M87, giant elliptical in the Virgo Cluster Extreme Galaxies: Part I Looking at the extremes in galaxy properties may tell us much about the systematics of galaxy formation and evolution and can provide a critical test of hierarchical merging. Dwarf galaxies account for a significant fraction of the mass in stars in galaxies.
VCC990, dwarf elliptical in the Virgo Cluster. Shown at the same scale as M87. These two galaxies likely differ by nearly 1000x in mass
Cole et al. MNRAS 1991 326 255 1 What’s Extreme? • Extreme in mass or volume (BCDs = Blue Compact Dwarfs) • LSB = Low Surface Brightness galaxies • Extreme in gas content: extreme only if very, very gas- rich; many galaxies are gas-poor • Extreme in chemical composition: extreme if very metal poor; many galaxies exist with super-solar metallicity • Star formation can have a huge range of properties but neither very little or huge amounts of star formation are usually considered “extreme”
Are galaxies extreme for fundamental reasons? Are they the products of special evolutionary processes?
Another important aspect of dwarf galaxy studies is understanding whether the apparent deficit of companion galaxies relative to ΛCDM predictions is real or an artifact of something missing in our knowledge. 2 Hierarchical Merging: Too Many Little Galaxies
Virgo
Also remember that small haloes are c= no merging denser haloes => star formation should happen earlier. =100x merging •= data Kauffman et al. 1993 MNRAS 264 201 3 Recall Lessons from the Local Group
=dSph/dE Dwarf galaxies are {/•=dIrrs likely to have c/U=dIrr/dSph subsolar metallicity with the smallest being very metal-poor.
Mateo 1998 ARAA 36 435 4 Recall the Local Group’s Content
17 dSph 10 Irr 3 Sph 1 E 3 S
van den Bergh 2000, PASP, 112, 529 5 Dark Matter in Dwarfs
•Dwarf galaxies appear to have much larger fractions of dark matter than big galaxies. • Is this the result of gas Local Group loss at early times, say at reionization?
Mateo 1998 ARAA 36 435 6 Fornax: A DM Case Study
• Early data on the velocities of stars in Local Group dSphs indicated at high velocity dispersion (Aaronson 1983) • These first studies were assumed to be contaminated by unresolved binary stars and/or tidal effects that increase the apparent velocity dispersion • Subsequent work with larger samples strongly imply that these galaxies contain large amounts of dark matter.
The Fornax dwarf spheroidal galaxy. 7 Fornax Case Study: Sample Selection
Walker et al. 2006 AJ 131 2114 8 Fornax Case Study: Spatial Distribution
9 Fornax Case Study: Results
1. Tidal effects not important. 2. Two component King model required for best fit – DM has a larger core radius than the visible matter. 10 Irregulars
• dIs are the dominant dwarf galaxy type in the field • tend to be gas-rich with sub-solar metallicity • Dominate by number count star-forming galaxies in the local universe • Surface brightness is typically well-fit by an IC5152: Nearby dwarf exponential disk (I(R)~I e-R/Ro) irregular(NED). o • Irrs extend very low luminosities and merge with spirals at the bright end • Are the bluest of all galaxies on average (B- V~0.4 as compared to B-V ~0.9 giant ellipticals)
NGC2537 from SDSS.
IZw18, galaxy with [Fe/H] ~1/50th solar (Thuan). 11 Van Zee 2001
12 Establishing that color gradients are small so that bulk photometry can be used to characterize these galaxies.
13 Constant star formation appears to be a good match to these galaxies.
14 Another way of looking at the colors versus SFR.
15 Current SFR ~ Past SFR also implies Constant Rate
Current SFR determined from Hα imaging:
SFR = 7.9x10-42L(Hα) (from Kennicutt 1998)
16 Hα Imaging
van Zee 2000 AJ 119 2757 Hα imaging shows that star formation is spread across the face of dwarf galaxies – suggests that stochastic self-propagating star
formation prevails in these galaxies. 17 Dwarf Spirals
• Existence of dwarf spirals as a class has been disputed • Small galaxies that show rotation imply that disk-like structures exist • Bona fide spiral arms are rare but can be found.
Data from Schombert et al. 1995 AJ 110 2067 18 NGC 536, D=54 kpc, and D564-15, D=6.6kpc.
19 UGC10445: Dwarf Spiral w/ Cold Dust
Hinz, J. et al. 2006 ApJ 651 874
20 Characteristics of UGC10445’s Dust
UGC10445 is an isolated galaxy. Its cool dust accounts for ~90% of its dust mass. The cool dust extends well beyond the visible extent of the galaxy => seen previously in cluster dwarfs where it was attributed to stripping but here must 3.9 kpc be a inherent to the galaxy. Cool dust is postulated to be heated by UV photons escaping from HII regions. 21 Dwarf Spheroidals, Ellipticals • The commonest type of galaxy in groups and in clusters are dSphs/dEs • The difference between dSphs and dEs is observational with dEs having a diffuse stellar background while dSphs are sets of individual stars • In the Local Group need to be careful because M32 A GALEX image of NGC205, a “nucleated” dwarf elliptical (from NED). surely has suffered from tidal effects from M31
22 Recall from Lec 5: Scaling laws • projected radial velocity dispersion (σ) tightly correlated with luminosity: “Faber-Jackson relation” • similar correlations observed
between σ and diameter (Dn) and linestrength (usually measured via Mg lines) Faber & Jackson 1976, ApJ, 204, 668
Pahre et al. 1998, AJ, 116, 1591
23 Are dEs and dSphs just small galaxies?
Kormandy 1985 ApJ 295 73 24 No, they lie off the fundamental plane.
25 Other Evidence on Status of dEs
Small galaxies appear to have a larger percentage of blue, presumably younger, globular clusters. Why? – authors suggest that dEs may have several formation mechanisms.
Strader et al. 2006 AJ 132 2333 26 Are dEs/dSphs an Extension of Irrs/ Spirals? • These authors look at these plots and conclude that the central dark matter density as indicated by ρo is a continuum across the range from dSph, MV ~-10, to Sc with MV~-22. • The dwarfs are underluminous with stars more spread out because baryons must have been Green = removed early in the history dEs.dIs of the dSphs. Blue,Red = Sc, Im
Kormendy & Freeman 1997, see also astro-ph 0407321 27 Looking for Disks in dEs/dSphs
• Virgo has nealy 3x as many dEs as would be expected from agglomerating groups (Conselice et al. 2001) => dEs likely result from something inhernet in the cluster environment such as a morphological transformation of galaxies falling into the cluster • Large sample (476 galaxies) of Virgo dwarf ellipticals were observed, ~40 found with definite or probable disks using unsharp masking
Luminosity function suggests that dEs with disks are a separate population from the main body of dEs.
Lisker et al. 2006 AJ 132 497 28 Are they building blocks?
Since dSphs in the Local Group have low mean metallicies and predominately old stars, it has been presumed that they may be very similar to objects that merged with the proto-Milky Way to form its halo.
But, these dSphs appear to fewer stars with [Fe/H]<-3 than the Milky Way halo Used a multi-fiber spectrograph on the VLT to which is the “HES” line in measure metallicities of red giant stars in four the plot. Local Group dSphs. Several 100 stars/galaxy were observed. N.B. This is a very new result so stay tuned. Helmi et al. ApJ 2006 651 L121 29 BCDs = Blue Compact Dwarfs
• BCDs are the opposite of LSBs: they have very high surface brightnesses and very high star formation rates • Current rate of gas consumption would lead to gas depletion in ~1 Gyr Ï IZw18 as observed by HST • Have very low metallicities which Ð IZw18 as observed in SDSS suggests that these galaxies may be undergoing an initial burst of star formation – existence of a dim, old population supports the alternative hypothesis that star formation in these objects is extremely episodic
30 BCDs: Surface Brightness Profiles
Gil de Paz & Madore 2005 ApJS 156 345 31 BCDs: Underlying Population
Some BCDs may be dEs undergoing a starburst. BCDs with an extended blue population may be a different type of galaxy altogether.
Squares = dEs, Stars = dIrrs Line = color-magnitude fit for dEs in Coma 32 Evolution in Clusters I
• Possible evolutionary ⎛ M ⎞ ρ v2 effects: ⎜ dyn ⎟ > ICM 100.4(Σ−26.8) – ram pressure stripping ⎜ ⎟ ⎝ LB ⎠ σ gal
– Tidal effects N ≈ nσvt
10 10 M galaxy 30 times more likely to 2/3 10 interact with another 10 M than ⎛ 3M ⎞ 8 2 gal with a 10 M galaxy. Dwarf galaxy σ ≈ r ⎜ ⎟ tidal stripping interactions are rare ⎜ ⎟ ⎝ M dwarf ⎠
33 Evolution in Clusters II -Supernova driven winds ⎛ M ⎞ 3 ⎜ dyn ⎟ 0.4(26.8−Σ) A viable gas stripping TW >10 ⎜ ⎟r10 mechanism ? ⎝ LB ⎠ Pressure confinement ? ⎛ M ⎞ (Babul and Rees 1992 3 ⎜ dyn ⎟ TW > 2×10 ⎜ ⎟ MNRAS 255 346) ⎝ LB ⎠ 5 - Harrassment – predicts TSN ≈10 larger galaxies than are observed
σ⎛ ⎞ ⎜ Galaxy ⎟ r ≈ RC ⎜ ⎟ ⎝σ Cluster ⎠
Moore et al. 1998 ApJ 495 139 34 Nuclear BHs in Dwarfs
• Very preliminary results suggest that a couple of dEs in Virgo may harbor nuclear black holes. • Much more evidence is needed before any N4486B detailed VCC128 interpretation can be made of these results.
Debattista et al. 2006 ApJ 651 L97 35