The Large-Scale Disk Structure of the LMC As Measured by the OUTER LIMITS SURVEY Olszewski, Saha, Smith, Olsen, Harris, Rest, Knezek, Brondel, Seitzer, Suntzeff, Subramaniam, Cook, Minniti, Dolphin, J. Claver

Main ideas for today: 1)Despite evidence that the LMC HAS felt tides of other objects, due north from the bar the structure is remarkably smooth, out to amazing distances. 2) The LMC and SMC are at the perfect distances to study from the ground. Let’s take advantage of this with DECcam and/or LSST to map it in entirety. 3) It would sure be nice to do even better with GLAO capable of superb photometry (GLAO alone Is not sufficient).

First, a quick review of our survey… 55 36x36 arcmin fields using Mosaic2 at CTIO. About ½ of a December night per field in 5 filters: C,M,DDO51, R,I This is roughly a 1% fill factor for the LMC. Exposure times calculated to reach below “ancient” turnoff. Exposure times for M and DDO 51 calculated to well-measure the giants. C gives metallicity information, and C-R or C-I gives a large wavelength baseline.

Roughly 30 4m nights and m nights Variety of exposure times to increase dynamic range of CMDs. Data resampled and coadded. Photometry takes into account changes in focal plane, and is appropriately coadded to give final Photometric star lists. Deep R image is the fiducial. Here is a sample field from 7-20 degrees North of LMC center. Ages 14,8,2,0.5 Gyr; metallicty increasing. Most stars 8Gyr or less.

Calibration done using 0.9m telescope, 2 0.9m fields per 4m field. This allows us to observe at 4m in light cirrus. Properly weighted transformation from many-stds 0.9m to 4m. Using just a zeropoint and color term maps calibrated 0.9m data onto 4m data.

Note those pesky galaxies. Now imagine that we are 10x farther away. Imagine using ACS. Imagine using wide-field GLAO and pulling out similarly superb photometry.

We probably all think of LMC/SMC or Milky Way/LMC/SMC as interacting galaxies. And there’s evidence: Magellanic Stream, other H I filaments, anomalous C star velocities, etc… But the several-year-old HST proper motion seems to say that either the Clouds are coming in for the first time, or that the orbital period is a substantial fraction of the Milky Way assembly time. Looking for evidence of tides in the outer stellar populations thus seems worthwhile.

There are in fact claims that the LMC stretches, at least in one stellar filament, at least 23 degrees, and is seen behind the Carina dwarf. We will be testing that claim in November. So we can map the density distribution of the LMC to large radii using main-sequence stars, giving us unprecedented sensitivity. We can measure LMC surface brightness, using star counts, to effective 34 mag per sq arcsec, and out to more than 10 disk scale lengths.

Count stars in specific loci of the CMD Compare with counts in control fields MS stars in box shown outnumber giants in box shown 60:1, and represent all ages and metallicities

Scale length ~ 1.35 degrees (~ 1.3 kpc !!!)

So the LMC is smooth, in this direction, as far out as we can measure.

Extended Region around LMC/SMC

Here are quite preliminary SMC pencil beams…

SMC dwarf surface count densities Nominal scale length ~ 0.75 degrees (~ 0.75 kpc) but…

Some other galaxies: DDO 170, Lake et al 1990 Disk scale length 0.7 kpc, 22’. Optical to 40’, HI to 190’ DDO 154, Carignan et al 1988 Disk scale length 0.5 kpc HI to 16 disk scale lengths NGC Begum et al 2005 Disk scale length 0.16 kpc, 10” HI to 38 optical scale lengths Most other dIrr galaxies disturbed at large radii.

But here we have detailed stellar populations reaching to the oldest stars.

What’s next? Equivalent study of SMC exterior is ongoing Construct “Hess diagrams” accounting for incompleteness- substantial artificial star studies ongoing Construct empirical Galaxy model “Hess diagram” from control fields. Model multi-component Hess diagrams using stellar evolution models using Bayesian methods and look for depth effects: Tolstoy & Saha 1996, ApJ 462, 672 Dolphin 2002, MNRAS, 332, 91 Identify giants associated with Clouds – follow up spectroscopy for velocities –use brighter MS stars?