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The M-81 Group of Galaxies

The M-81 Group of Galaxies. Brian Fleming Stephanie LaMassa Seamus Riley (The Chunky Ponies). Outline. Overview Makeup of M81 – Major Members & Structure Kinematics of the group Interactions. The M-81 Group. One of the nearest to the LG Similar Morphology and Binary Structure

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The M-81 Group of Galaxies

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  1. The M-81 Group of Galaxies Brian Fleming Stephanie LaMassa Seamus Riley (The Chunky Ponies)

  2. Outline • Overview • Makeup of M81 – Major Members & Structure • Kinematics of the group • Interactions

  3. The M-81 Group • One of the nearest to the LG • Similar Morphology and Binary Structure • Bordered on either side by bright galaxies NGC 2403 and NGC 4236 • Between LG and M81 is 3MPc of nothing <-9mag • Until 1998 only the distances to M81 and NGC 2403 were known via Cephids – limited research, kinematcs • Angular Radius ~ 8 degrees • Dominated by M81 Galaxy – similar to MW or M31 • MB=-20.47 • Vrot=250km/s • R~500 kpc • 25 probable companions known

  4. M81 in Galactic Coordinates • M-81 Cluster flanked by NGC 2403 and NGC 4236 • Filament has angular size ~ 25o

  5. Central M81 • 25 known satellites of M81 • Sample biased towards brightness and location • 11 Sph • 12 Irr (M82, NGC 3077) • 1 Sb (M81) • 2 Sm (NGC 2574, NGC 2976) • At least 44 GC • Missing mass suggests many more undiscovered Dwarves & GC

  6. Properties of M81 • Karachentsev et al. 2001 – The M81 Group of Galaxies: New Distances, Kinematics, and Structure • Using HST WFPC2 and Tip of the Red-Giant Branch method, determined distances to 13 of the late type galaxies around M81 • 15 CMD (V-I) with anywhere from 1,000 to 20,000 stars per image • Distances and Radial Velocities allow determination of cluster mass and study of 3-D structure and kinematics

  7. TRGB for M82 • Sakai & Madore 1999 • Method effective for most objects in M81 since we can still resolve stars in the halo • Age and Metalicity independent • Slight reddening by dust

  8. Mass Estimates • Karachentsev et al. 2001: 16 of the M81 objects have measured radial velocities, so a mass estimate is possible: • Virial Mass Estimate: • N=# of objects, σ2=dispersion of radial velocities w/respect to group centroid (σv=84km/s), and RH=Harmonic radius of the group (57kpc) • Mvir(M81)=0.94x1012M๏ • NGC 2403 with 4 DIrr and 1 DSph have Mvir(NGC 2403)=0.26x1012M๏ • Groups with dominant central galaxies can use orbital motion: • MORB(M81)=1.63x1012M๏ • MLG=1.29x1012M๏

  9. Globular Cluster Properties • Schroder et al. – Spectra of 44 M-81 GC using Keck I, 2002 • Similar to MW GC: • No bimodality detected yet

  10. Kinematics: Filament structure: does not fit free expansion models NCG 2403: approaching M81 NGC 4236: moves away from M81

  11. Relative Velocities Velocities relative to M81 demonstrate: • Cosmological expansion at distances R > 1 Mpc (No galaxies approaching M81) • Galaxies only approaching M81 for 1>R>.55 Mpc (infall zone) • Objects moving both towards and away for R<.55Mpc (“Virialized” Zone) • Zero Velocity surface is R0= (1.05 ±0.07) Mpc

  12. Arp Paradox (1982) • The number of M81 Companions with positive and negative peculiar velocities has a ratio of 13:3 • Suggested that selection occurs when “HI line emission of galaxies with Vh< 0 km/s is blended with strong Galactic emission” (Karachentsev et al 2002) -this would imply many missing galaxies • Could also be explained by peculiar velocity of M81 (~50 km/s) with respect to group center of mass

  13. Hot Gas on Outskirts of M81 (2003) Bureau at el. examined HoII using the VLA to determine nature of HI morphology •Could be the result of tidal interactions with the low mass M81dwA • M81dwA may be a tidal dwarf of HoII, formed by the gravitational collapse of tidal debris from interactions with a third galaxy, UGC 4483 • Tails could be a result of ram pressure stripping through an IGM encounter Search for stellar counterpart to HI in arms would resolve issue

  14. HI in the M81 Group • Distribution of gas between galaxies is affected by tidal interactions • Yun et al. (1994) mapped the H1 distribution in the M81 group using the VLA M82 “north tidal bridge” M81 NGC3077 “south tidal bridge” Palomar sky survey Integrated H1 Map

  15. HI in the M81 Group (cont.) • Results of Yun et al.'s study: • filamentary structures seen in HI image indicate tidal interactions have disrupted this system • M81 • HI spiral arms trace the optical arms • HI spiral arms beyond the optical disk • M82 • H1 streamers > 30 kpc emerging from stellar disk; evidence for tidal disruption of outer H1 disk by massive companion

  16. HI in the M81 Group (cont.) • Boyce et al. (2001) used the HI Jodrell All-Sky Survey (HIJASS) to survey HI in the M81 Group • two dIrr galaxies (Holmberg IX and A952+69) are within the tidal bridge between M81 and M82 • They suggest these galaxies are “tidal dwarf galaxies,” formed from the tidal debris between M81 and M82 • dIrr galaxy Garland is in the tidal tail between M82 and NGC 3077 and may also be a tidal dwarf galaxy.

  17. H1 in the M81 Group (cont.) • Study of stellar distributions within the H1 regions undertaken by Sun et al. (2005) using Beijing-Arizona-Taipei-Connecticut (BATC) survey • Image from survey with the Yun et al. (1994) H1 map overlaid • Region contains M82, most of M81 and half of the north tidal bridge

  18. HI in the M81 Group (cont.) • Results from survey • coexistence of stellar and HI components in the intergalactic region • stars are probably tidal in origin • further evidence of disruption of M82 disk from M81 and M82 passing by each other

  19. Bibliography Arp, H. 1982, ApJ, 256, 54 Bureau, M., Walter, F., van Gorkom, J., Carignan, C. International Astronomical Union Symposium no. 217, held 14-17 July, 2003 in Sydney, Australia. Edited by P.-A. Duc, J. Braine, and E. Brinks. San Francisco: Astronomical Society of the Pacific, 2004., p.452 Karachentsev, I. D., et al. 2002a, A&A, 383, 125 Schroder et al. – Spectra of 44 M-81 GC using Keck I, 2002

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