Astro 6590: Dwarf galaxies

Astro 6590: Dwarf galaxies Overview of dwarf galaxies Distinctive features of dwarfs Dwarfs vs giants Dwarfs vs globular clusters Dwarfs: challenges to CDM Missing satellite problem: how many dwarfs? The core-cusp problem: dark matter halos Morphological sequence of dwarfs: de, dsph, di Morphological segregation Local Group dwarfs Perseus dwarfs Virgo dwarfs Transition dwarfs Star formation history Metallicity-luminosity relation Blowout in dwarfs Stellar streams and disrupted satellites near the Milky Way DDO 154: an almost dark galaxy HST Survey of the Perseus Cluster core 12 Orbits centered around the core of Perseus (Conselice (PI), Held, DeRijke) 1

Dwarf ellipticals exist in clusters and morphologically appear similar to Local Group dwarf ellipticals Discovered in 1950s by G. Reaves through a Virgo cluster imaging survey There are however 5-10 as many low-mass cluster galaxies per giant galaxy as there are in Looser groups Low-Mass Galaxies in the Perseus Cluster (Conselice, Ringberg conf.) Perseus Cluster HST survey (Conselice, Gallagher & Wyse 03 sample) Real Dwarfs Compact dwarfs Background Spirals? Ambiguous cases 2

Cluster des deviate from the color-magnitude relation Clues for this deviation are old (e.g., Held & Mould 1994) Low L galaxies in clusters are a heterogenous population Color-Magnitude diagram for Perseus galaxies with Local Group des plotted. Both 'blue' and 'red des Conselice, Gallagher & Wyse (2003) Cluster des likely have a mix of stellar populations some might be metal rich Three stellar synthesis-modeled age tracts at constant metallicity (3 different ones) for different age populations. Conselice et al. (2003) 3

Kinematics of des Kinematics of dwarfs 4

Kinematics of dwarfs Internal Dynamics of NGC 205 CFHT12K mosaic imag (Demers et al. 2003) Keck/ DEIMOS slitmasks chosen to lie along tidal extensions. Spectroscopic targets chosen to be likely RGB stars in NGC 205. 5

NGC 205 Inner rotation = 15 km s -1 velocity (km/s) radius (arcmin) Geha, Guhathakurta, Rich & Cooper (2006) Local Group rotation curves NGC 205 Geha et al. (2006) velocity (km/s) NGC 147 Geha et al. in prep vrot = 17 km/s NGC 185 Bender et al (1991) radius (arcmin) 6

dsphs in the LG Rotation of des in Virgo Unlike des in the Local Group, some des in Virgo are rotationdominated. Long slit stellar absorption line spectroscopy from Palomar 5m Rotation curve Velocity dispersion van Zee et al 2004 7

de s as stripped di s? Rotationdominated des appear to avoid the cluster center Non-rotating des are found in the core van Zee et al 2004 Velocity distribution of des is large, with more substructure than giant cluster galaxies Conselice et al. (2001) A sign of their formation mechanism? 8

des in Virgo des are widely distributed in Virgo. Do not cluster around giants Different dynamics from Es des versus de,ns At the faint end, M B ~ -12, only 10% of des are nucleated. At the bright end, M B -16, almost 100% are. A significant fraction of des in Virgo are rotation dominated. Binggeli et al. 2000, A&A, 359, 447 Conselice et al. 2001, ApJ 559, 791. Oh & lin 2000, ApJ 543, 620 van Zee et al. 2004, AJ Fornax dsph 9

Fornax CMD Carina Dwarf: Episodic SF Plots from Smecker-Hane 2004 10

Observational Constraints All dwarfs so studied have ancient stars (> 10 Gyr) IC1613 Dolphin et al. LeoA Dolphin et al. M32 Alonso et al. LG dwarfs: wide range of SFH 11

Substructure in the Local Group Galaxies mainly clustered around the two principal galaxies MW & M31 Giant spirals dsph (+dell) dirr dirr/dsph Diagram from Eva Grebel Transformations?? Different types of dwarfs: de dsph di } different scaling relations in r, M, L, SB, etc (Kormendy 1985; Kormendy and Freeman 2004). Hybrid/Transition cases (eg, LGS3, Phoenix) What are the evolutionary relationships? 12

Characteristics of the satellite systems (Some) simulations suggest that: abundance and dynamics of the satallites depend on the host halo predictions accurate for satellites with v>50 km/s VDF significantly different at low v the Milky Way should have ~50 satellites with v>20km/s with M>3E8 and within 570kpc Issues: Resolution in the simulations Gastrophysics Reionization Transition type dwarfs Grebel, Gallagher & Hardbeck 2003 dsphs: Filled circles des: Open circles dirr/dsph: Filled diamonds dirr: Open diamonds General trend for HI masses to increase with increasing D dsphs typically < 10 5 M dirrs typically > 10 7 M Transition types intermediate Distance to nearest massive galaxy 13

Transition type dwarfs Grebel, Gallagher & Hardbeck 2003 dsphs & dirrs Show similar exponential radial surface brightness profiles Different versions of the same? dsphs vs dirrs dsphs are more metal-rich at same L Evident both in PNe and stellar Z Gas poor Different histories? Phoenix, DDO 210, LGS 3, Antlia, KKR 25, Leo T Locus in L-Z diagram is consistent with dsphs Have mixed dirr/dsph morphologies Low stellar mass Low angular momentum Small HI mass Would closely resemble dsphs if gas were removed Progenitors of dsphs? The case of Leo T Discovered by Irwin et al. 2007 MNRAS 384, 535 D = 420 kpc V = +35 km/s M T,V = -7.1 R plummer ~ 170pc R HI ~ 300 pc CMD has both RGB (6-8 Gyr) and young blue *s (200 Myr) Morphologically dsph but SF transition dwarf Two phase HI(500/6000K) Stellar mass: 1.2 x 10 5 M HI mass: 2.8 x 10 5 M Peak N HI = 7 x 10 20 cm -2 M dyn (R<R HI ) = 3.3 x 10 6 M M/L > 50 Kopylov et al. 0803.1107 14

The case of Leo T Ryan-Weber et al. 2008, MNRAS 384, 535 Outer contour 2 x 10 19 cm -2 HI radius = 2.5 = 300 pc Cold and warm components seen but no organized rotation The case of Leo T Discovered by Irwin et al. 2007 MNRAS 384, 535 D = 420 kpc V = +35 km/s M T,V = -7.1 R plummer ~ 170pc R HI /R plummer ~ 1.8 CMD has both RGB (6-8 Gyr) and young blue *s (200 Myr) Morphologically dsph but SF transition dwarf Two phase HI(500/6000K) Stellar mass: 1.2 x 10 5 M HI mass: 2.8 x 10 5 M Peak N HI = 7 x 10 20 cm -2 M dyn (R<R HI ) = 3.3 x 10 6 M M/L > 50 Forming stars at slow rate 1.5-2 x 10-5 M /yr Gas consumption time ~ 5 x 10 9 yr Low N H but also low central σ ~2 km/s => form *s Ryan-Weber et al. 2008, MNRAS 384, 535 15

Environmental effects Astro-ph/0403222 Morphological Segregation Grebel 2005 Gas-poor, low-mass dwarfs (dsphs) tend to cluster around massive galaxies (Cetus & Tucana are exceptions) Gas-rich, high-mass dwarfs (dis) are found to be widely distributed Observed in nearby groups and clusters as well as the Local Group Do these trends result from morphological transformations due to the influence of the massive primary galaxy? (i.e. tidal or ram pressure stripping) info on orbits of satellites would help 16

1) Not a single dwarf galaxy studied lacked an old population although how dominant that population was varied. 2) Evidence was found for a common episode of star formation (ancient Pop II stars found in Galactic halo and galactic dsphs to be same age within 1 Gyr). 3) Even the least massive dsphs showed evidence of some kind of continuous star formation (but with decreasing intensity) over several Gyr - no cessation of star formation during or after reionization. 4) No two histories look the same. Grebel & Gallagher Dwarf galaxies & metallicity 17

Blow out in Holmberg II Bureau & Carignan 2002, AJ 123, 1316 Blow out in Holmberg II Rhode et al. 1999 AJ 118, 323 18

Blow out in Holmberg II Rhode et al. 1999 AJ 118, 323 Extremely low mass galaxies may lose most or all of their ISM after a vigorous star formation episode. Starbursting and Blue Compact Dwarfs (BCDs) Small size Very blue Optical light dominated by line emission Young stellar population (and old stars?) Low metallicity => 1/40 th solar 19

IZw 18: A Young Galaxy? Small size Very blue Optical light dominated by line emission Young stellar population (and old stars?) Low metallicity => 1/40 th solar Not a young galaxy just one which did not form many/any stars until recently I Zw 18 van Zee et al. 1998 The Magellanic Clouds LMC SMC The Magellanic Clouds are contained within a common HI envelope. The Magellanic Stream traces their interaction with the MW. Best fit by tidal encounter with MW (but parameters still uncertain) 20

The Sagittarius Dwarf Discovered in 1994 by Ibata et al. Small, relatively metal rich About 14 kpc away Being deformed by the Milky Way In 1998-2MASS shows that stream circles whole sky David Martinez-Delgado (MPIA) & Gabriel Perez (IAC), The Sagittarius Dwarf Tidal Stream Drawing. From: http://antwrp.gsfc. nasa.gov/apod/ap050529.html Sagittarius streams in 2MASS Used M-Giant Stars from the 2MASS database Computer modeling: N-body simulations Mass and morphology of the Milky Way, mass and location of Sgr Used physical constraints to rule out certain models with out a full trial Picked the best fitting oblate, prolate, and spherical models Law, Johnston, Majewski, 1998 21

Tidal debris in the Milky Way? Tidal Tidal debris is usually assumed to contribute to the spheroidal component of the galaxy like the Sagittarius stream but it may also contribute to the Galactic disk(s Tidal debris in the disk of the Milky Way ω Cen The J z distribution of metal poor stars in the vicinity of the Sun suggests the presence of distinct kinematical groups. 22

The Milky Way Field of Streams SDSS SEGUE and other surveys designed to measure spectrum of millions of stars in MW to get velocities, metallicities => fossil record of MW history Ring around the Galaxy: its progenitor? Ibata et al. 2003 23

Tidal Remnants Durrell & DeCesar; +Yun 1994 HI traces event history even in absence of stellar debris. HI in the M81/M82 system Yun et al 1994 VLA-D De Mello et al. 2007, astro-ph/0711.2685: GALEX +ACS => modest SF intermediate between low levels in Magellanic Bridge and SF tails in major mergers like Antennae. 24

HI in the M81/M82 system 5 previously unidentified clouds M HI = (0.7-8.4) x 10 7 M Chynoweth et al 2008 astro-ph/0803.3631 A few examples of TDGs These are young, massive, gaseous, TDG candidates Duc et al., 1996,1997, 1998,1999; Hibbard etal., 1996; Malphrus et al., 1997 25

Tidal Dwarf Galaxies Hibbard et al. 1994 Expected to form in dynamically cool, gas-rich tidal tails Should be a mix of pre-existing stars from tidally disrupted material and a new generation of young stars produced as HI gas condenses => fall off the metallicity-luminosity relation Should contain little dark matter (assuming no DM in galaxy disks) Although young TDGs may be prominent due to the burst of star formation, their long-term fate remains unclear Other types of tidal objects Giant star-forming regions along tidal tails Detached «intergalactic» star forming regions A1367 - Gavazzi et al. 2003, Sakai et al., 2003 Tadpole galaxy (HST/ACS) All called «Tidal Dwarf Galaxies candidates» 26

VCC 2062: an old tidal dwarf? Faint, low SB dwarf Kinematically detached rotating condensation within HI tail Linked to disturbed N4694 (SB0 pec) VCC 2062 has strong CO and high O/H => deviates from metallicity - luminosity relation VLA map (blue) superposed on true color (BVR) image of N4694 (left) and VCC 2062 (right) with GALEX (red) overlaid. Duc et al. 2007, A&A 475, 187 Ultracompact Dwarf Galaxies Cluster UCD s(hilker et al. 1999; Drinkwater et al. 1999, 2000) First identified by spectroscopic survey of Fornax; also found in Virgo To date, found only in cluster centers (but see last slide) -13.5 < M B < -11.5 (intermediate between GCs and de/dsph) Unresolved in ground-based images so r ½ 50 pc versus normal dwarfs (r ½ ~ 300pc) or typical MW GCs (r ½ ~3 pc) Formation scenarios: Massive (intra-cluster) GCs Remnant nuclei of stripped ( threshed ) de,n Evolved products of YMGCs, massive superstar clusters formed in galaxy interactions Ultracompact BCDs identified in SDSS (Corbin et al. 2006) D(r band) < 6 => D < 1 kpc z < 0.01 (to allow for high resolution imaging) Only 9 in SDSS DR2 (rare!) Reside in voids Very metal poor 27

UltraCompact BlueDwarf Galaxies Color images (U, V narrow, I) Composite populations Starburst regions embedded in envelopes of red stars Corbin et al. 2006, ApJ 651, 861 Sample identified from SDSS UltraCompact BlueDwarf Galaxies Young massive cluster at its center Number of red supergiants Underlying, low surface brightness component UM 463 M = -14.2 D = 27 Mpc D(neigh) ~ 2.4 Mpc Corbin et al. 2006, ApJ 651, 861 Sample identified from SDSS 28

Ultracompact BlueDwarf Galaxies Left: fraction of light at 4020Å from stellar components by age Right: fraction of mass contained in different stellar population components All UBCDscontain significant population of stars ~10 Gyr old. HI content shows no clear pattern; some rich, some not Corbin et al. 2006, ApJ 651, 861 Sample identified from SDSS Compact objects near M87 Hasegan et al 2005, ApJ 627, 203 : HST-ACS Virgo Survey 10 compact, high-l (-11.8<M V <-10.8) dwarf globular transition objects 29

Compact objects near M87 Hasegan et al 2005, ApJ 627, 203 : HST-ACS Virgo Survey 10 compact, high-l (-11.8<M V <-10.8) dwarf globular transition objects Circles indicated inferred tidal radii Compact objects near M87 Hasegan et al 2005, ApJ 627, 203 : HST-ACS Virgo Survey 10 compact, high-l (-11.8<M V <-10.8) dwarf globular transition objects Get r ½, σ, M/L => 2 objects like predictions of population synthesis models for old, metal-rich, high L GCs 3 are much larger with r ½ ~ 20 pc and 6 < M/L V < 9. Resemble nuclei of nucleated des in Virgo 1 uncertain: stellar supercluster? 30

Compact objects near M87 Hasegan et al 2005, ApJ 627, 203 : HST-ACS Virgo Survey 10 compact, high-l (-11.8<M V <-10.8) dwarf globular transition objects Half-light radius r ½ vs magnitude for 2000 sources in M87 field. DGTO s near M87 Hasegan et al 2005, ApJ 627, 203 : HST-ACS Virgo Survey 10 compact, high-l (-11.8<M V <-10.8) dwarf globular transition objects Central velocity dispersion σ vs M for hot stellar systems 31

DGTO s near M87 Hasegan et al 2005, ApJ 627, 203 : HST-ACS Virgo Survey 10 compact, high-l (-11.8<M V <-10.8) dwarf globular transition objects Representation of the Virial Theorem for hot stellar systems Lower dashed line shows VT for M/L V =1.45. Upper line shows VT for M/L V =5 Compact objects near M87 Hasegan et al 2005, ApJ 627, 203 : HST-ACS Virgo Survey 10 compact, high-l (-11.8<M V <-10.8) dwarf globular transition objects Scaling relations for low-mass hot stellar systems 32

Compact objects near M87 Hasegan et al 2005, ApJ 627, 203 : HST-ACS Virgo Survey 10 compact, high-l (-11.8<M V <-10.8) dwarf globular transition objects DGTOs: Some similar to old, metal-rich, high L globular clusters Some are larger: ultra compact dwarfs Transition in scaling relations between two types ~ 2 X 10 6 M Three are embedded in low surface brightness envelopes. Presence of DM is fundamental property distinguishing globular clusters from UCDs 5/13 DGTOs in Virgo are associated with M87 Proximity to Virgo center may be critical UCDs form through tidal stripping of nucleated dwarfs? But Mieske et al. 2006 AJ 131, 244 found that Fornax de nuclei have lower metallicity than UCD s Fornax vs Virgo UltraCompact Dwarf Galaxies Most luminous UCDs: -14<M V <-12 Most/all have shallow or steep cusps; only 1 shows flat King core None show tidal cutoffs (to limit) ACS images and residuals from fits Evstigneeva et al. 2007, A.J. 133, 1722 Virgo and Fornax images+spectra 33

Comparison of internal dynamics of UCDs and GCs UltraCompact Dwarf Galaxies No gap between bright GCs and UCDs Virgo UCDs have velocity dispersions and luminosities similar to those of Fornax UCDs Evstigneeva et al. 2007, A.J. 133, 1722 Virgo and Fornax images+spectra Fundamental plane for dynamically hot stellar systems κ-space: axes are combination of central velocity dispersion, effective radius and mean intensity within r eff κ 1 log Mass κ 2 log SB 3 x M/L κ 3 log M/L Top: UCDs lie on same line as bright GCs but fainter GCs show large spread Bottom: UCDs not on main GC relation UltraCompact Dwarf Galaxies Evstigneeva et al. 2007, A.J. 133, 1722 Virgo and Fornax images+spectra 34

Comparison of data with model grids for different samples. UltraCompact Dwarf Galaxies Majority of de nuclei are consistent with solar [α/fe] ~ 0.0 Majority of UCDs have supersolar ratios (+0.3 to +0.5) Evstigneeva et al. 2007, A.J. 133, 1722 Virgo and Fornax images+spectra Comparison of data with model grids Hβ index: age sensitive [MgFe] index: total metallicity UltraCompact Dwarf Galaxies Virgo UCDs are old Stellar pops older than present-day de,n nuclei Evstigneeva et al. 2007, A.J. 133, 1722 Virgo and Fornax images+spectra 35

Ultra Compact Dwarf Galaxies Keck Echelle spectroscopy: v(helio), σ, Lick indices (stellar pops) Virgo UCDs are old (older than 8 Gyr) and have [Z/H] = -1.35 to +0.35 dex. 5 Virgo UCDs have supersolar [α/fe] abundances 1 Virgo UCD has a solar abundance ratio Virgo UCDs have structural and dynamical properties similar to Fornax UCDs Masses ~ (2 to 9) x 10 7 M M/L V ~ (3 to 5) Virgo UCD s M/L consistent with simple stellar model predictions => do NOT require dark matter Origin? Could be M/L extreme of known globular cluster population Couldresult from simple removal of halo from the nuclei of nucleated dwarf galaxies => But ages/metallicities argue against simple stripping Evstigneeva et al. 2007, A.J. 133, 1722 Virgo and Fornax images+spectra Ultra Compact Dwarf Galaxies Previous studies have found UCDs only in the centers of rich clusters; do they exist elsewhere? Survey of 6 galaxy groups using 2dF spectrograph on AAT: Dorado, N1400, N681, N4038 (Antennae), N4697, N5084 Only 1 UCD candidate found; 2 others maybe. Compare to simulations of Kravtsov & Gnedin (2005, ApJ623, 650) who give relation between total mass of the cluster population and mass of the host galaxy: M max = 2.9 x 10 6 M (M h /10 11 M ) 1.29 ±0.12 Find that this relation works for Fornax UCDs/N1399 and Virgo UCDs/M87 What about dominant galaxies in these groups? Would expect to find UCDs in Dorado, N1400 and N5084 group but not others Need deeper observations to provide firm test Evstigneeva et al. 2007 36

DDO 154 Are there totally dark galaxies? Arecibo map outer extent [Hoffman et al. 1993] DDO154 M H M stars M Dyn = 3.0 x 10 8 M = 5.0 x 10 7 M = 4.0 x 10 9 M Extent of Optical image Carignan & Beaulieu 1989 VLA D HI 37

HI 1225+01 Optical galaxy M L > 200 Giovanelli, Williams & Haynes 1989 Dwarf galaxies: summary Dwarf galaxies of the Local Group : Mateo 1998 ARAA 36, 435 Dwarf galaxies exist in large numbers, especially in large clusters The Local Group contains a mix of dirr and dsph/de s among its low L galaxies; they are distributed very differently. Low L dwarfs tend to be metal poor; the lowest L dwarfs are still composed largely of primordial material. Dwarfs are the simplest galactic systems known. But the dwarfs in the LG show complex star formation and chemical enrichment histories. Dwarf galaxies may be among the darkest single galaxies known; they provide important constraints on the distribution and nature of DM. Tides/interactions are important to the evolution of dwarfs and, possibly, their formation. 38