plate 3 · the local group
galaxies come in groups. ours is a modest one: two big spirals — the milky way and andromeda — a third, smaller spiral in triangulum, and a hundred-odd dwarf galaxies, several of which were discovered within the last decade. we are still finding them; they are dim, and the sky is large.
the milky way's halo, 600 kpc across
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' ' . leo II' '
' . leo I '
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' sextans . '
' . ursa minor '
. draco
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' sagittarius o @ milky way '
' LMC O '
carina .o
' SMC '
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. sculptor
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' . fornax dsph '
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' ' ' = the dark-matter halo's virial radius, ~250 kpc.
supergalactic plane from above; positions & distances real.
the milky way’s own retinue: the two magellanic clouds (the LMC is on its first infall, dragging the smaller cloud with it), and a shell of dwarf spheroidals. the sagittarius dwarf is currently being eaten — its stars are smeared in a stream around the whole sky. this is how big galaxies grow.
the local group, ~3 Mly across
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' . leo A '
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' milky way o@ '
LMC+SMC o IC 10
' . NGC 185/147 '
' NGC 6822 o '
@o andromeda (M31)
sag DIG . . phoenix '
' # triangulum (M33)
' aquarius dw '
.'tucana . IC 1613 . . pegasus dw '
' . cetus '
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' . WLM '
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' ' ' = zero-velocity surface: inside it, gravity has beaten
cosmic expansion. everything here is bound to us.
andromeda is 2.5 million light-years away and approaching at about 110 km/s. the textbook says head-on collision in five billion years; the 2025 simulations (sawala et al.) say the odds of a merger within ten billion years are roughly a coin flip — the LMC’s pull actually helps us dodge. either way, everything inside the dotted line is bound: the local group is the one piece of the universe that expansion will never take from us.