The M-81 Group of The M-81 Group of Galaxies Galaxies

Brian FlemingBrian FlemingStephanie Stephanie LaMassaLaMassaSeamus RileySeamus Riley(The Chunky Ponies)(The Chunky Ponies)

OutlineOutline

OverviewOverview Makeup of M81 – Major Members & Makeup of M81 – Major Members &

StructureStructure Kinematics of the groupKinematics of the group InteractionsInteractions

The M-81 GroupThe M-81 Group

One of the nearest to the LGOne of the nearest to the LG Similar Morphology and Binary StructureSimilar Morphology and Binary Structure Bordered on either side by bright galaxies NGC 2403 and NGC Bordered on either side by bright galaxies NGC 2403 and NGC

42364236 Between LG and M81 is 3MPc of nothing <-9magBetween LG and M81 is 3MPc of nothing <-9mag Until 1998 only the distances to M81 and NGC 2403 were Until 1998 only the distances to M81 and NGC 2403 were

known via Cephids – limited research, kinematcsknown via Cephids – limited research, kinematcs Angular Radius ~ 8 degreesAngular Radius ~ 8 degrees Dominated by M81 Galaxy – similar to MW or M31 Dominated by M81 Galaxy – similar to MW or M31

MMBB=-20.47 =-20.47 VVrotrot=250km/s =250km/s R~500 kpc R~500 kpc 25 probable companions known25 probable companions known

M81 in Galactic CoordinatesM81 in Galactic Coordinates

•M-81 Cluster flanked by NGC 2403 and NGC 4236

•Filament has angular size ~ 25o

Central M81Central 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

Properties of M81Properties of M81

Karachentsev et al. 2001 – The M81 Karachentsev et al. 2001 – The M81 Group of Galaxies: New Distances, Group of Galaxies: New Distances, Kinematics, and StructureKinematics, and Structure Using HST WFPC2 and Tip of the Red-Using HST WFPC2 and Tip of the Red-

Giant Branch method, determined Giant Branch method, determined distances to 13 of the late type distances to 13 of the late type galaxies around M81 galaxies around M81

15 CMD (V-I) with anywhere from 15 CMD (V-I) with anywhere from 1,000 to 20,000 stars per image1,000 to 20,000 stars per image

Distances and Radial Velocities allow Distances and Radial Velocities allow determination of cluster mass and determination of cluster mass and study of 3-D structure and study of 3-D structure and kinematicskinematics

TRGB for M82TRGB 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

Mass EstimatesMass Estimates

Karachentsev et al. 2001: 16 of the M81 objects have Karachentsev et al. 2001: 16 of the M81 objects have measured radial velocities, so a mass estimate is possible:measured radial velocities, so a mass estimate is possible: Virial Mass Estimate: Virial Mass Estimate:

N=# of objects, N=# of objects, σσ22=dispersion of radial velocities w/respect to =dispersion of radial velocities w/respect to group centroid (group centroid (σσvv=84km/s), and R=84km/s), and RHH=Harmonic radius of the group =Harmonic radius of the group (57kpc)(57kpc)

MMvirvir(M81)=0.94x10(M81)=0.94x101212MM๏๏ NGC 2403 with 4 DIrr and 1 DSph have MNGC 2403 with 4 DIrr and 1 DSph have Mvirvir(NGC 2403)=0.26x10(NGC 2403)=0.26x101212MM๏๏

Groups with dominant central galaxies can use orbital motion:Groups with dominant central galaxies can use orbital motion:

MMORBORB(M81)=1.63x10(M81)=1.63x101212MM๏๏ MMLGLG=1.29x10=1.29x101212MM๏๏

Hvvir RGNNM 211)1(3

212

1 )3

21(

3

32RPORB VR

eGM

Globular Cluster PropertiesGlobular Cluster Properties

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

yetyet

Kinematics:Kinematics:Filament Filament structure: structure:

does not fit does not fit free free

expansion expansion modelsmodels

NCG 2403: NCG 2403: approaching approaching

M81M81

NGC 4236: NGC 4236: moves away moves away from M81 from M81

Relative VelocitiesRelative VelocitiesVelocities 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

Arp Paradox (1982)Arp Paradox (1982)

The number of M81 Companions with The number of M81 Companions with positive and negative peculiar velocities positive and negative peculiar velocities has a ratio of 13:3 has a ratio of 13:3

Suggested that selection occurs when “HI Suggested that selection occurs when “HI line emission of galaxies with line emission of galaxies with VVhh < < 0 km/s 0 km/s is blended with strong Galactic emission” is blended with strong Galactic emission” (Karachentsev et al 2002)(Karachentsev et al 2002)

-this would imply many missing galaxies-this would imply many missing galaxies• • Could also be explained by peculiar Could also be explained by peculiar

velocity of M81 (~50 km/s) with respect to velocity of M81 (~50 km/s) with respect to group center of massgroup center of mass

Hot Gas on Outskirts of M81 Hot Gas on Outskirts of M81 (2003)(2003)Bureau at el. examined HoII using the VLA Bureau at el. examined HoII using the VLA

to determine nature of HI morphologyto 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

HI in the M81 GroupHI in the M81 Group

Distribution of gas between galaxies is Distribution of gas between galaxies is affected by tidal interactionsaffected by tidal interactions

Yun et al. (1994) mapped the H1 Yun et al. (1994) mapped the H1 distribution in the M81 group using the VLAdistribution in the M81 group using the VLA

M81

M82

NGC3077

Integrated H1 Map Palomar sky survey

“south tidal bridge”

“north tidal bridge”

HI in the M81 Group (cont.)HI in the M81 Group (cont.) Results of Yun et al.'s study:Results of Yun et al.'s study:

filamentary structures seen in HI image indicate filamentary structures seen in HI image indicate tidal interactions have disrupted this systemtidal interactions have disrupted this system

M81M81 HI spiral arms trace the optical armsHI spiral arms trace the optical arms HI spiral arms beyond the optical diskHI spiral arms beyond the optical disk

M82M82 H1 streamers > 30 kpc emerging from stellar disk; H1 streamers > 30 kpc emerging from stellar disk;

evidence for tidal disruption of outer H1 disk by evidence for tidal disruption of outer H1 disk by massive companionmassive companion

HI in the M81 Group (cont.)HI in the M81 Group (cont.) Boyce et al. (2001) used the HI Jodrell All-Boyce et al. (2001) used the HI Jodrell All-

Sky Survey (HIJASS) to survey HI in the Sky Survey (HIJASS) to survey HI in the M81 GroupM81 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.

H1 in the M81 Group (cont.)H1 in the M81 Group (cont.)

Study of stellar distributions within the H1 Study of stellar distributions within the H1 regions undertaken by Sun et al. (2005) regions undertaken by Sun et al. (2005) using Beijing-Arizona-Taipei-Connecticut using Beijing-Arizona-Taipei-Connecticut (BATC) survey(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

HI in the M81 Group (cont.)HI in the M81 Group (cont.)

Results from surveyResults from survey coexistence of stellar and HI components coexistence of stellar and HI components

in the intergalactic regionin the intergalactic region stars are probably tidal in originstars are probably tidal in origin further evidence of disruption of M82 further evidence of disruption of M82

disk from M81 and M82 passing by each disk from M81 and M82 passing by each otherother

BibliographyBibliography

Arp, H. 1982, ApJ, 256, 54Arp, H. 1982, ApJ, 256, 54Bureau, M., Walter, F., van Gorkom, J., Carignan, Bureau, M., Walter, F., van Gorkom, J., Carignan,

C. International Astronomical Union C. International Astronomical Union Symposium no. 217, held 14-17 July, 2003 in Symposium no. 217, held 14-17 July, 2003 in Sydney, Australia. Edited by P.-A. Duc, J. Sydney, Australia. Edited by P.-A. Duc, J. Braine, and E. Brinks. San Francisco: Braine, and E. Brinks. San Francisco: Astronomical Society of the Pacific, 2004., Astronomical Society of the Pacific, 2004., p.452p.452

Karachentsev, I. D., et al. 2002a, A&A, 383, 125Karachentsev, I. D., et al. 2002a, A&A, 383, 125Schroder et al. – Spectra of 44 M-81 GC using Keck I, Schroder et al. – Spectra of 44 M-81 GC using Keck I,

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