Telling Tails About Galaxies (Stellar Halos, Satellites and Hierarchical Structure Formation) Kathryn V Johnston (Wesleyan) & James S Bullock (Harvard)

The point of the talk… We see substructure in galactic stellar halos (e.g. NGC Shang et al 2000). We think galaxies form hierarchically (e.g. Moore et al 1999). What can the former tell us about the latter?

Why stellar halos? substructure long-lived, dynamics simple (phase-mixing) => easy to interpret

Overview 1.Introduction: Observations of stellar halos 2.Our study: Motivation and methods 3.Sanity checks 4.Results I: Substructure in halos 5.Results II: Properties of satellites vs halos 6.Summary

Observations - Properties of Milky Way’s halo Stellar density falls as r -3 E.g. RR Lyraes in SDSS, Ivezic et al 2003 (See also Siegel et al 2003, Wetterer & McGraw 1996…..)

Observations - Properties of Milky Way’s halo Beyond ~10kpc, substructure rules e.g. Newberg et al blue-colored turnoff stars along celestial equator, g*=19.4~11kpc g*=22.5~45kpc (See also Majewski team, Morrison’s “Spaghetti” group, Century Survey, QUEST…) Pal 5

Observations - Properties of Milky Way’s halo Most of that substructure is associated with the Sagittarius dwarf galaxy E.g. M-giants selected from 2MASS data (Majewski et al 2003) See also Ibata et al (1995, 1997….), SDSS, QUEST…

Observations - Other Galaxies Mostly only upper limits to densities of other stellar halos. Several examples of single streams. E.g. M31, Ibata et al 2001 Sources classified as “star-like” in I and with colors and magnitudes consistent with M31 RGB stars. See also Malin & Hadly1997, Shang et al 1999, Forbes et al 2003 Martinez- Delgado et al 2003

Observations - Local Sources for Accretion Events 11 satellites of Milky Way Similar number for M31 ~20 Local Group field dwarfs

Questions Within a given hierarchical cosmological model, where the halo is built from satellites: Just how lumpy do we expect the Galactic stellar halo to be? What is the expected frequency of low surface brightness features around other galaxies? To what extent can we reconstruct recent accretion history? How should stellar halo properties compare with the properties of surviving satellites?

What’s Missing from Previous Studies? Semi-analytic models can’t follow dynamics very accurately. N-body models of individual events miss the cosmological context. Cosmological simulations are too expensive to track: –Low surface brightness features –More than one galaxy. All of the above track the dark matter - i.e. assume mass-follows-light

Our Approach Approach: – Semi-analytic cosmological accretion history.Semi-analytic cosmological accretion history. – Analytic model for ~90% of parent galaxy we AREN’T interested in.Analytic model for ~90% of parent galaxy we AREN’T interested in. – N-body models for ~10% of galaxy we ARE interested in (ie the satellites).N-body models for ~10% of galaxy we ARE interested in (ie the satellites). –Analytic prescriptions to assign baryons associated with each satellite.Analytic prescriptions to assign baryons associated with each satellite. –Analytic star formation historiesAnalytic star formation histories –Analytic prescription to assign varying M/L to N-body particles to mimic embedded King modelsAnalytic prescription to assign varying M/L to N-body particles to mimic embedded King models Dark matter - set by cosmology - simulations run once Baryons - assigned by prescriptions - allowed to vary

Our Approach time Dark matter modeled via merger tree. N-body model run for each satellite accreted. Light mattter painted on subsequently (Incorrect for minor/major mergers. Restrict study to disk galaxies likely to have suffered <10% accretions for several Gyears.) (Luminous component not followed self- consistently.)

Formation of a Halo? Embedded King models Luminosity-weighted dark matter Color bar mag/arcsec 2

Sanity Checks Do our prescriptions produce reasonable Milky Way halo and luminous satellite population? “Free” (?) parameters: A.Epoch of reionization - z re B.Star formation rate f cold M gas /t * set by - f cold fraction of baryons in cold gas - t * star formation timescale C.Degree of concentration of baryons within dark matter

Sanity Checks: 1. Size of satellite population Solve the “missing satellites” problem (Kauffmann, Moore et al 1999, Klypin et al 1999) through feedback at reionization (Bullock, Kratsov & Weinberg, 2000). feedback at reionization

Sanity Checks: 1. Size of satellite population Stellar satellites accreted Stellar Halo (10 9 L sun ) Surviving satellites Milky Way?~111 Halo Halo Halo Halo ………..depends on A. - z re cannot be much greater than 10

Sanity Checks: 2. Star fraction in accreted satellites Assume satellites infalling today should look like Local Group field dwarfs.

Sanity Checks: 2. Star fraction in accreted satellites Depends on B. Assume f cold =0.15, need long t * (15 Gyears) to be consistent with Local Group field dwarfs

Sanity Checks: 3. Properties of Surviving Satellites Assume that gas in satellites is immediately stripped on accretion (and star formation halted): …depends on B.

Sanity Checks: 3. Properties of Surviving Satellites..depends on C. - concentration of baryons within each halo. Set by Local Group relations (e.g. see Dekel & Woo 2003)

Sanity Checks: 4. Size of stellar halo Stellar satellites accreted Stellar Halo (10 9 L sun ) Surviving satellites Milky Way?~111 Halo Halo Halo Halo ………..depends on A. and B. - if satellites form stars too rapidly then stellar halo is too big.

Sanity Checks: 5. Stellar halo radial profile Depends on C…..

Results I: Substructure Note: Color Bar 2<LOG(fluctuations)< L sun /degree 2 ~ 20 giants/degree 2 Shell thickness = 50% of radius Level of substructure in inner regions significantly overestimated - concentrate on r > 50 kpc

Results I: Substructure Substructure increases with radius at r > 50 kpc - level of fluctuations factor of 10 or more - size of order 10’s of degrees => should be apparent in any survey of sufficient depth covering hundreds of sq degrees provided an appropriate tracer.

Results I: Substructure Morphology of substructure: Angular scale relates to mass of progenitor Great Circles - apparent at intermediate radii - signature of circular or moderately eccentric orbit Blobs (shells?) - apparent at large radii - signature of apocenters of highly eccentric orbits => Reconstruct recent accretion history from this…(work with Sam Leitner). Observational collaborations: Steve Majewski - Deep Grid Giant Star Survey, 2MASS M-giants…..

Results I: Substructure Luminosity- weighted dark matter, color bar mag/arcsec 2 Stars, color bar mag/arcsec 2 Stars, color bar mag/arcsec kpc 200kpc

Results I: Sub- structure => 0-few detectable features within 100kpc of each galaxy Observational collaborations: Penny Sackett (ANU)

Results II: Stellar Content of Satellites vs Halo Satellites look chemically different from the halo (e.g. Unnavane, Wyse & Gilmore, 1996) - data from compilation of Venn et al (2004)

Results II: Stellar Content of Satellites vs Halo Does this make sense if halo is built from satellites?

Results II: Stellar Content of Satellites vs Halo Halo built inside out Surviving satellites accreted recently. => local halo (i.e. observed) from satellites accreted much earlier than surviving population.

Results II: Stellar Content of Satellites vs Halo % Halo from Surviving Satellites: AllWithin 20kpc Halo Halo Halo Halo Moreover - negligible fraction of halo came from surviving satellites => Stars in halo likely to be chemically different from those in surviving satellites….

Results II: Stellar Content of Satellites vs Halo Work with Andreea Font: e.g. assuming a closed box model of chemical enrichment in our simulated satellites…. Observational collaborations: Guhathakurta, Rich & Majewski (US), Ferguson, Irwin & Ibata (Europe).

Outlook - More Dimensions ….to our analysis…and in observations: large scale spectroscopic surveys: RAVE, SEGUE, various proposed MOS (Supporting feasibility studies….) astrometric missions: SIM, GAIA….. variability studies: LSST

Summary Models: reproduce radial profile of stellar halo, number and properties of surviving satellites and gas fraction in Local Group dwarfs. Provide: Testable predictions for level of substructure around Milky Way and other galaxies. Training set for recovering accretion histories from observations. Suggest: Stars in satellites expected to chemically different than those in local halo

Cosmological Background Standard  CDM  m =0.3,   =0.7, h=0.7,  8 =0.9,n=1 Mass accretion histories generated via the Extended-Press-Schechter method

The Simulations Background potential Fixed bulge and disk. Spherical halo growing & NFW profile evolving according to smoothed accretion history (Wechsler et al. 2002) Parent galaxy does not respond to satellites - Chandrasekhar-based dynamical friction calculated for each satellite (Hashinoto, Funato & Makino, 2003) Only those halos that have suffered no accretion event >10% in last 7 Gyears considered % of luminous halo mass accreted in that time.

The Simulations Satellites 100K particles initialized as NFW density profiles Scales, masses, orbits and accretion times chosen semi-analytically from cosmologically- motivated history. Self gravity calculated using a basis-function expansion code (Hernquist & Ostriker, 1992)

Assigning Gas and Forming Stars Satellite baryon content assigned from model considering feedback from ionizing background: –Systems with Vc<15km/s at z re are photoevaporated (e.g. Barkana & Loeb 99) –Systems with Vc<30km/s contain gas in proportion to mass in place at reionization, z re =10 (e.g. Bullock, Kravtsov, & Weinberg 00). –Systems with Vc>30km/s are able to accrete gas subsequently in proportion to dark matter accretion rate A fraction f cold of the gas is capable of forming stars Star formation rate given by f cold M gas /t *

Embedding stars within dark matter Embed dwarf galaxies within host dark matter hosts using particle energies. Assign M/L weights to produce King light profiles that match Local Group dwarf population.