheless, lack of proper motion and variability will allow quasars to be efficiently separated from stars in these redshift ranges.
Fig 1: Colour-colour plots of known quasars from SDSS (coloured dots) and stars (black dots) in the LSST photometric system. The colour coding indicates the redshift of the quasars and the dot size is inversely proportional to the expected surface density as a function of redshift. Since there is no y-band filter in the SDSS system, a random Gaussian colour offset has been added to the z − y color according to the width of the stellar locus in the i − z color. The quasar colours are degenerate with F stars at 2.5 < z < 3. Adapted from Fig 10.1 of the LSST Science Book.
AGN selection via variability
Most AGN exhibit broad-band aperiodic, stochastic variability across the entire electromagnetic spectrum, on timescales ranging from minutes to years. Continuum variability arises in the accretion disk of the AGN, providing a powerful probe of accretion physics. The main LSST Wide Fast Deep (WFD) survey will obtain ∼ 108 AGN light curves (i.e., flux as a function of time) with ∼ 1000 observations (∼ 200 per filter band) over 10 years. The Deep Drilling Fields will provide AGN lightcurves with much denser sampling for a small subset of the objects in the WFD survey. The science content of the lightcurves will critically depend on the exact sampling strategy used to obtain the light curves. For example, the observational uncertainty in determining the color variability of AGN will depend strongly on the interval between observations in individual filter bands.