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The Impact of JWST on the Stellar Initial Mass Function (IMF)
Marla Geha
Yale University

Tom Brown (STScI) Jason Tumlinson (STScI) Jason Kalirai (STScI) Josh Simon (OCIW)


The Stellar Initial Mass Function (IMF)
The IMF parameterizes the relative number of stars formed as a function of stellar mass. The IMF is fundamental to all calculations of galaxy star formation rates and galaxy mass.
Bochanski et al (2010)

N

star

low mass stars

0.1

0.2

intermediate mass stars

0.5

1.0

See recent review by Offner et al (2013)


The Stellar Initial Mass Function (IMF)
The IMF parameterizes the relative number of stars formed as a function of stellar mass. The IMF is fundamental to all calculations of galaxy star formation rates and galaxy mass.
Bochanski et al (2010)

Salpeter ! = 2.35

Direct measurements suggests IMFtars well fit by Chabrier or Kroupa Ns i in varying environments throughout Milky Way -> IMF is universal.
Chabrier Kroupa M > 0.5 ! = 2.3 M < 0.5 ! = 1.3

low mass stars

0.1

0.2

intermediate mass stars

0.5

1.0

See recent review by Offner et al (2013)


The Stellar Initial Mass Function (IMF)
Indirect estimates suggest that the IMF depends on galactic properties

Indirect results consistently suggest that higher mass galaxies have steeper IMFs (more low mass stars). IMF depends on global galaxy properties.

Indirect estimates of the IMF include: -- Spectral line fitting
(e.g., Conroy & van Dokkum 2012, Conroy et al 2013, La Babera et al 2013)

-- Dynamical modeling
(e.g., Cappellari et al. 2012, Treu et al 2011, Dutton et al 2013)


The Stellar Initial Mass Function (IMF)
Direct measurements suggests IMF is universal in varying environments throughout Milky Way Indirect results consistently suggest that IMF depends on global galaxy properties Need to directly measure IMF in galaxies distinct from the Milky Way

The dwarf galaxy satellites offer opportunity to directly measure IMF beyond the Milky Way.

Dwarf


Direct Measurements of the IMF
MW: Bochanski et al (2010) SDSS star counts SMC: Kalirai et al (2013) 121 ACS orbits
Ursa Minor: Wyse et al (2002) 12 WFPC2 orbits

MW SMC UMi

IMF slopes are inconsistent with MW, but at 2-sigma level


The Satellites of the Milky Way
The dwarf galaxy satellites of the Milky Way offer opportunity to directly measure IMF in a galaxy other than the Milky Way.

Sgr

SMC

UMi

The ultra-faint galaxies discovered in SDSS since 2006 offer new chance to measure IMF in environment different from Milky Way.

20

50

200

500


Direct Measurements of the IMF
MW: Bochanski et al (2010) SDSS star counts SMC: Kalirai et al (2013) 121 ACS orbits
Ursa Minor: Wyse et al (2002) 12 WFPC2 orbits Herc: Geha et al (2013) 20 ACS orbits Coma: in prep 24 ACS orbits

MW SMC UMi Herc Coma


Direct Evidence for IMF Variations
Direct evidence for systematic IMF variations with galactic environment.
Geha et al (2013)
Indirect estimates (Conroy priv comm)

MW M

UMi

Herc
Coma

This provide strong motivation for further observations: 1. Directly probe IMF down to 0.1 Msun 2. Directly measure IMF in a larger number of systems

IMF slope

SMC SM


IMF Studies in the Infrared
Low mass main sequence stars produce more flux in the infrared

HST
J-band stellar mass = 0.1 Msun

-- At optical wavelengths, the goal of 0.1 Msun is beyond HST capability -- In the infrared, 0.1 Msun can be achieved for only nearest Milky Way satellites

V-band stellar mass = 0.1 Msun

30

500


IMF Studies in the Infrared - WFC3
HST Cycle 21 WFC3 award to measure IMF of one UFD down to 0.17 Msun

Stay tuned: Data taken last week!


IMF Studies in the Infrared -- WFC3 vs. JWST
Low mass main sequence stars produce more flux in the infrared

HST
J-band stellar mass = 0.1 Msun

-- At optical wavelengths, the goal of 0.1 Msun is beyond HST capability -- In the infrared, 0.1 Msun can be achieved for only nearest Milky Way satellites

V-band stellar mass = 0.1 Msun

JWST

30

500


The Impact of JWST on the Stellar Initial Mass Function (IMF)
Sgr Magellanic Clouds

HST
UMi

JWST

20

50

200

500

JWST will open a new era of direct IMF studies Key Question: Does IMF depend on global galaxy properties, if so why?