SMASH (Survey of the Magellanic Stellar History)
Description
The Survey of the Magellanic Stellar History (SMASH) used the Dark Energy Camera (DECam) to map 480 square degrees of sky to depths of ugriz ∼24th mag with the goal of identifying broadly distributed, low surface brightness stellar populations associated with the stellar halos and tidal debris of the Magellanic Clouds. It contains measurements of approximately 360 million objects distributed in discrete fields spanning an area of about 2400 square degrees. The first data release (DR1) contained ∼100 million objects from 61 observed fields, while the second data release contains all 360 million objects in 197 fields. Browse this page to learn more about SMASH and how to access the data. The SMASH overview paper (Nidever et al. 2017) describes the survey in detail, including its goals, survey strategy, reduction, and calibration.
The smash_dr2.object table has been crossmatched against our default reference datasets (except SDSS due to 0 matches) within a 1.5 arcsec radius, nearest neighbor only. These tables will appear with x1p5 in their name in our table browser. Example: smash_dr2.x1p5__object__gaia_dr3__gaia_source.
| SMASH Summary | |
|---|---|
| Area covered | 480 deg² spanning ∼2400 deg² |
| Bands | ugriz |
| Depth (5σ, ugriz) | 23.9, 24.8, 24.5, 24.2, 23.5 mag |
| Seeing (ugriz) | 1.22, 1.13, 1.01, 0.95, 0.90 arcsec |
| Number of fields | 197 |
| Number of DECam exposures | 5,982 |
| Number of objects | 360,201,921 |
| Number of measurements | 4,155,114,664 |
| Photometric precision | ∼1% in u and 0.5-0.7% in griz |
| Photometric calibraton accuracy | ∼1.3% in all bands |
| Astrometric accuracy | ∼20 mas |
Scientific Goals
Using old main sequence stars as tracers, SMASH is revealing the relics of the formation and past interactions of the Clouds down to surface brightnesses equivalent to Σ_g=35 mags arcsec². The main project goals are:
- Search for the stellar component of the Magellanic Stream and Leading Arm. The detection of stellar debris in these structures would make them the only tidal streams with known gaseous and stellar components in the Local Group. This would not only be invaluable for understanding the history and observable consequences of the Magellanic interaction, but would give us a dynamical tracer of the MW's dark halo and a way to probe the MW's hot halo gas via ram pressure effects.
- Detect and map the smooth components of the Clouds, including their extended disks and potential stellar halos. The size of the LMC's stellar disk is a direct probe of the tidal radius of the LMC, with which we can explore the dark matter halos of the LMC and MW.
- Detect and map potential streams and substructure in the Magellanic periphery not associated with HI features. These would probe stages in the formation and interaction of the Clouds at times earlier than the HI dissipation timescale.
- Derive spatially resolved, precise star formation histories covering all ages of the MCs and to large radii, thus providing detailed information on their complete evolution.
- Enable many community-led projects, including studies involving the LMC/SMC main bodies, Galactic structure, discovery of variable objects, and background galaxy populations.
Data Releases
Second Data Release (SMASH DR2)
The figure below shows the 197 SMASH fields which are being publicly released in the second SMASH data release (DR2) which contains ∼4 billion measurements of ∼360 million objects in fields sampling the ∼2400 deg² region of the SMASH survey (green hexagons in the figure below). The main data access is through the Astro Data Lab. There are seven main SMASH DR2 tables in the database, described in the table below. The "field" table includes summary information for each field.
| SMASH DR2 Tables | |
|---|---|
| Table Name | Description |
| chip | Information on each chip image (359,393 rows). |
| deep | Same as object but only using the deepest exposures for each field (357,395,383 rows). |
| exposure | Information on each exposure (5,982 rows). |
| field | Information on each field (197 rows). |
| object | Average values for each unique object (360,201,921 rows). |
| source | All of the individual source measurements (4,155,114,664 rows). |
| xmatch | Crossmatches between SMASH and Gaia DR2, 2MASS and ALLWISE (4,155,114,664 rows). |
Main Bodies
DR2 contains the first release of SMASH data from the central regions of "main bodies" of the Large and Small Magellanic Clouds. There are 1177 exposures in 68 contiguous SMASH fields of the LMC covering 15 deg x 18.5 deg. 25 fields have deep exposure while 43 only have shallow (∼60 sec) exposures. There are 412 exposures in 13 deep contiguous SMASH fields of the SMC covering 8.5 deg x 7.4 deg. Three color mosaics are shown below (click on them for a zoomable view). They were presented as a poster at the ESO workshop A synoptic view of the Magellanic Clouds: VMC, Gaia and beyond in Garching, Germany on September 9-13, 2019.
First Data Release (SMASH DR1)
| SMASH DR1 Tables | |
|---|---|
| Table Name | Description |
| chip | Information on each chip in the frame (149,165 rows). |
| exposure | Information on each exposure (2,480 rows). |
| field | Information on each target field (61 rows). |
| galaxies | View of object table to select for galaxies (5,937,616 rows). |
| object | Average photometry of each unique object (101,425,210 rows). |
| source | All of the individual source measurements (722,653,189 rows). |
| stars | View of object table to select for stars (24,074,789 rows). |
| xmatch | Crossmatch of object against GAIA DR1 and WISE (4,443,547 rows). |
Data Reduction and Calibration
The SMASH data reduction of the DECam data makes use of three separate software packages: (1) the Community Pipeline for instrument signature removal, (2) PHOTRED for PSF photometry, and (3) SMASHRED, custom software written for PHOTRED pre- and post-processing of the SMASH data.
- Instrument Signature Removal: The NOIRLab DECam Community Pipeline (Valdes et al. 2014) was jointly developed by the Dark Energy Survey Data Management (DESDM) team and NOIRLab to produce reduced images for the community. The CP performs bias and crosstalk correction, masking of bad pixels and other artifacts, linearity correction, flat field and illumination calibration, fringe pattern subtraction, astrometric and photometric calibration, sky pattern subtraction, remapping and coaddition.
- PSF Photometry: PHOTRED is an automated and robust PSF photometry pipeline based on the DAOPHOT suite of programs (Stetson 1987, 1994). It performs WCS fitting, single-image PSF photometry (ALLSTAR), source matching across multiple images, forced PSF photometry across multiple exposures using a master source list created from iterative detection from a deep multi-band "detection" stack (ALLFRAME), aperture corrections, and dereddening. PHOTRED was run on each nightly separately after the CP-reduced images were pre-processed using the "SMASHRED_PREP.PRO" script. The precision of the photometry for bright stars is ∼1% in u and 0.5-0.7% in griz.
- Calibration: Since most of the SMASH data are in disconnected "island" fields and little prior ugriz data exists in the southern sky, we used traditional techniques of calibrating our data with observations of standard star fields (on photometric nights; SDSS equatorial fields) and extra calibration exposures (for non-photometric nights) from the CTIO 0.9m telescope. The DECam standard star field exposures were processed with STDRED, a sister package to PHOTRED, which works in a similar manner. Then, custom software (SOLVE_TRANSPHOT.PRO) was used to derive the photometric transformation equations for all of the DECam standard star field data combined. Finally, the DECam science data were calibrated using custom SMASH software (SMASHRED_CALIBRATE_FIELD.PRO) that ties all chip data for a given field onto the same photometric zero-point (via overlaps) using an ubercal technique and calibrates the zero-point using the DECam transformation equations for photometric nights or the 0.9m data for non-photometric nights. The accuracy of the photometric calibration is ∼1.3% in all bands.
Data Access
The SMASH data are accessible by a variety of means:
Data Lab Table Access Protocol (TAP) service
TAP provides a convenient access layer to the SMASH catalog database. TAP-aware clients (such as TOPCAT) can point to https://datalab.noirlab.edu/tap, select the smash_dr2 database, and see the database tables and descriptions. You can also view the SMASH tables and descriptions in the Data Lab table browser.
Data Lab Query Client
The Query Client is available as part of the Data Lab software distribution. The Query Client provides a Python API to Data Lab database services. These services include anonymous and authenticated access through synchronous or asynchronous queries of the catalog made directly to the database. Additional Data Lab services for registered users include personal database storage and storage through the Data Lab VOSpace.
The Query Client can be called from a Jupyter Notebook on the Data Lab Notebook server. Example notebooks are provided to users upon creation of their user account (register here), and are also available to browse on GitHub at https://github.com/astro-datalab/notebooks-latest.
Image Cutouts
The Data Lab Simple Image Access (SIA) service provides a fast way to retrieve cutouts from SMASH images. See our Jupyter notebook on how to use the SIA service. For SMASH DR2, the SIA URL is:
https://datalab.noirlab.edu/sia/smash_dr2
For SMASH DR1, the SIA URL is:
https://datalab.noirlab.edu/sia/smash_dr1
Jupyter Notebook Server
The Data Lab Jupyter Notebook server (authenticated service) contains examples of how to access and visualize the SMASH catalog:
- Detecting the Hydra II dwarf galaxy in SMASH DR1
- Exploring SMASH DR2
- Time-series analysis of a RR Lyrae star
Results
Hydra II
The SMASH data have already produced some exciting results. In Martin et al. (2015), we presented the discovery of a compact and faint Milky Way satellite, Hydra II (in Field169), with morphological and stellar population properties consistent with being a dwarf galaxy (also see Kirby et al. 2015). Interestingly, comparison with simulations suggests that at Hydra II's position in the sky and distance of 134 kpc (from blue horizontal branch stars) it could be associated with the Leading Arm of the Magellanic Stream, although proper motion information is needed to confirm. We obtained follow-up time-series data on Hydra II to study its variable stars. This work yielded one RR Lyrae star in Hydra II that gave a slightly larger distance of 151 kpc as well as the discovery of dozens of short period variables in the field (Vivas et al. 2016).
SMASH 1
Further sensitive searching for overdensities in the SMASH data yielded the discovery of a compact and very faint (Mᵥ=-1.0) stellar system (SMASH 1) ∼11 degrees away from the LMC. SMASH 1 is consistent with being an old globular cluster in the LMC periphery likely associated with the LMC disk and potentially on the verge of tidally disrupting (Martin et al. 2016).
Extended LMC Stellar Populations
One of the on-going SMASH projects is to map out the extended stellar populations of the LMC. An analysis of the Hess diagrams indicates that LMC stellar populations can be detected in SMASH data out to 21.1 degrees from the LMC center, or ∼18.4 kpc, and to surface brightness levels of ∼33.3 mag/arcsec² (Nidever et al. 2017).
Star Formation Histories
One of the main goals of SMASH is to use the data in the central LMC/SMC fields to derive spatially-resolved star formation histories. The Hess diagram of Field55 in the figure below is an example of the wealth of information in the data. This field, and other nearby ones, in particular, show two subgiant branches which indicates two periods of peak star formation. This was previously only seen in star formation rate diagrams from detailed star formation history modeling (Meschin et al. 2014), but now is visually clear just in the Hess diagrams. Full star formation history modeling still await computationally intensive artificial star tests for the SMASH data, which will be a focus of on-going SMASH processing efforts in the near future.
Citizen Science Cluster Detection
The deep and multi-band data in the main bodies of the Magellanic Clouds are also very useful for detecting faint star clusters. We have developed a citizen science project (led by L .Cliff Johnson) based on the SMASH data under the Zooniverse platform which currently has roughly one million users and hosts many citizen science projects in multiple scientific disciplines. The project is called "Local Group Cluster Search" The citizen scientists will inspect our deep co-add ugriz images and visually identify (a) star clusters (open and globular), (b) galaxies behind the LMC/SMC main bodies, (c) and new dwarf galaxies of MW or the MCs. The SMC portion was completed in early-mid 2019 and the LMC portion will start in late 2019.
