Searching for Satellites Around Galactic AnalogsThe SAGA Project

Risa Wechsler, Ben Weiner, Erik Tollerud, Yao-Yuan Mao, Ben Hoyleand the SAGA team

Marla Geha

The Milky Way’s Satellite Population ~1000 satellite galaxies predicted 2005: 11 satellite galaxies

Dwarf Galaxies (pre-2005)

The “Missing Satellite Problem”

2014: 25 satellite galaxies2015: 41 satellite galaxies

Via Lactea

5020

Small Scale Problems: Missing Satellites

Observations

Predictions

Mass of Satellite Galaxy

Num

ber o

f Sat

ellit

e G

alax

ies

5020

Observations

Predictions

new satellites

Num

ber o

f Sat

ellit

e G

alax

ies

Mass of Satellite Galaxy

Small Scale Problems: Missing Satellites

SDSS suggests LMC/SMC are unusual for a MW-mass halo,

but not uncomfortably so.

Velocity Dispersion (km/s)1006020 1006020

Velocity Dispersion (km/s)

Milky Way: Observed Milky Way: Predicted

Small Scale Problems: TBTF

MV

-16

-12

-8

-20

N (MV< -13) = 4 N (MV< -13) = 12

Many papers written comparing one observation (Milky Way) to one simulation!

The SAGA Project

Need a statistical sample (~100) of MWs,requires a volume out to 50 Mpc.

The properties of satellites observed around our Milky Way Galaxy disagree with model predictions.

Are models incorrect? Or is our Milky Way galaxy an outlier?

@ 30 Mpc, Rvir ~1 deg

Photometric redshifts for z < 0.05 are useless!

@ 30 Mpc, MV = -13 => mv = 21

The SAGA Project

Confirmed Satellites

Confirmed Background Galaxies

To move forward, need a training set of galaxies

w/spectra down to r < 21

NGC 6181 field: >3000 spectra to r=21.Most are background galaxies or stars. There are 8 satellites in here at z=0.008!

Building a Spectroscopic Training Set

Slide from B. Weiner

The SAGA Project

Candidate Satellites w/in virial radius:

-- All galaxies: ~10,000-- Basic Color cuts: ~1000-- Aggressive cuts: ~500

B. Hoyle: Machine learning using all photometric properties from SDSS has improved selection significantly

-- Machine Learning: ~150

Confirmed Satellites

Confirmed Background Galaxies

The SAGA Project: Early results

The data so far:-- Spectroscopic training set of ~3000 redshifts 17.7 < r < 21.

-- Additional spectra using cuts: 12,000

The results so far:-- 19 satellites across 8 MW systems

-- All but one satellite is star forming!

-- Significant variation in luminosity function.