Surveying the Extremities Surveying the Extremities of the Magellanic Cloudsof the Magellanic Clouds

A work in progressA work in progress

A. Saha

Collaborators: Ed

Olszewski Chris

Smith Knut Olsen Jason

Harris Armin Rest Pat Knezek Brian

Brondel Pat

Seitzer

Nick SuntzeffA SubramaniamKem CookDante Minniti(Andrew

Dolphin)

The SurveyThe Survey Imaging with MOSAIC on Blanco 4m of ~40 fields ( plus ~20 Imaging with MOSAIC on Blanco 4m of ~40 fields ( plus ~20

control fields) between 7 and 15 degrees from LMC and/or control fields) between 7 and 15 degrees from LMC and/or SMC, and along Magellanic stream (both, trailing and leading SMC, and along Magellanic stream (both, trailing and leading arms)arms)

5 bands: C, R, I, M, DDO515 bands: C, R, I, M, DDO51

Gets to R ~24, C~25, I~23.5, well past oldest possible TO. Gets to R ~24, C~25, I~23.5, well past oldest possible TO. Shallower in M and DDO51Shallower in M and DDO51

Study distribution of stars + abundances + ages as data allowStudy distribution of stars + abundances + ages as data allow

Identify giants at LMC/SMC like distances and in Galactic halo Identify giants at LMC/SMC like distances and in Galactic halo for follow up kinematic studies and spectroscopic abundancesfor follow up kinematic studies and spectroscopic abundances

Why study extremities?Why study extremities? In our Galaxy the most metal poor (and oldest?) stars are In our Galaxy the most metal poor (and oldest?) stars are

distributed in a spheroidal halo extending beyond 25 kpc.distributed in a spheroidal halo extending beyond 25 kpc.

Their spatial distribution, chemical composition, and Their spatial distribution, chemical composition, and kinematics provide clues regarding the Galaxy’s early history, kinematics provide clues regarding the Galaxy’s early history, as well as its continued interaction with neighboring galaxies as well as its continued interaction with neighboring galaxies -- e.g. what is the distribution of the -- e.g. what is the distribution of the oldestoldest stars along the line stars along the line between the LMC and SMC, and what does that tell us about between the LMC and SMC, and what does that tell us about the history of interaction between these objects?the history of interaction between these objects?

An extended stellar structure provides dynamical clues about An extended stellar structure provides dynamical clues about a Dark Matter halo a Dark Matter halo

Ignorance of the extended stellar structure (or lack thereof) in Ignorance of the extended stellar structure (or lack thereof) in our nearest neighbor is an embarassment!our nearest neighbor is an embarassment!

The nature of stellar halosThe nature of stellar halos

• Characterized by extended spatial distribution, high Characterized by extended spatial distribution, high velocity dispersion, and low chemical enrichment velocity dispersion, and low chemical enrichment

---- but until now all 3 only measured in the Galaxy ---- but until now all 3 only measured in the Galaxy

• What makes a stellar halo? What makes a stellar halo? -- original component ?-- original component ? -- mainly accreted?-- mainly accreted? -- mixture of both?-- mixture of both?Studies within the Galaxy are inconclusiveStudies within the Galaxy are inconclusive HST studies in M31 still argue about what the real halo isHST studies in M31 still argue about what the real halo is

• What relationship is there between a stellar halo and a What relationship is there between a stellar halo and a DM halo?DM halo?

L/SMC specific issuesL/SMC specific issues

How old are stars in the extremities of the Clouds, and what are How old are stars in the extremities of the Clouds, and what are their chemical properties?their chemical properties?

How are stars in the L/SMC extremities distributed? ---How are stars in the L/SMC extremities distributed? ---

--- Disk-like (exponential)? --- Disk-like (exponential)?

--- Halo-like (DeVaucouleurs or power law?) --- Halo-like (DeVaucouleurs or power law?)

--- How far do they extend? --- How far do they extend?

--- Continuity between LMC and SMC?--- Continuity between LMC and SMC?

--- Interaction with Galaxy halo?--- Interaction with Galaxy halo?

Interaction if any between stars in Galaxy halo with stars in outlying Interaction if any between stars in Galaxy halo with stars in outlying LMC/SMC regions -- kinematic signatures?LMC/SMC regions -- kinematic signatures?

Evidence of tidal stripping of Evidence of tidal stripping of starsstars from the L/SMC by Galaxy from the L/SMC by Galaxy

Structure of the LMC diskStructure of the LMC disk 2MASS and DENIS2MASS and DENIS

-- shows elongation of the -- shows elongation of the LMC disk and its tilt (van LMC disk and its tilt (van der Marel 2001, AJ 122, der Marel 2001, AJ 122, 1827)1827)

-- yields disk scale length-- yields disk scale length

-- not useful beyond ~8 -- not useful beyond ~8 degrees from LMC centerdegrees from LMC center

-- not much information on -- not much information on metallicitiesmetallicities

Star count densities de-projected to the plane of the LMC Note intrinsic elongationDisk scale length = 1.2 - 1.4 kpc

Relative Positions of LMC - SMC- GalaxyRelative Positions of LMC - SMC- Galaxy

…van der Marel(2001) AJ, 122, 1827

Indirect Evidence for LMC haloIndirect Evidence for LMC halo(an incomplete discussion)(an incomplete discussion)

Feast (1964, 68) - PN velocity dispersion ~22 km/s in LMC exceeds that of Feast (1964, 68) - PN velocity dispersion ~22 km/s in LMC exceeds that of HHIIII regions (~10 km/s) regions (~10 km/s) a less flattened system a less flattened system

Schommer et al. (1992) - used kinematics of old clusters (instead of PN) to Schommer et al. (1992) - used kinematics of old clusters (instead of PN) to show the same. But also argued that an ~30 km/s dispersion is insufficient for show the same. But also argued that an ~30 km/s dispersion is insufficient for an isothermal halo an isothermal halo

Hughes et al (1991) - Short period (~180 days) Miras in the LMC have Hughes et al (1991) - Short period (~180 days) Miras in the LMC have velocity dispersion ~ 33 km/s. But Hartwick & Cowley (1989, 91) -- CH stars velocity dispersion ~ 33 km/s. But Hartwick & Cowley (1989, 91) -- CH stars have dispersion of only ~24 km/s have dispersion of only ~24 km/s

Kinman (1992) -- RR Lyraes as far out as 15 degrees have mags and Kinman (1992) -- RR Lyraes as far out as 15 degrees have mags and velocities consistent with LMC membership. Observed distribution fits a King velocities consistent with LMC membership. Observed distribution fits a King model with central density of 22 RRLs/sq degree. Suntzeff (1992) and Alves model with central density of 22 RRLs/sq degree. Suntzeff (1992) and Alves (2004) argue show they are also consistent with an extended disk model, a (2004) argue show they are also consistent with an extended disk model, a possibility also discussed by Kinman.possibility also discussed by Kinman.

Minniti et al (2003) - Velocity dispersion of RRLs over the LMC Minniti et al (2003) - Velocity dispersion of RRLs over the LMC barbar of 53 of 53 10 10 km/skm/s

Direct detection of extended Direct detection of extended structurestructure

Irwin (1991) -- star counts from UKSTUIrwin (1991) -- star counts from UKSTU

Stryker (1984) -- photometry from 4m PF Stryker (1984) -- photometry from 4m PF plates of sky around NGC 2257plates of sky around NGC 2257

Gallart et al (2004) -- Deep CMD 8 Gallart et al (2004) -- Deep CMD 8 degrees from the LMCdegrees from the LMC

Designing a new SurveyDesigning a new Survey

What we want to map:What we want to map:1.1. Spatial Distribution of star densities at very low levels Spatial Distribution of star densities at very low levels

-- needs separation from Galactic fore-ground stars -- needs separation from Galactic fore-ground stars solution: use MS stars: solution: use MS stars: has stars of all ages and has stars of all ages and abundances -- UNBIASED !!abundances -- UNBIASED !!

2.2. Metallicities and ages of component populationsMetallicities and ages of component populations

3.3. Kinematics of representative stars - requires Kinematics of representative stars - requires identification of individual stars as members of identification of individual stars as members of LMC/SMC/Galaxy-haloLMC/SMC/Galaxy-halo

4.4. Distinction from background galaxiesDistinction from background galaxies

Survey requirementsSurvey requirements

Need at least 4 bands to separate Need at least 4 bands to separate temperature, metallicity, age, reddeningtemperature, metallicity, age, reddening

Giant vs dwarf separationGiant vs dwarf separation

Mitigate age-metallicity degeneracyMitigate age-metallicity degeneracy

Need to control systematic errors, especially in R and I !!

Preliminary Results

7 degrees North of LMC

9 degrees North

11 degrees North

12.5 degrees North

Control Field : l = 225, b = -75

Preliminary ConclusionsPreliminary Conclusions CRI photomtery (2 hrs per pointing) very cleanly CRI photomtery (2 hrs per pointing) very cleanly

identifies the MS stars in LMC and SMC. identifies the MS stars in LMC and SMC.

Provides a handle to study differences in metallicities Provides a handle to study differences in metallicities and agesand ages

At first sight, the age of the extended structure is metal poor and old, At first sight, the age of the extended structure is metal poor and old, but not “ancient”but not “ancient”

Can be counted -- after corrections for FG and BG Can be counted -- after corrections for FG and BG pollution -- and used as probes of stellar density …. pollution -- and used as probes of stellar density …. Expected to work past 15 kpcExpected to work past 15 kpc

Could be used to search for stars formed in the wake of Could be used to search for stars formed in the wake of the Magellanic streamthe Magellanic stream

Survey PracticalitiesSurvey Practicalities

Need R ~24 & I ~23.5 w S/N ~ 20; C ~25 w S/N ~5 ==> 4m class Need R ~24 & I ~23.5 w S/N ~ 20; C ~25 w S/N ~5 ==> 4m class telescope. T/his also provides S/N > 50 at R~21 in all above bandstelescope. T/his also provides S/N > 50 at R~21 in all above bands

M and DDO51 @ S/N ~ 50 at R ~21M and DDO51 @ S/N ~ 50 at R ~21

Need large areas ==> MOSAIC on Blanco 4m is the best available Need large areas ==> MOSAIC on Blanco 4m is the best available

Observing times ~ 4hrs per field Observing times ~ 4hrs per field

Area per field = 36x36 arc-min square = 0.36 sq degreesArea per field = 36x36 arc-min square = 0.36 sq degrees

““Reasonable” time request: 30 nights ==> ~ 60 fieldsReasonable” time request: 30 nights ==> ~ 60 fields

Need control fields: wide range of Galactic latitudesNeed control fields: wide range of Galactic latitudes

An Early ResultAn Early Result Counting stars in the MS in the 4 LMC outskirt Counting stars in the MS in the 4 LMC outskirt

fields shown:fields shown:

Another Early Result…Another Early Result…

StatusStatus

Feasibility for attaining science goals Feasibility for attaining science goals demonstrated demonstrated

Data collection in progress in earnest -- approx Data collection in progress in earnest -- approx half way throughhalf way through

Processing and photometry to requisite precision Processing and photometry to requisite precision proved feasibleproved feasible

Production processing to begin soonProduction processing to begin soon

Early ResultsEarly Results

Extended structure to 10 disk scale Extended structure to 10 disk scale lengths appears disk-likelengths appears disk-like

Dominant age in LMC periphery (albeit Dominant age in LMC periphery (albeit limited sample) is ~8 Gyrs: significantly limited sample) is ~8 Gyrs: significantly younger than oldest clustersyounger than oldest clusters

Early hints that Magellanic Stream has Early hints that Magellanic Stream has stars after allstars after all

Complete Coverage?Complete Coverage?

Depending on what complexities this Depending on what complexities this survey reveals, there will likely be a survey reveals, there will likely be a strong case for doing a “filled” survey over strong case for doing a “filled” survey over ~1000 square degrees.~1000 square degrees.

Ideal case for LSST !!!Ideal case for LSST !!!

27 degrees West