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Optical Galactic Plane Surveys

Optical Galactic Plane Surveys

a seminar for Cardiff University on 29 May 2013

Geert Barentsen

May 29, 2013
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  1. Optical Galactic Plane Surveys
    a seminar by Geert Barentsen for Cardiff University on 29 May 2013
    Image: NGC 3293/3324 in VPHAS (credit: Hywel Farnhill)

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  2. Hi. I’m Geert Barentsen
    @GeertMcTwit
    blog.barentsen.be
    github.com/barentsen

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  3. My favourite picture of our Galaxy
    (Credit: Nick Risinger / skysurvey.org)

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  4. INT/IPHAS & UVEX
    VST/VPHAS
    UKIDSS/GPS
    VISTA/VVV
    SCUBA-2/JPS & SASSy
    Herschel/Hi-GAL
    Janet Drew Phil Lucas Mark Thompson
    In Hertfordshire, we <3 Milky Way surveys

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  5. This talk
    1. Introduction
    The IPHAS/UVEX/VPHAS surveys are mapping
    the entire Galactic Plane in visible light.
    2. Scientific rationale
    These projects are a necessary counterpart to
    infrared surveys. Also a key complement to Gaia.
    3. Demonstration
    Discovery and parameter inference of young
    stars using survey photometry.

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  6. IPHAS+UVEX+VPHAS = EGAPS
    • EGAPS stands for “European Galactic Plane Surveys”;
    key members are from institutes in UK/NL/Spain.
    • Composed of two surveys in the North (IPHAS/UVEX)
    and one survey in the South (VPHAS+).
    • Covers the entire Galactic Plane at |b| < 5°
    in u’, g’, r’, i’, Hα (near-simultaneous).
    • 5σ-depth typically at g’ > 22 / r’ > 21.
    (complete to r’ ~ 19; saturated at r’ ~ 13.)

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  7. IPHAS/UVEX surveys
    (Northern Plane)
    Wide Field Camera
    Isaac Newton Telescope (La Palma)

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  8. Isaac Newton Telescope
    • Originally located in Herstmonceux, Sussex (1967-1979).
    • “Moved” to La Palma in 1984 (new dome, mount & primary).
    • Surveys apply for time each semester, like anyone else.

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  9. IPHAS
    INT Photometric Hα Survey
    (Northern Plane)
    r’ i’

    www.iphas.org

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  10. U g’ r’
    astro.ru.nl/uvex
    UVEX
    UV Excess Survey
    (Northern Plane)

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  11. = 15 270 pointings

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  12. NGC 2244 (3°×2°)
    IPHAS H-alpha Credit: Nick Wright

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  13. NGC 2237 (30’×20’)
    IPHAS H-alpha Credit: Nick Wright

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  14. IC 1396 (30’×20’)
    IPHAS Ha+r+i Credit: Nick Wright

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  15. Omegacam
    VLT Survey Telescope (Paranal)
    VPHAS+
    VST Photometric Hα Survey
    (ESO Public Survey)
    www.vphasplus.org

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  16. g’ r’
    u’ i’

    VPHAS+
    VST Photometric Hα Survey www.vphasplus.org

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  17. VPHAS+ Footprint

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  18. NGC 6611 (60’×40’)
    VPHAS H-alpha

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  19. Eta Carinae (7°×4°)
    VPHAS H-alpha Credit: Hywel Farnhill

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  20. NGC 3293 (40’×30’)
    VPHAS H-alpha

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  21. NGC 3293 (40’×30’)
    VPHAS u’

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  22. NGC 3293 / NGC 3324
    VPHAS u’+g’+H-alpha Credit: Hywel Farnhill

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  23. NGC 6530
    VPHAS H-ALPHA Credit: Hywel Farnhill

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  24. VPHAS+ status
    • Data taking started early 2012.
    • Nearly 20% done so far.
    • Data quality is looking superb
    • 0.8” median seeing in r’ (unguided);
    • 0.05 median ellipticity.
    • Reduced data of the first semester was handed over to
    ESO on 30 April - release imminent.

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  25. IPHAS/UVEX status
    • Initial data releases in 2008 (IPHAS) and 2011 (UVEX).
    • IPHAS Data Release 2 (DR2) is imminent
    • 95% of the Northern Plane;
    • 159 milion sources (80% detected at >2 epochs);
    • photometric calibration consistent with SDSS at the
    level of 3%.

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  26. Single-band
    catalogues
    Band-merged
    catalogues
    Source
    catalogue
    Images
    Flag duplicate detections
    Cross-match
    Catalogue
    generation
    Quality
    control
    Reduction pipeline
    (Cambridge / CASU)
    Recalibrate

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  27. Let Δij
    be the magnitude offset
    between exposures i and j;
    minimise ∑∑(Δij + ZPi - ZPj)2
    Photometric re-calibration
    Exploit the overlaps between exposures
    (cf. Glazebrook et al. 1994)

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  28. Let Δij
    be the magnitude offset
    between exposures i and j;
    minimise ∑∑(Δij + ZPi - ZPj)2
    Photometric re-calibration
    Exploit the overlaps between exposures
    (cf. Glazebrook et al. 1994)
    Solve

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  29. Validation against SDSS
    Galactic Longitude
    Galactic Latitude
    Photometric residuals
    (IPHASr’ - SDSSr’) = 0.03

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  30. Secret weapon
    #!/usr/bin/env python
    import numpy
    import multiprocessing
    import astropy.io.fits
    import astropy.wcs
    cf. http:/
    /github.com/barentsen/iphas-dr2

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  31. IPHAS source density near Cygnus
    Credit: Hywel Farnhill

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  32. Completeness

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  33. Scientific rationale

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  34. r’-i’ / r’-Hα diagram
    H-alpha emission is
    common for a wide
    range of rare objects.
    (Corradi et al. 2008)

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  35. r’-i’ / r’-Hα diagram
    2005MNRAS.362..753D
    (Drew et al. 2005)
    Main sequence
    Reddening
    Breaks degeneracy
    between spectral
    type and reddening
    (e.g. Sale et al. 2009).
    Complement to Gaia:
    need to constrain
    extinction to get
    luminosities!

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  36. u’-g’ / g’-r’ diagram
    Credit: Michael Smith
    Powerful tool to
    reveal O/B-types
    and white dwarfs.

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  37. This talk
    1. Introduction
    The IPHAS/UVEX/VPHAS surveys are mapping
    the entire Galactic Plane in visible light.
    2. Scientific rationale
    These projects are a necessary counterpart to
    infrared surveys. Also a key complement to Gaia.
    3. Demonstration
    Discovery and parameter inference of young
    stars using survey photometry.

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  38. Demonstration: T Tauri stars
    Shock emission
    (UV/optical excess)
    Hot inner disk
    (near-infrared excess)
    Warm dust & gas
    (infrared/radio)
    Hot gas; emission lines
    (including H-alpha)
    = young, solar-like stars; < 10 Myr; < 2 Msun

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  39. 2 degrees = 30 pc (d = 900 pc)
    IC 1396 in IPHAS
    Massive star (O6V)

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  40. Area investigated using spectroscopy
    (Sicilia-Aguilar et al. 2003, 2004)
    Area investigated using
    IPHAS (Barentsen et al. 2011)

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  41. r’-i’ / r’-Hα diagram

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  42. Image: Nick Risinger
    Observation bias: 88% of T Tauri stars
    known by SIMBAD are located at |b| > 5,
    where spectrocopic surveys are cheaper
    b = +5
    b = -5
    Orion

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  43. Different environments at larger distances
    150 pc 1000 pc >> 1000 pc
    Taurus
    log N = 2
    IC 1396
    log N = 3
    Tarantula Nebula
    log N = 6?
    Sun’s birth environment thought to be log N = 3-4? (Adams 2010)

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  44. Spectroscopy is the gold standard,
    but survey photometry is cheap & deep:
    • Readily available up to 20th mag;
    • Homogeneous: few biases between regions;
    • Narrow-band filters provide “a low-res spectrum”.
    => Use photometric surveys to analyse objects across
    environments in a homogeneous way.

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  45. NGC 2264
    (Barentsen et al. 2013)

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  46. T Tauri stars
    Goal
    Infer age/mass/extinction using r’ / i’ / J
    Infer accretion rate using H-alpha

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  47. T Tauri spectra ordered
    by accretion rate:

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  48. H-alpha luminosity traces the accretion luminosity

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  49. r’ - i’ / r - Ha diagram traces the H-alpha luminosity

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  50. H-alpha equivalent width can be quantified from the diagram
    AV
    = 5

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  51. ... albeit only if you know the extinction

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  52. Similarly, colour-magnitude diagrams trace ages and masses;
    but masses are also degenerate with extinction

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  53. Typical reddening law (Cardelli et al. 1989)

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  54. r’ - i’ / i’ -J diagram:
    extinction can be constrained for low-mass stars
    (Black line: NextGen model track)

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  55. Data
    Generative model
    Data
    Physics
    what you want:
    Physics
    what you know:
    Inference
    Parameter estimation

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  56. P(physics | data) ∝ P(data|physics) P(physics)
    P(data | physics)
    Bayes’ theorem
    “posterior”
    “likelihood”
    “prior”

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  57. Application
    Given IPHAS/UKIDSS photometry
    {r', Hα, i', J, σr', σHα, σi', σJ}
    we aim to constrain
    {extinction, mass, age, accretion rate }
    taking “nuisance parameters” into account
    uncertain distance (760 ± 5 pc)
    uncertain inner disc radius (5 ± 2 R*)
    uncertain log LHα ~ log Laccretion (±0.43 dex)

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  58. Application (cont.)
    We might assume that the model residuals are Gaussian, i.e.:
    P(data | physics) ∝ exp[ ∑(SEDmodel - SEDobs)2 / σ2 ]
    ... and assume a uniform prior:
    P(physics) ∝ 1
    We want to know the parameter-space regions where the posterior
    is high... In this case, the peak correspond to a chi-squared fit.

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  59. So, chi-squared fitting is great?

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  60. Chi-squared fitting is usually
    not what you want!
    • Your model is rarely ever “just Gaussian”. There are
    often a bunch of nuisance parameters with known
    distributions.
    • Astronomical data is sparse and hence there is often a
    family of degenerate solutions. A maximum-likelihood
    fit does not capture this.
    • Generic solution: write down your posterior and
    compute its distribution in full.

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  61. Probabilistic Graphical Model
    (Barentsen et al. 2013)

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  62. MCMC
    Computing the posterior with “brute force” is often intractable.
    Instead, perform a “biased random walk”
    => Markov Chain Monte Carlo (MCMC) sampling methods

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  63. Example result
    Mass
    Extinction
    Mass
    Age

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  64. Conclusions
    • IPHAS/UVEX/VPHAS are mapping the entire Galactic
    Plane in u’, g’, r’, i’, H-alpha.
    • Get in touch if you would like to use the data,
    or keep an eye out for “IPHAS DR2”.
    • Take-away message: Python and Graphical
    Probabilistic Models are key tools for mining surveys.

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  65. Survey team
    Consortium:
    University of Hertfordshire (IPHAS/VPHAS PI)
    University of Nijmegen (UVEX PI)
    University of Cambridge (pipeline)
    University of Graz
    Other members:
    Instituto de Astrofísica de Canarias, Harvard/Smithsonian CfA, University
    College London, Imperial College London, University of Warwick, University
    of Manchester, University of Southampton, Armagh Observatory, Macquarie
    University, Tautenburg Observatory, ESTEC, University of Valencia.
    Key individuals:
    Janet Drew, Hywel Farnhill, Geert Barentsen, Robert Greimel, Mike Irwin,
    Eduardo Gonzalez-Solares, Romano Corradi, Paul Groot (UVEX lead),
    Danny Steeghs.

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