Transcript Document

Advanced Lectures on Galaxies (2008 INAOE): Chapter 1 and 3a
Local Group Galaxies
Divakara Mayya
INAOE
http://www.inaoep.mx/~ydm
Galaxies in the Universe: An Introduction
Reference Linda S. Sparke and John S. Gallagher
Ultra Deep Field
Binggeli
Adopted from
Eva K. Grebel
Astronomical Institute
University of Basel
Astro-ph/0508147
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Grebel 1999
LMC: Multi-wavelength view
LMC and SMC: Milky Way’s satellites
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Grebel 1999
The Local Group
dSphs
dEs
dSph/dIrrs
dIrrs
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Why the Local Group?
• Proximity
 Resolution (individual stars)
 Depth (faintest absolute luminosities)
 Measurements of:
 Lowest stellar masses
 Oldest stellar ages
 Metallicities, element abundances
 Detailed stellar and gas kinematics
 Highest level of detail and accuracy
• Variety (of galaxy types)
 Range of masses, ages, metallicities
 Range of morphological types
 Range of environments
• Tests of galaxy evolution theories
• Understanding distant, unresolved galaxies
Ultra Deep Field
Buonanno et al. 1998
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Global star formation histories
in a synthetic
colormagnitude
diagram
Shown:
Constant star
formation rate
from 15 Gyr to
the present, no
photom. errors.
Gallart et al. 1999
Age structure
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105 Star CMDs from WFPC2:
LMC Star Formation Histories
Disk
Bar
Smecker-Hane, Gallagher, Cole, Stetson, 2002, ApJ, 566, 239
CMDs: Galactic bulge vs LMC disk
Fornax dwarf spheroidal galaxy
Carina dwarf spheroidal galaxy: M/L=74
Luminosity function of Ursa Minor:
Indistinguishable from Galactic globulars
Feltzing et al. 1998; Wyse et al. 2002
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Morphological Segregation
Gas-poor, low-mass dwarfs
Gas-rich, higher-mass dwarfs
Grebel 2000
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1. The Earliest Epoch of Star Formation
Cold dark matter
models predict:
Kravtsov & Klypin (CfCP & NCSA)
• Low-mass systems:
first sites of star
formation (z ~ 30)
• Larger systems
QuickTim eﾪ and a
YUV420 codec decom pressor
are needed t o see t his pict ure.
form through
hierarchical merging
of smaller systems
• Re-ionization may
squelch star formation in low-mass substructures
• Galaxies less massive than 109 M lose starforming material during re-ionization
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1. The Earliest Epoch of Star Formation
Consequences:
 Low-mass galaxies must form stars prior to reionization; must contain ancient populations
 Sharp drop / cessation of star formation activity
after re-ionization, may resume only much later
 High-mass galaxies’ oldest populations must be as
old as low-mass galaxies’ populations or younger
Testable predictions!
• Redshifts of 20 - 30 not (yet?) accessible
• Dwarf galaxies at those redshifts would be
extremely difficult targets anyway
 Exploit fossil record in nearby Universe instead
 Local Group ideal target since oldest
populations resolved and accessible with HST
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1. The Earliest Epoch of Star Formation
Results (largely based on HST):
• Old populations ubiquitous but fractions vary
• Evidence for a common epoch of star formation
 Globular clusters with main-sequence
photometry (Galactic halo & bulge,Sgr, LMC,For)
 Field populations with main-sequence
photometry (Sgr,LMC,Dra,UMi,Scl,Car,For,LeoII)
 Inferred from globular clusters
(e.g., BHBs, spectra): M31, WLM, NGC 6822)
 Inferred from BHBs in field populations:
Leo I, Phe, And I, II, III, V, VI, VII, Cet, Tuc
• Possible evidence for delayed formation?
 Inferred from GC MS: SMC’s NGC 121 (2-3 Gyr).
(However, lack of ancient globulars does not imply absence
of ancient field population.)
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1. The Earliest Epoch of Star Formation
Limitations:
• Deep data for direct (MSTO) age measurements
lack in dwarfs beyond ~ 300 kpc.
• True fraction of old stars still poorly known
(incomplete area coverage & unknown tidal loss)
• No data on Population III stars and their ages
Confirmed:
• Ancient Population II in Milky Way, LMC, and dwarf
spheroidal galaxies ~ coeval (± 1 Gyr)
 Consistent with building block scenario
• All galaxies studied in sufficient detail so far
contain ancient populations
In contrast to CDM predictions:
• No cessation of star formation activity in low-mass
galaxies during re-ionization
• Considerable enrichment: Episodes of several Gyr
Grebel & Gallagher 2004
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Star formation activity in low-mass galaxies (~107 M)
Grebel & Gallagher 2004
Cosmology: flat, m = 0.27, H0 = 71 km/s/Mpc
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Correlation between SFH and distance
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Star formation history - distance correlation
Faint (MV > -14) Milky Way companions:
Increasingly higher intermediate-age population
fractions with increasing distance from the MW
 Environmental influence of Milky Way?
Star-forming material might have been removed
earlier on from closer companions via
 ram-pressure stripping
 SN-driven winds from Milky Way
 high UV flux from proto-Milky Way
 tidal stripping
(van den Bergh 1994)
If environment is primarily responsible for gaspoor dSphs, then existence of isolated Cetus &
Tucana is difficult to understand.
Caveat: Argument considers only present-day
distances; orbits still poorly known / unknown.
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If the apparent trend of low-mass galaxy properties with
distance from the primary generally holds, we should also
find it for M31’s low-mass companions…
Harbeck, Gallagher, & Grebel 2004
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No obvious distance correlation for M31 dSphs
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3. Harrassment and Accretion
Can we find evidence for this
• in the surroundings of massive galaxies in the
Local Group?
• in the massive galaxies of the Local Group
themselves?
 Structural properties of nearby galaxies
 Stellar content and population properties of
nearby galaxies (including abundance patterns)
 Streams around and within massive galaxies
Dwarf galaxies might be considered the few
survivors of a once more numerous dark matter
“building block” galaxy population.
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Hierarchical structure formation:
Numerous mergers leave imprint on halo (and disk)
Thus expected:
 Overdensities
 Lots of streams
 Identification photometrically / kinematically
2MASS + Johnston streams
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3. Dwarf galaxy accretion:
Sagittarius’ tidal stream within the Milky Way
Majewski et al. 2003
2MASS: Detection of Sgr’s tidal stream across the entire sky
(area coverage advantage of shallow of all-sky survey).
Recent detection of second dSph in state of advanced
accretion: Monoceros (SDSS, Newberg et al. 2002); “CMa dSph”.
Ibata et al. 2001, Ferguson et al. 2002
+ ongoing
HST follow-up
Zucker et al. 2004
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Extremely
deep HST
imaging of
M31’s halo
Old
populations
present, but
intermediate
-age,
metal-rich
populations
dominate.
Brown et al. 2003
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Essential science: The Local Group
as a test case for galaxy evolution theories
What we know now:
• All nearby galaxies contain ancient populations;
fractions vary; ~ coeval Population II.
• No two galaxies alike in star formation histories,
population fractions, mean metallicities and
abundance spreads.
• But: global correlations (e.g., mass-metallicity)
Environmental impact and CDM building blocks:
But:
 Morphology-density
 Distance - HI content
 Accretion events
 Coeval ancient SF
• Tucana, Cetus
• Uncertain distance - SFH
• Number and [ / Fe]
• Extended SF in low-mass
galaxies (vs. reionization)
Galaxies (Class III): Types
Giant galaxies
Dwarf galaxies
Galaxies with a
prominent nucleus
E
Sp
IrrI, IrrII
Peculiar
LSB
dE
dSph
dIrr
HII, BCD, Haro …
Starburst, post-starburst
AGN
Ellipticals, lenticulars, spirals and irregulars fit into the classical
Tunic-fork diagram
What about the rest?
Irregular II or Amorphous galaxies
Ring galaxies
(Romano et al 2008)
Ellipticals: 4 Flavors
• Giant cDs, centers of clusters/groups,
masses 10^13-10^14 Msolar
• Normal Es: Masses from 10^8 (not many,
M32 holds down the low mass range of
most correlations…) to 10^13 Msolar
• Spheroidals: dSphs in the local group,
lower surface brightness dwarfs
(10^7-10^9) in clusters
•Dwarf Ellipticals
•