Extreme Galaxies: Part I

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$provide a critical test of hierarchical merging. Dwarf galaxies account for a significant fraction of the mass in stars in galaxies. VCC990, dwarf elliptical in the Virgo Cluster.$

1 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

2 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 gasrich; 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

3 Hierarchical Merging: Too Many Little Galaxies Virgo Also remember that small haloes are denser haloes => star formation should happen earlier. Kauffman et al MNRAS = no merging =100x merging = data 3

4 Recall Lessons from the Local Group =dsph/de / =dirrs / =dirr/dsph Dwarf galaxies are likely to have subsolar metallicity with the smallest being very metal-poor. Mateo 1998 ARAA

5 Recall the Local Group s Content 17 dsph 10 Irr 3 Sph 1 E 3 S van den Bergh 2000, PASP, 112, 529 5

6 Dark Matter in Dwarfs Local Group Dwarf galaxies appear to have much larger fractions of dark matter than big galaxies. Is this the result of gas loss at early times, say at reionization? Mateo 1998 ARAA

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

7 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

8 Fornax Case Study: Sample Selection Walker et al AJ

9 Fornax Case Study: Spatial Distribution 9

10 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

star-forming galaxies in the local universe Surface brightness is typically well-fit by an exponential disk (I(R)~I o e -R/Ro ) Irrs

11 Irregulars IC5152: Nearby dwarf irregular(ned). 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 exponential disk (I(R)~I o e -R/Ro ) 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

12 Van Zee

13 Establishing that color gradients are small so that bulk photometry can be used to characterize these galaxies. 13

14 Constant star formation appears to be a good match to these galaxies. 14

15 Another way of looking at the colors versus SFR. 15

16 Current SFR ~ Past SFR also implies Constant Rate Current SFR determined from Hα imaging: SFR = 7.9x10-42 L(Hα) (from Kennicutt 1998) <SFR> past trickier: <SFR> past = M stars / T SF Need to measure the mass in stars and the time over which star formation has been occurring. 16

17 Hα Imaging van Zee 2000 AJ 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

18 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 AJ

19 NGC 536, D=54 kpc, and D564-15, D=6.6kpc. 19

20 UGC10445: Dwarf Spiral w/ Cold Dust Hinz, J. et al ApJ

21 Characteristics of UGC10445 s Dust 3.9 kpc 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 be a inherent to the galaxy. Cool dust is postulated to be heated by UV photons escaping from HII regions. 21

22 Dwarf Spheroidals, Ellipticals A GALEX image of NGC205, a nucleated dwarf elliptical (from NED). 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 surely has suffered from tidal effects from M31 22

23 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 (D n ) and linestrength (usually measured via Mg lines) Faber & Jackson 1976, ApJ, 204, 668 Pahre et al. 1998, AJ, 116,

24 Are des and dsphs just small galaxies? Kormandy 1985 ApJ

25 No, they lie off the fundamental plane. 25

26 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 AJ

27 Are des/dsphs an Extension of Irrs/ Spirals? Green = des.dis Blue,Red = Sc, Im These authors look at these plots and conclude that the central dark matter density as indicated by ρ o is a continuum across the range from dsph, M V ~-10, to Sc with M V ~-22. The dwarfs are underluminous with stars more spread out because baryons must have been removed early in the history of the dsphs. Kormendy & Freeman 1997, see also astro-ph

into the cluster Large sample (476 galaxies) of Virgo dwarf ellipticals were observed, ~40 found with definite or probable disks using

28 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 AJ

29 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. Used a multi-fiber spectrograph on the VLT to measure metallicities of red giant stars in four Local Group dsphs. Several 100 stars/galaxy were observed. Helmi et al. ApJ L121 But, these dsphs appear to fewer stars with [Fe/H]<-3 than the Milky Way halo which is the HES line in the plot. N.B. This is a very new result so stay tuned. 29

BCDs = Blue Compact Dwarfs IZw18 as observed by HST IZw18 as observed in SDSS BCDs are the opposite of LSBs: they have very high surface brightnesses and very high star formation rates Current rate

30 BCDs = Blue Compact Dwarfs IZw18 as observed by HST IZw18 as observed in SDSS 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 Have very low metallicities which 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

31 BCDs: Surface Brightness Profiles Gil de Paz & Madore 2005 ApJS

32 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

33 Evolution in Clusters I Possible evolutionary effects: ram pressure stripping M L dyn B > ρ σ ICM gal v ( Σ 26.8) Tidal effects M galaxy 30 times more likely to interact with another M than with a 10 8 M galaxy. Dwarf galaxy tidal stripping interactions are rare N σ n r σ vt 2 3 M M gal dwarf 2/3 33

34 -Supernova driven winds A viable gas stripping mechanism? Pressure confinement? (Babul and Rees 1992 MNRAS ) - Harrassment predicts larger galaxies than are observed Evolution in Clusters II T T T W W SN > 10 3 > M L 5 3 dyn B M L r10 dyn B 0.4(26.8 Σ) r R C σ σ Galaxy Cluster Moore et al ApJ

35 Nuclear BHs in Dwarfs VCC128 N4486B Very preliminary results suggest that a couple of des in Virgo may harbor nuclear black holes. Much more evidence is needed before any detailed interpretation can be made of these results. Debattista et al ApJ 651 L97 35

Stellar Populations in the Local Group

Stellar Populations in the Local Group Recall what we ve learned from the Milky Way: Age and metallicity tend to be correlated: older -> lower heavy element content younger -> greater heavy element content