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Die Lokale Gruppe

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Die Lokale Gruppe Die „lokale Gruppe“ Vortrag im Rahmen des Seminars „Entstehung und Entwicklung der Galaxis“ Vortrag imVortrag Rahmen im des Rahmen Seminars „Entstehung des Seminars und Entwicklung „Entstehung der Galaxis“ und Entwicklung der Galaxis“ Marius Halosar 31.5.2013 Übersicht • Definitionen • Struktur • Mitglieder der verschiedenen Untergruppen • Satellitenscheiben (DoS) • Umgebung und Dynamik der lokalen Gruppe • Morphologie und Eigenschaften von Zwerggalaxien • Populationsboxen • Vorstellung ausgewählter Mitglieder • Nachschlag Lokale Gruppe: Definitionen und Allgemeines • Der Begriff „Lokale Gruppe“ (LG) stammt aus dem Buch von Edwin HUBBLE „the realm of the Nebulae“ (E. Hubble, 1936), in dem er bereits die Mitgliedschaft von 12 Galaxien postulierte. • Eine Gruppe ist eine Ansammlung von 20 – 60 Galaxien mit einer Ausdehnung 7 13 von bis zu 10 Lj, enthält ca. 10 MO. Ihre Mitglieder sind gravitativ gebunden. • Galaxienhaufen bestehen aus bis zu 1000 Galaxien, die sich zu Superhaufen zusammenschließen. Ihre Abgrenzung zu den Gruppen ist nicht ganz eindeutig. • Die LG ist eine Galaxiengruppe, die Bestandteil des Virgo – Galaxienhaufens ist und der Andromeda (M 31) und die Milchstraße (MS) als dominante Komponenten angehören. Diese beiden enthalten 80% der Leuchtkraft der LG. • Die LG enthält 75 Galaxien und hat eine Ausdehnung von 1.8 – 2.4 Mpc. • Da die meisten Zwerggalaxien nur sehr geringe Oberflächenhelligkeiten aufweisen, wurden viele von ihnen erst kürzlich entdeckt. Es ist zu erwarten, dass durch verfeinerte Technik insbesondere in niedrigen galakt. Breiten in naher Zukunft noch weitere gefunden werden. 12 • Gesamtmasse beträgt ca. 2.10 MO • Geschwindigkeitsdispersion innerhalb der LG beträgt σ = 61 8 km/s • In einer Galaxie ist die Verteilungsfunktion des Phasenraums innerhalb des Alters des Universums inkompressibel. Struktur der Lokalen Gruppe Einteilung der LG in 4 Untergruppen: • MS - UG +26 Satelliten (Listen auf getrennter Folie) • M31- UG +35 Satelliten incl. M33 + And I – And XXX + IC 10 • NGC 3109 - UG + 3 Satelliten (Sextans A + B, Antlia-Zwerg) Diese Gruppe ist vom Baryzentrum der LG ca. 1,8 Mpc entfernt; ihre Zugehörigkeit zur LG ist daher fraglich. • Lokale Gruppenwolke besteht aus 8 Satelliten. Es ist eine Gruppe von Zwerggalaxien, die den kompakteren Gruppen nicht unmittelbar zuordenbar ist. Diese sind: dIrr......... Leo A, IC 1613, SagDIG, WLM (Wolf- Lundmark-Melotte) dSph...... Cetus, Tucana, Aquarius (DDO 210) IB(s)m Galaxie NGC 6822 (Barnard`s Galaxie) • Sonderfälle: And IV ist wahrscheinlich keine Galaxie Leo A (Leo III) steht sehr isoliert IC 1613 wird auch zur M31 – UG gezählt. GR 8 (dIrr) liegt extrem weit (2,8 Mpc) und isoliert, ihre Mitgliedschaft wird noch debattiert! (nicht in dieser Liste) Ausdehnung und Mitglieder der LG bezogen auf die gal. Ebene Bekannte Mitglieder der Untergruppe MS-Satelliten aus P.Kroupa, G.Hensler et al.“Local-group tests of dark-matter concordance cosmology“, 2010 and references therein Bekannte Mitglieder der Untergruppe M31 - Satelliten Im Jahr 2013 sind bereits 4 Paper erschienen, die sich mit der Dynamik bzw. der Scheibenstruktur der Begleitsysteme von M31 auseinandersetzen. daraus: M31 EB (μ⍺,μδ) = (34.3, - 20.2) ± 8 μas/yr (van der Marcel et al.,2012) weitere EBs sind nur von M33 und IC 10 bekannt. PAndAS ist ein Survey, der 2008 mit dem Canada France-Hawaii Telesc.von M31+Sat. erstellt wurde. aus L. Watkins et al (2013): „A census of orbital properties of the M31 satellites“ PAndAS = PAN-Andromeda Archeological Survey Entfernungsindikatoren für die Lokale Gruppe • Für die relevanten Distanzen von ~30 kpc bis ~ 3,0 Mpc von Zwerggalaxien innerhalb der Lokalen Gruppe bieten sich in erster Linie folgende photometrischen Methoden der Entfernungsmessung an: 1. Klassische Cepheiden und RR Lyrae Sterne (metallizitätsabhängig) 2. TRGB (Tip of Red Giant Branch) im I-Band 3. Absolute Helligkeit des HB (Horizontal Branch) bes. für alte Sterne. • Für größere Distanzen gut geeignet: 4. Tully – Fischer Beziehung (für Spiralgalaxien) 5. Faber – Jackson Beziehung (für elliptische Galaxien) 6. SBF (Surface Brightness Fluctuation) DoS (disc of satellites) um die MS und um M31 Die Positionen und Distanzen der Begleitgalaxien wurden in ein galaktozentrisches, kartesisches Koordinatensystem transformiert und von der gal. Ebene aus bei bestimmten ℓMS betrachtet. Durch Variation von ℓMS ergeben sich die Parameter einer scheibenförmigen Struktur geringster Dicke: MS Normalenvektor.. ℓMS = 156,4°, bMS = -2,2° also fast perfekt polar! Halbe min. Scheibendicke Δmin = 28,9 kpc, DP = 8,2 kpc (DoS – MS) --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Aus (1):“Here we report the existence Andromeda (99.998% significance) of a planar sub- group of satellites in the Andromedagal axy“ Hiebei wurden 15 Sat. Galaxien zu einer Scheibe zusammengefasst, deren Dicke nur Δmin =14,1 kpc beträgt. (rms) Incl. sind die Ellipsen NGC 147 u. 185, nicht aber M 32 u. M110. Bemerkenswert ist die Orientierung der Scheibe im Raum: aus Sicht der MS liegt sie perfekt „edge on“! Außerdem liegt sie recht genau normal auf die galaktische Ebene! Karte der 3-D Verteilung der MS - DA = 400 kpc R. Ibata et al. (2013) „A vast thin plane Satellitengalaxien (Kroupa et al. of co-rotating Dwarf Galaxies orbiting 2010 „Local group tests of dark...) the Andromeda Galaxy“. (1) Umgebung der Lokalen Gruppe • Die Lokale Gruppe ist unmittelbar von 5 weiteren Galaxiengruppen umgeben. • Wie bei der LG gibt es auch bei diesen Gruppen i.a. 2 beherrschende Galaxien. • Im Umkreis von 7 Mpc gibt es weitere Gruppen mit knapp 30 großen Galaxien. Gruppe ~N Dist. Zentral- Richtg. Mpc objekte Canes 42 4,0 - M 94 Jagd- Venatici I 5,5 M 106 ? hunde M 81 60 3,0 - M 81 Großer 4,5 NG 2403 Wagen Maffei 24 3,0 Maffei 1 Cas IC 342 Cam M 83 50 3,7- M 83 Centaur NGC 5128 5,2 Cen A Hydra Sculptor 22 2-6 NGC 253 Galakt. NGC 55 Südpol Supergalaktische Koordinaten und die supergalaktische Ebene • De Vaucouleurs definierte 1991 ein Kugelkoordinatensystem SGL, SGB, dessen Äquator annähernd in der supergalaktischen Ebene (SGP) lag, wie sie zu dieser Zeit bekannt war. • Schon William Herschel wies vor über 200 Jahren auf die planare Verteilung von Nebeln hin. • Die SGP beschreibt eine planare Struktur, die durch die Anordnung des galaktischen Virgo – Haufens, des Perseus - Pisces -Haufens und des großen Attraktors gebildet wird. • Tatsächlich lässt sie sich aber nicht als ein homogenes Ellipsoid beschreiben. Z.B. variiert die kleine Achse bis zu einem Radius von 80 Mpc gegenüber der normalen SGP z-Achse um 30°. Der SG Nordpol (SGB = 90°) liegt bei l = 47,37°, b = +6,32° (gal.) und bei ⍺ = 18h 55`, δ = +15° 42` (äqu.) Der Nullpunkt (SGL = 0, SGB = 0) bei l = 137,37°, b = 0 (gal.) und bei ⍺ = 02h 55`, δ = +59° 32` (äqu.) Der Virgohaufen liegt bei: Aitov-Projektion des IRAS Survey SGL = 103°, SGB = -2°, D = 17 Mpc Die DYNAMIK der Lokalen Gruppe • Durch die an anderer Stelle beschriebenen, immer genauer gewordenen Distanzmessungen zu den Galaxien im expandierenden lokalen Universum können die Bewegungen relativ zur LG viel exakter gemessen werden. • Sonnengeschwindigkeit rel. zum galakt. Zentrum: VGSR = 219 km/s • Sonnengeschw. rel . zum Schwerpunkt der LG: VLG = 318 km/s • Vektordifferenz V = VLG – VGSR = 135 25 km/s Mit dieser Geschw. fällt die Galaxis in Richtung M 31 (17° Offset) • Das „Local sheet“ (LS) ist ein an die Lokale Gruppe angrenzendes Gebiet aus benachbarten Gruppen mit einem Radius von 7 Mpc. • Es entspricht einem Filament, das die Region vom angrenzenden lokalen Void trennt mit einer internen Geschwindigkeitsdispersion von nur 40 km/s. • Pekuliargeschwindigkeiten VPek = VObs – H0.d • Die LG bewegt sich relativ zum LS mit 66 24 km/s → l = 349, b = +22 • Das LS bewegt sich im System der anderen Galaxien, die max. 3000 km/s Fluchtgeschwindigkeit aufweisen, aber weiter als die 7 Mpc entfernt sind, mit v = 323 25 km/s in Richtung SGL = 80, SGB = -52. (Lokaler Superhaufen) • Eine Komponente dieses Vektors in Richtung Virgo-Haufen beträgt 185 km/s. • Die zweite Komponente von 260 km/s zeigt nach SGL = 11, SGB = -72 nach B.Tully et al., „Our peculiar motion away from the local void“, 2008 Das „local sheet“, das lokale Void und der Virgo-Cluster Das local sheet in der supergalaktischen Ebene (3 Mpc dick!) • Diese Komponente zeigt genau in die Richtung der negativen supergalaktischen Z-Achse und zeigt nicht auf etwas Besonderes hin, sondern weg vom Local Void! (min. 45 Mpc weit u. leer) • Das Dipol-Muster in der CMB zeigt eine Bewegung des LS mit v = 631 22 km/s Richtung l = 276 3, b = 30 3 (SGL=139, SGB = -31). • Diese Bewegung kann zerlegt werden in die schon bekannte Komponente zum lokalen Superhaufen (v = 323 km/s) und . • eine zweite die den großskaligen Einfluss des Hydra - Centaurus - Superhaufen und des noch weiter entfernten Shapley - Superhaufens im Hintergrund berücksichtigt. v = 455 15 km/s (SGL = 80, SGB = -52) ← auf die Sphäre projizierte Bewegungskomponenten des local sheet. nach B.Tully et al., „Our peculiar motion away from the local void“, 2008 Galaxiengruppen im Umkreis von 100 MLj (supergalakt. Ebene) Morphologie von Zwerggalaxien S(a) = Spiralgalaxie dIrr = irreguläre Zwerggalaxie ← UV → M33 = NGC 598, D = 850 kpc IC 1613 = DDO 8, D = 730 kpc (RR Lyrae) dE = ellipt. Zwerggalaxie dSph = sphärische Zwerggalaxie HST → ← 2 MASS NGC 185 = UGC 396, D = 628 kpc Cassiopeia = And VII, D = 790 kpc Eigenschaften
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