Download Dwarf Galaxies

Dwarf Galaxies:
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Low-luminosity: 106 – 1010 L!
Low-mass: 107 – 1010 M!
Small in size, ~few kpc
Often low surface brightness, so they
are hard to find!
•! Why are dwarf galaxies important??
Schechter Function by environment
Bingelli (1988)
Field – dominated by
Spirals, faint end dIrr
Clusters – many more
E/S0 galaxies, faint end
dE, more dwarfs than in
field
Dwarf Galaxies:
•! Why are dwarf galaxies important??
–! Majority of galaxies are dwarfs!!
–! Dwarf galaxies may be remnants of galaxy
formation process: “proto-dwarf” gas
clouds came together to form larger
galaxies (hierarchical formation)
–! Dwarf galaxies are currently being
“absorbed” by larger galaxies
–! Dwarf galaxies are relatively simple
systems, not merger products
Dwarf Galaxies:
•! Different types of dwarf galaxies
–! Dwarf ellipticals (dE): Note that these are structurally very different
from luminous E’s. Gas-poor, old stellar population. Note that many
dE’s have nuclei (dE,N).
–! Dwarf spheroidals (dSph): Gas-poor, diffuse systems. Low
luminosity (low surface brightness end of dE’s.
–! Dwarf irregulars (dIrr): Extreme end of late type spirals. Active, ongoing star-formation but low surface brightness (like dSph’s). Gasrich. Note that there are no dwarf spirals!!
•! Is there an evolutionary link between dE’s and dIrr’s??
NGC205 (dE,N)
M31
M32 (cE)
NGC205 (dE)
Leo I, dSph
Pegasus,
dSph
Sagittarius, dIrr
Dwarf Ellipticals:
Sagittarius, dIrr
Surface brightness of Virgo dE’s,
Binggeli & Jerjen 1998
•! dE’s are structurally very different than luminous ellipticals.
–! Probably means different formation history.
•! dE’s approximately follow exponential profiles,
–! I(r) = I0 exp{-r/re}
–! Not r1/4 law!!
–! More generally, follow a Sersic profile: I(r) = I(re) exp{-b(r/re)1/n –1}
with varying n
•! Note that compact ellipticals (cE’s) do have similar structural
properties to luminous ellipticals. They may have lost their outer
portions due to tidal stripping. (e.g., M32 interacting with M31)
1/n values
Effective radius vs Absolute magnitude
Central surface brightness & core radius relations
(Kormendy)
Average surface brightness vs Absolute magnitude
Dwarf Irregulars:
•! Show knots of star formation
•! Surface brightness profile (in IR where it is tracing old stars) is
exponential
•! HI gas is more extended relative to stellar distribution than in
spirals
•! Blue compact dwarfs (BCDs) are extreme dwarf irregulars only a
few regions of star formation
–! BCDs may be young objects forming stars for the first time
•! Star formation in dIrr’s and BCDs leads to bubbles of HII gas
which can cause significant mass loss
Cannon et al. 2002 WFPC2 observations of I Zw 18
H!
Wilcots et al, Observations of IC10
H"
HI
Meurer et al. 1992, 1998 Observations of NGC 1705
Blue=HI
Green=optical (B)
Pink=H!
H!
Star formation histories:
•! In the Local Group, we can study the resolved stellar population
(color magnitude diagrams) to determine the star formation
histories of dwarf galaxies
•! Dwarf ellipticals are generally old (stars formed > 10 Gyr old), but
some may have had more recent (a few Gyr ago) weaker
episodes of star formation
•! Dwarf irregulars tend to have quasi-continuous star formation
(perhaps interspersed with bursts). Lower luminosity dIrr’s more
likely to have a bursty history
•! Environmental effects may play a role (e.g., tidal stripping
removing gas from dSph’s)
•! No two galaxies have the same star formation history!
Theoretical isochrones
and CMD for IC 1613
Skillman et al. 2003
Star formation history and
metal enrichment history
of IC 1613
Skillman et al. 2003
From
Grebel (2000)
Population Box (after Hodge, 1989)
Abundances:
From
Grebel (2000)
Abundances in dwarf galaxies, Mateo 1998
•! Dwarf galaxies have low metallicities (significantly
less than solar)
•! Dwarf galaxies obey a metallicity luminosity relation,
but dIrr’s and dE/dSph’s follow separate tracks
•! Evidence against a evolutionary path from dIrr to
dSph
Kinematics:
•! Measure kinematics of dE’s and dSph’s via highresolution spectroscopy of absorption lines in
individual stars
•! dIrr galaxies are dominated by rotation, can measure
rotation curves of the HI gas
•! All dwarf galaxies are dark matter dominated
(throughout the galaxy), with M/L ~ 3- 100
Square = dE or dSph
Triangles = “transition objects”
Circles=dIrr
Filled=[Fe/H] from stars
Open=[O/H] from HII regions and PNe
Geha et al. 2003
Central M/L
Velocity dispersions of
dE’s in Virgo
Total M/L
Filled square = dSph
Open square=dIrr
Note M/L is probably
underestimated for dSph
Using dwarf galaxies to measure dark
halos of massive galaxies:
•! We can use isolated satellite dwarfs as “tracer
particles” to measure the dark matter content of
massive galaxies at large galactocentric radii
–! Why do we need isolated galaxies to do this?
•! Measure radial velocities in large statistical samples,
find that velocities are large, even at several 100 kpc,
implying large dark halos!
–! What are potential problems with this analysis?
Zaritsky et al 1997
Future of dwarf galaxies:
SDSS
Prada et al.
2004
•! Dwarf galaxies are easily disrupted via interactions
with larger galaxies
•! We see this happening in the Local Group!
•! Larger galaxies may be built up by “eating” dwarf
galaxies
•! The Milky Way is a cannibal!!
Escape velocity
Sagittarius dwarf, shredded by the Milky Way
Majewski et al. 2003
Canis Major, shredded by the Milky Way
Ibata et al. 2003, nearest galaxy to MW, recently discovered!
Andromeda is also a
cannibal -- Ibata et al
2004
NGC 4013, Gabany et al (2008) -- including C.
Palma (PSU)
Tidal Dwarfs:
•! Dwarf galaxies can form from the debris torn from
more massive galaxies during interactions & mergers
•! Do not contain dark matter
•! May have high metallicities
The Antennae
WFPC2 imaging of the tail of the Antennae
HI observations of the tail of the Antennae
Saviane et al.
2004
Saviane et al.
2004
Yellow=young
stellar assoc.
Blue=bright
blue stars
Yellow cross=
faint blue stars
Red=bright red
stars (supergiants)
The number of dwarf galaxies increases
with redshift …
Galaxy groups & interactions:
•! Galaxies are social creatures, almost all are found in pairs,
groups, and clusters
•! Groups have < 50 galaxies, sizes ~1-2 Mpc, # ~ 100-500 km/s
•! In contrast, clusters have 50 – several thousands of galaxies,
sizes ~ few Mpc, # ~ 700-1200 km/s
•! Nearby groups:
–! The Local Group!
–! M81 group
–! Sculptor group
•! Groups have HI gas and x-ray halos
Properties of groups:
Neighborhood of the Milky Way
•! Prototypical group has 3-6 bright members and 30-40
faint ones
•! Velocity dispersions ~100-500 km/s
•! X-ray luminosities ~ 1042-1043 erg/s
•! M/L ~200 !!!
•! Groups show morphological segregation, spirals and
irregulars tend to lie on the edges of the groups, dE’s
and dSph’s are companions to massive galaxies
•! Groups are dominated by spirals and irregulars
Map of the Local Group
Local Group to scale
The Leo I (or M96) Group
The M81 Group