The Large-Scale Disk Structure of the LMCAs 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 tidesof other objects, due north from the bar the structureis remarkably smooth, out to amazing distances.

2) The LMC and SMC are at the perfect distances tostudy from the ground. Let’s take advantage of thiswith DECcam and/or LSST to map it in entirety.

3) It would sure be nice to do even better withGLAO capable of superb photometry (GLAO aloneIs not sufficient).

First, a quick review of our survey…

55 36x36 arcmin fields using Mosaic2at CTIO. About ½ of a December night perfield in 5 filters: C,M,DDO51, R,IThis is roughly a 1% fill factor for the LMC.

Exposure times calculated to reach below“ancient” turnoff.Exposure times for M and DDO 51 calculatedto well-measure the giants.

C gives metallicity information, and C-R or C-Igives a large wavelength baseline.

Roughly 30 4m nights and 20 0.9m nights

Variety of exposure times to increase dynamicrange of CMDs.

Data resampled and coadded. Photometrytakes into account changes in focal plane,and is appropriately coadded to give finalPhotometric star lists. Deep R image isthe fiducial.

Here is a sample field from 7-20 degreesNorth 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 lightcirrus.

Properly weighted transformationfrom many-stds 0.9m to 4m. Using just a zeropointand color term maps calibrated 0.9m dataonto 4m data.

Note those pesky galaxies.

Now imagine that we are 10x farther away.

Imagine using ACS.

Imagine using wide-field GLAO and pullingout similarly superb photometry.

We probably all think of LMC/SMC or Milky Way/LMC/SMCas interacting galaxies.

And there’s evidence: Magellanic Stream, other H Ifilaments, anomalous C star velocities, etc…

But the several-year-old HST proper motion seemsto say that either the Clouds are coming in for the first time,or that the orbital period is a substantial fraction of theMilky Way assembly time.

Looking for evidence of tides in the outer stellar populationsthus seems worthwhile.

There are in fact claims that the LMC stretches, atleast in one stellar filament, at least 23 degrees,and is seen behind the Carina dwarf. We willbe testing that claim in November.

So we can map the density distribution of the LMCto large radii using main-sequence stars, givingus unprecedented sensitivity.

We can measure LMC surface brightness, usingstar 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 farout 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 1990Disk scale length 0.7 kpc, 22’.Optical to 40’, HI to 190’

DDO 154, Carignan et al 1988Disk scale length 0.5 kpcHI to 16 disk scale lengths

NGC 3741- Begum et al 2005Disk 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 populationsreaching 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?