Extragalactic AO Science
James Larkin AOWG Strategic Planning Meeting September 19, 2004
Galaxies quickly shrink below 1” in size making ground-based observations difficult, but their sub-structures like bulges remain above the Keck diffraction limit to arbitrary redshift.
WM=0.25, WL=0.75, Ho=70 km/s/Mpc
Good Optical/NIR Seeing
Keck Diffraction Limit @ 1.6mm
Sb Galaxy @ z=0.5
0 0 1 2 Redshift 3
At high redshift, optical spectral lines shift into the infrared where AO correction is best and HST has had limited impact.
Magic redshift ~ 2.3
Ha & NII in K band OIII & Hb in H band OII, 4000 Break in J band This is probably the formation epoch of MW-like disks (1” diameter).
Most gravitational lenses occur in areas under a couple of arcseconds, and weakly lensed galaxies are elongated by of order an arcsecond. Even for extended sources, AO on Keck provides increased sensitivity. Especially powerful in identifying point-like sources within galaxy. Crowding of stars in nearby systems prevents accurate analysis of stellar populations. The internal structure of most nearby active nuclei is unresolved with one arcsecond resolution.
Guide star brightness
Very few galaxies have sufficiently bright cores for high-order AO systems. Only ~10-4 of objects are near bright foreground stars Curvature systems are currently doing most of the extragalactic science, but with limited Strehl. Sensitivity increases rapidly with Strehl for point sources, but extended targets gain much less. AO systems produce additional background in Near-IR and reduce throughput further making it difficult to observe faint extended sources. Normal galaxy disks only achieve a maximum SB of K~16 mag/sq arcsec and this fades as (1+z)4. This means all normal disks are fainter than 22.5 mag within 0.05x0.05”.
Galaxy evolution improves this affect.
Observations take hours even for imaging.
What will the laser do…
Provide consistent performance on variety of sources. Allow for target selection by characteristics. Open up HST deep fields and ground based redshift fields.
Brightest star within ultra deep field is R~15 mag
Opens up the study of rare but important objects such as Lyman-break galaxies, sub-mm galaxies, and ultraluminous infrared galaxies. Allow studies of stellar populations as a controlled function of radius. Improves Strehl since extragalactic sources have depended on off-axis guide stars. Generally beneficial to all areas of extragalactic science.
What would higher order do for you without a laser
Reduce fraction of sky available, probably becoming totally dependent on foreground off-axis stars. Increased sensitivity to point sources, and better contrast. Probably only beneficial to a few areas of stellar population studies if still dependent on natural guide stars.
Other areas that will benefit extragalactic science…
Cleaner (or better coatings) and colder AO systems, and better throughput.
K–band is probably the most important filter Local thermal background can devastate faint object work.
Integral field spectroscopy
Avoids slit losses. Samples complex geometry. Multiplex advantage on resolved stellar populations. SINFONI is commissioned on VLT.
9 out of 12 approved science verification programs are extragalactic
Some big questions future AO could address
Assembly of galaxy masses. Complex kinematics at z~1, Lyman break kinematics at z~3. Modern mass disks at z~2? Variations within NLR of individual AGN, and detailed comparisons of many AGN. Testing standard paradigm. Evolutionary (or not) linkages between ULIRGS, Quasars and normal galaxies. Cosmological constant – High redshift type-Ia supernovae. Formation of bulges and tie to central black hole.
Central velocity dispersions in local galaxies. Bulge formation tied to quasar epoch? Test new CDM models of galaxy formation.
Technology with biggest impact
Laser, especially with faint TT magnitudes PSF Characterization (stability,telemetry)– accurate photometry and morphology General improvements: better wavefront sensor CCD, faster reconstructor, cleaner optics.