Highlighting Ultra Diffuse Galaxies: VCC 1287 and Dragonfly 44 AS DISCUSSED IN VAN DOKKUM ET AL. 2016, BEASLEY ET AL. 2016 PRESENTED BY MICHAEL SANDOVAL Overview §Brief Overview of Galactic Structure and Globular Clusters (GCs) §Background of Ultra Diffuse Galaxies (UDGs) §The Search Process: Challenges and Importance §Dragonfly 44 §VCC 1287 §Big Picture: what do the results mean? Crash Course: Galactic Structure and GCs §Focusing on two features: § Globular Clusters (GCs) § Dark Matter Halo (DMH) §GCs extend to the outer halo and are used for dynamics measurements §DMH extends passed the visible structure What is a UDG? •Galaxies the size of the Milky Way, but significantly less bright •Problem: Act like they have very high mass, but have very little mass….? •Large dark matter fractions •Relatively featureless, but round and red •Not sure how they are formed • Perhaps they are “failed” galaxies? • Maybe from tidal forces? Reference: van Dokkum et al. 2016 Reference: Sandoval et al. 2015 Search Process – The Dragonfly •The Dragonfly Telephoto Array • Collaboration between Yale and University of Toronto in 2013 • Designed by Pieter van Dokkum & Roberto Abraham • Multi-lens array designed for ultra-low brightness visible astro • 400mm Canon lenses – as of 2016 Dragonfly has 48 lenses • Cold Dark Matter (CDM) cosmology Reference: Abraham & van Dokkum 2014 Search Process – Computational Method •Utilize images from Canada France Hawaii Telescope (CFHT) •MegaPrime/MegaCam is the wide-field optical imaging facility at CFHT. •Comparable to the Sloan Digital Sky Survey’s (SDSS) navigation tool. •Search through optical images looking for diffuse galaxies. •Analyze photometry The Early Discoveries •47 UDGs found with the Dragonfly in 2014 in the Coma cluster • Combined data with SDSS, CFHT • Radial velocity measurements (Coma: cz ∼ 7090 km/s) •Stony Brook, National Astronomical Observatory of Japan (NAOJ) follow up • Discovery of 854 UDGs in the Coma cluster • Analyzed data from the Subaru Reference: van Dokkum et al. 2015a, Koda et al. 2015 Dragonfly 44: Fun Facts •Chosen because it’s the second brightest, and one of the largest •Apparent magnitude: mg ~ 19.4 •Spectrum obtained with Keck I •No tidal features Reference: van Dokkum et al. 2015a,2016 Dragonfly 44: Spectrum •Radial velocity calculated from best fit • cz = 6280 ± 120 km s-1 •Falls within Coma cluster velocity distribution • Re ~ 4.3 kpc Reference: van Dokkum et al. 2015b Reference: van Dokkum et al. 2015a Dragonfly 44: Mass Calculations •Follow up spectrum obtained with Keck II • Similar radial velocity / effective radius obtained • σ ~ 47 km s-1 • Re ~ 4.3 kpc •Enclosed mass within the half light radius: à •Very high mass considering stellar population •Mass to light ratio is typical for low mass dwarf galaxies, but not for a galaxy this massive Reference: van Dokkum et al. 2016 Dragonfly 44: Dark Matter and GCs 8 •Stellar mass calculated from i-band luminosity: M* ≈ 3 × 10 M⦿ •fdm = [M(r < r1/2 ) − 0.5M*]/M(r < r1/2 ) ≈ 98 % !!!!! •Has ~36 observableGCs, which is similar to what galaxies of comparable mass have • Say RGC = Re × 1.5 ≈ 6.5 kpc • Cutoff magnitude of mg = 27.2 • Estimate a total population of ~96 after corrections 11 •Halo mass estimation: Mhalo ≈ 8 × 10 M⦿ •Ratio of stellar mass to halo mass: ~ 0.0003 • Off by a factor of 100! Reference: van Dokkum et al. 2016, Beasley et al. 2016 Reference: van Dokkum et al. 2016 VCC 1287: Fun Facts •In Virgo, which is known for its abnormalities •Photometry obtained with CFHT / MegaCam •Re ≈ 2.4 kpc Tidally disrupted M85 in Virgo •GC spectra method •No tidal features VCC 1287 Reference: Beasley et al. 2016 Reference: Beasley et al. 2016 VCC 1287: Mass Estimates from GCs •22 ± 8 GCs • Cutoff magnitude of mi = 23.1 •σ ≈ 33 km s-1 •Two approaches for mass estimates 1. Assume GC dispersion is indicative of the system 2. Tracer Mass Estimator: estimate mass out to outermost GC à 7 à M* ≈ 2.8 × 10 M⦿ Reference: Beasley et al. 2016 VCC 1287: Dark Matter Estimates 10 •From simulated mass profiles: Mhalo = (8 ± 4) × 10 M⦿ -5 •Harris et al. 2013: MGCs / Mhalo = 6 × 10 10 à accounting for the ~ 22 GCs… Mhalo ≈ 7.3 × 10 M⦿ … consistent! •Has a halo mass similar to the Large Magellanic Cloud (LMC) •Dark matter fraction 99%! •Ratio of stellar mass to halo mass is ~ 3.5 × 10-4 • Normal would be 10-2 Reference: Beasley et al. 2016 Conclusion and Next Steps •“Matching up” stellar mass with halo mass is risky •Perhaps UDGs were dwarf galaxies that were halted early due to gas starvation as it fell into their cluster, and were unable to grow in stellar mass as it has in dark matter •Counting up GCs vs. direct spectrum •Need more data!.