Telling Tails About Galaxies (Stellar Halos, Satellites and Hierarchical Structure Formation)...

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. NGC5907 - 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

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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…..)


Beyond ~10kpc, substructure rules

e.g. Newberg et al 2002 - 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


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.

– 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).

– Analytic prescriptions to assign baryons associated with each satellite.

– Analytic star formation histories

– Analytic prescription to assign varying M/L to N-body particles to mimic embedded King models

Dark matter - set by

cosmology -

simulations run onceBaryons - assigned

by prescriptio

ns - 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 modelsLuminosity-

weighted dark matter

Color bar 34-24 mag/arcsec2

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Sanity Checks

Do our prescriptions produce reasonable Milky Way halo and luminous satellite population?

“Free” (?) parameters:

A. Epoch of reionization - zre

B. Star formation rate fcoldMgas/t* set by

- fcold 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).

Sanity Checks: 1. Size of satellite

populationStellar satellites accreted

Stellar Halo (109 Lsun)

Surviving satellites

Milky Way

? ~1 11

Halo 2 115 1.3 20

Halo 5 102 1.3 6

Halo 7 106 1.1 16

Halo 9 97 1.5 11………..depends on A. - zre cannot be much greater than 10

Sanity Checks: 2. Star fraction

in accreted satellite

s

Assume satellites infalling today should look like Local Group field dwarfs.

Sanity Checks: 2. Star fraction

in accreted satellite

sDepends on B.

Assume fcold=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 (109 Lsun)

Surviving satellites

Milky Way

? ~1 11

Halo 2 115 1.3 20

Halo 5 102 1.3 6

Halo 7 106 1.1 16

Halo 9 97 1.5 11

………..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)<2

• 104 Lsun/degree2 ~ 20 giants/degree2

• 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: SubstructureMorphology 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 40-30 mag/arcsec2

• Stars, color bar 40-30 mag/arcsec2

• Stars, color bar 34-24 mag/arcsec2

-200kpc 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)


Does this make sense if halo is built from satellites?


• Halo built inside out

• Surviving satellites accreted recently.

=> local halo (i.e. observed) from satellites accreted much earlier than surviving population.


% Halo from Surviving Satellites:

All Within 20kpc

Halo 2

0.5 0.01

Halo 5

0.02 0.0002

Halo 7

0.1 0.0

Halo 9

0.3 0.001

• Moreover - negligible fraction of halo came from surviving satellites

=> Stars in halo likely to be chemically different from those in surviving satellites….


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 SimulationsBackground 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.

• 10-40% 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 zre are photoevaporated (e.g. Barkana & Loeb

99)– Systems with Vc<30km/s contain gas in proportion to mass in place at

reionization, zre=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 fcold of the gas is capable of forming stars • Star formation rate given by fcoldMgas/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.

Telling Tails About Galaxies (Stellar Halos, Satellites and Hierarchical Structure Formation)...

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