What the UV SED Can Tell UsAbout Primitive Galaxies

Sally HeapNASA’s Goddard Space Flight Center

Outline of Talk1. The UV SED: introduction to , why is important

2. The challenge: interpreting f(age, Z, Fneb, dust)

3. Meeting the challenge: using the full SED to identify the various contributors to via case study of galaxy, I Zw 18

4. Results of case study: • The full SED is needed to make a quantitative interpretation of • Improvements will be possible through:

– New stellar evolution/spectra models– Inclusion of nebular gas & dust in model SED’s

is the power-law index in F( ~

Calzetti + 94

I Zw 18

ff

The UV SED is the basis of our knowledge about very high-redshift galaxies

F ~

phot = 4.29(J125-H160)

= -2.77

Age < 100 Myr

Metallicity – low

Extinction – low LFUV

SFR = 40 M☉/yr

M* = 7.8x108 M☉

F ~

phot = 4.29(J125-H160)

= -2.77

Age < 100 Myr

Metallicity – low

Extinction – low LFUV

SFR = 40 M☉/yr

M* = 7.8x108 M☉

ACS i’ ACS z’ WFC3 Y WFC3 J WFC3 H

ff

obs (m)=8.32rest

F

(nJy)

Finkelstein + 10

is sensitive to:

• stellar age

• metallicity

• dust extinction

• nebular emission

is sensitive to:

• stellar age

• metallicity

• dust extinction

• nebular emissionbeta_age_Z.jou

is sensitive to many factorsis sensitive to many factors

(Duration of Star Fomation)

Use the full SED to identify contributors to

Stars HII Emission Dust

Ly [CII]

HST/WFPC2WFPC2He II F469N[OIII] F502NH F656N

HST/WFPC2WFPC2He II F469N[OIII] F502NH F656N

HST/STIS Far-UVHST/STIS Far-UV

VLA 21-cm with optical image superposedVLA 21-cm with optical image superposed

H II Region Young, massive stars H I Envelope

Use the full SED of I Zw 18 as a test caseUse the full SED of I Zw 18 as a test case

I Zw 18 has been observed at all wavelengths

xray 21cm (Chandra) (VLA)

The spectrum reveals MXRB’s (xray), stars (UV-optical), HeIII and HII regions (UVOIR lines & continuous emission), dust (IR), HI envelope (far-UV, 21 cm)

Property I Zw 18 z=7-8 Galaxies

Stellar Mass (M☉) 2:x106 108 - 109

HI Gas Mass (M☉) 2.6x107

Dynamical mass (M☉) 2.6x108

SFR (M☉/yr) 0.1 10-100

Age of young stars (Myr)Age of older stars (Myr)

15:≤500? ≥1000?

<200

Metallicity (Z/Z☉) < 0.03 < 0.05

Dust low Low

Measured -2.45 -2.13 (H160<28.5)-3.07 (H160>28.5)

I Zw 18 is similar to high-redshift galaxies

• Birth Phase: Galaxies affected by photoionization.Mhalo<~109 M

• Growth Phase: Star formation fueled by cold accretion, modulated by strong, ubiquitous outflows.Mhalo<~1012+ M

• Death Phase: Accretion quenched by AGN, growth continues via dry mergers.Mhalo>~1012 M

Phases of Galaxy Formation

R. Dave et al. (2011) “Galaxy Evolution Across Time” Conference: Star Formation Across Space and Time, Tucson AZ April 2011

Evolutionary phase of I Zw 18 vs. WFC3 z=7-8 galaxiesEvolutionary phase of I Zw 18 vs. WFC3 z=7-8 galaxiesI Zw 18 is in the “birth phase” of galaxy evolution• Dynamical mass (halo mass) < 109 M☉

• No evidence of strong outflows

• Strong stellar ionizing radiation regulating star formation

• Huge HI cloud enveloping optical system suggesting SF in its early phase

WFC3 z=7-8 galaxies are in the “growth phase”• Stellar mass ~ 108 M☉, so halo mass (Mstar + Mgas + DM) must be >109 M☉

• High SFR (10-100 M ☉ per year)

• Large (negative) suggests incomplete absorption of stellar ionizing radiation

➙ HI envelope is perforated, thin, or non existent

• Mass inflow rate ~ (1+z)2.25 (Dekel+09) so that SFR is higher in higher-z galaxies of the same mass

• Maximum possible age of stars

Redshift-dependent differencesRedshift-dependent differences

Geneva evolutionary tracksCastelli+Kurucz spectral grid

Nebular geometry – spherical Dust treatment – dust included

ZAgeIMFSFH (iSB vs. CSF)

Z, grainsH density (HI, HII, H2)Inner radiusOuter radius: log NHI=21.3

Model stellar SED

iso_geneva

cloudy

Galaxy SEDGalaxy SED

Construct model SED’s to compare with observation

Stellar Models. I. Evolutionary tracks don’t account for rotation

Brott et al. (2011) astro-ph 1102.0530v2

Rotation is a bigger factor at lower metallicity (Maeder+2001, Meynet+2006)• Low-Z stars are more compact, so on average are born rotating faster • Low-Z stars retain their angular momentum since their rates of mass-loss are low• Rotational mixing is more efficient at low Z• Stars rotating above a certain threshold will evolve homogeneously• Stars evolving homogeneously move toward the helium MS (higher Teff)

C&K 03

II. Spectral grids for very hot stars (Teff>50 kK) are unavailable

Tef

f=50

kK

Tef

f=30

kK

Isochrones for log Z/Zsun=-1.7 (Lejeune & Schaerer 2002)

UV CMD for

Izotov+97

RRest Wavelength (A)RRest Wavelength (A)

NW

HST/COS Spectrum of I Zw 18-NW

III. Spectral grids for massive stars with winds e.g. WC stars, are unavailable

CMFGEN model spectra for low-Z stars are on the way!

Comparison of model SED to observations of I Zw 18Comparison of model SED to observations of I Zw 18

Comparison of model UV SED to observationsComparison of model UV SED to observations

Conclusions1. The spectra of star-forming galaxies near and far are composite, with contributions

from stars, HII region, HI region, and dust.

2. The flux contributions of these components are prominent at different spectral regions

• Young, massive stars: UV• Nebular emission: near-IR• Dust: thermal IR

• HI cloud: absorption (e.g. Ly) and emission lines (e.g. [CII] 158 )

3. A robust understanding of a star-forming galaxy requires the full SED

4. Progress in our understanding of high-redshift galaxies requires

• Evolutionary tracks & spectra of very hot stars (Teff>50,000 K) at low Z

• Inclusion of nebular emission in model SED’s