The Resolved Stellar Content of Nearby “Young” Galaxies

Regina E. Schulte-LadbeckUniversity of Pittsburgh

Time

Cosmology

Early Universe

Cosmic Microwave Background

Today

Adapted from Dekel 2003

Observational Cosmology

• Study the global properties of the Universe

– e.g. Ho, m

• Study the local properties of the Universe which are significantly affected by the global properties

– e.g. large-scale-structure and galaxy formation from primordial density fluctuations

Dwarfs, dwarfs, dwarfs ...

• The substructure problem of CDM

• Formation of dwarf galaxies

• Dwarf galaxies, DLA systems, and the chemical evolution of the Universe

The Dwarf Galaxy Crisis

or Searching for Stars in High Velocity Clouds

The Issue: Substructure

MWG's virial radius 6 0 0 kpc

Font et al. 2002

CDM Predictions vs Local Group Dwarfs

● CDM simulations predict the existence of >10x more dark matter subhaloes than there are dwarf satellites of the Milky Way Galaxy. (From Font et al. 2002)

Proposed Solutions• Particle physicsParticle physics (e.g. Spergel & Steinhardt

2000, Colín et al. 2000)

• AstrophysicsAstrophysics (e.g. Bullock et al. 2001,

Somerville 2002)

• Observational BiasObservational Bias– Dwarf galaxies have been overlooked– High Velocity Clouds are candidates

(Blitz et al. 1999)

High Velocity Clouds

• Are concentrations of neutral hydrogen• Have high radial velocities inconsistent with

Galactic rotation models• Origins (e.g. Wakker & van Woerden 1997)

– Galactic Fountain– Magellanic Stream– ““Others”Others” => Missing satellites if at extragalactic

distances (Blitz et al. 1999) => Search for stars

Our Approach• Search for stars in Compact HVCs

– Deep V, I imaging with the VLTVLT centered on the highest column density HI (from EffelsbergEffelsberg).

– Small FOV of FORS 6.8’x6.8’, but reach even faint stellar populations throughout the entire Local Group (RGB to 2 Mpc).

– 2MASS2MASS archival data in J, H, K.– Appropriate, 2o fields, adjacent fields used for

galactic foreground contamination, data sensitive to RBG/AGB within 300 kpc.

K-band Position Plots of 2o Fields centered on dSphs

Sensitivity of 2MASS Data

• MS stars to 125 kpc => none are seen• RGB and AGB to 300 kpc => simulations

– used Sculptor (80 kpc) RGB, KS test on LFs– could add up to 8 times the Sculptor RGB– V of 21.4 arcsec-2

– such a high-surface brightness galaxy would have been seen on POSS plates (cf. Simon & Blitz 2002)

The Optical CMD of a “Transition Dwarf”

The Optical CMDs of Four CHVCs

Sensitivity of VLT Data

• MS stars to 1 Mpc => none are seen• RGB stars to 2 Mpc => simulations

– used Phoenix (450 kpc) RGB, KS test on LFs – could add up to 70 Phoenix RGB stars– 1.5 RGB stars arcmin-2 at 0.5 Mpc– V of 29 mag arcsec-2

– Mv of -5.8

Statistics

• If all CHVCs contain stars, then null detection of stars in 5 CHVCs can occur by chance with a probability of <10-8.

• There is a 50-50 chance that we missed the stars in 5 CHVCs if only 13% of CHVCs have formed stars.

Papers on No Stars in HVCs• Hopp, Schulte-Ladbeck & Kerp 2003, MNRAS,

339, 33

• Simon & Blitz (2002) - POSS plates• Willman et al. (2003) - SDSS ERD• Lewis et al. (2002) - one HIPASS cloud• Irwin et al. (2003) - the M31 HVC• No stars=> need to look for gravitational lensing

The Formation of Dwarf Galaxies

orExtremely metal-poor

star-forming dwarfs in

the present epoch

Galaxy Formation under the Cold Dark Matter Hierarchical Structure Formation Paradigm

Adapted from Dekel 2003

Dwarf Galaxy Building Blocks• Halo formed early – stars formed early (overcooling?)

– Non-merged remnants – old (+ young) stellar population

• Halo formed early – SF suppressed/delayed

– Non-merged remnants – dark or intermediate-age (+young) population

• Halo formed late – stars formed late => young today

– Isolated dwarf galaxies – young stellar population

cf. Roukema 1998, Babul & Ferguson 1996, White & Springel 2000

Blue Compact Dwarf Galaxies

Each square is 5 kpc on the side.

Gil de Paz, Madore & Pevunova , 2003

Primary N => Primeval BCDs?● Izotov & Thuan (1999)

interpret BCDs with low N/O at low O/H as “primeval” galaxies.

● O is produced in massive stars only.

● N could be produced in massive stars or in intermediate-mass stars; it is primary if there are no “seeds” for the CNO cycle.

● What about DLAs? 12 + log O/H

log N/O

Co-Is on BCDs/SFHs Regina Schulte-Ladbeck Ulrich Hopp

Igor Drozdovsky Laura Greggio

Mary Crone Claus Goessel

Jan Snigula

SFRs in the Izotov&Thuan sample

• Note the absence of any local BCDs with SFR > 0.1 M⊙/yr in the “primordial He” sample.

(Hopkins, Schulte-Ladbeck & Drozdovsky 2002)

CMD MorphologyCMD Morphology

Aparicio et al. 1996SimulationSimulation of an OLD Galaxy with continuing star formation

CMD Morphology of Distant, BCDDistant, BCD

----

VII Zw 403 from the Ground with the 48” (1.2m) DSS Telescope

VII Zw 403 DSS

FOV: 4’x 4’

VII Zw 403 from the Ground with the 140” (3.5m) Calar Alto Telescope

VII Zw 403 DSSHopp & S-L 1995

Prime-focusR-band image

256”x160”

S is up

VII Zw 403 with the 95” (2.4m) Hubble Space TelescopeHubble Space Telescope

VII Zw 403 HST

The CMD of VII Zw 403

Spatial Distribution of the Stellar Populations

Resolving the Core-Halo Structure

How We Derive the Star-Formation History

The Star-Formation History

HST/NICMOS CMDs

HST CMDs of BCDs

● More distant BCDs exhibit shallower WFPC2 CMDs.

● In all cases, stars with ages as old as the limiting magnitudes of the data are seen.

Extremely Metal-Poor SFGs

• Increasing metallicity: – I Zw 18, – Leo A, – SBS 1415+437, – VII Zw 403

• Increasing distance:– Leo A– VII Zw 403– SBS 1415+437– I Zw 18

Leo

A

SBS

141 5

+ 437

I Zw

18

VII

Zw

40 3

DLAs

Extreme SFGs

HII regions& galaxies

HST WFPC2 over Wendelstein Fields GO 5915

Tolstoy et al. 1998SFH core < 2 Gyr

GO 8575Schulte-Ladbeck et al. 2002SFH “halo” ≥9 Gyr

Wendelstein 0.8 mHopp et al. 2004Cepheids and other variable stars

Distance, metallicity, age•We find a distance modulus of 24.5 or 795 kpc using the TRGB method.•Globular cluster ridgelines indicate the metallicity (Fe/H) of RGB stars is extremely low, 1% Solar, smaller than that of the ionized gas.•But if we assume that the ISM of Leo A indicates the metallicity of the RGB stars, then their age is only 5 Gyr.

What have we learned?

• All of the candidate “young” galaxies studied, even those in extremely underdense regions, contain old stars. They are old.

• If there are still any “new” galaxies coming into being, we have not identified them yet.

• The “leftover” building blocks have had a complex star-formation history of their own, but strong bursts or long gaps are not seen.

• They are quite different from the building blocks that merged into large galaxies at early times.

Star-forming Galaxies, Lyman Alpha Absorbers,

andThe Chemical Evolution of the

Universe

SFGs causing Ly absoprtion

With Co-I Rao, Turnshek, Pettini, et al.

• Study the chemical evolution of the universe.• Study the relation between emission-line and

absorption-line diagnostics.• Study the relation between ionized phase and

neutral phase ISM in galaxies giving rise to DLA systems.

• Study the nature of DLAs.

Hopkins et al. 2001

Lilly et al. 2002Lilly et al. 2002

SFGs

DLAs

Data from Pettini et al. 2001, Jansen et al. 2000, Kulkarni & Fall 2002

SBS1543+593/HS1543+5921• This is a unique system.• QSO (z=0.807) impact parameter is small,

close to the center of the foreground type Sm dwarf galaxy (z=0.0096).

• Galaxy previously “misclassified” as a Seyfert – chance alignment discovered by Reimers & Hagen (1998) during the HS QSO survey.

• They found the galaxy contains an HII region.• HST spectrum shows Ly line is damped

(Bowen et al. 2001).

Star-Formation Rate

• H luminosity of #5 -> 9.1h70 -2 x 10-4 M⊙yr-1

• For total SFR, scale R-band luminosity of the 33 HII regions = 4.4 times that of #5.

• Total SFR about 0.006 M⊙yr-1

• SFR per unit area about 1.4 x 10-4 M⊙yr-1 kpc-2

12 + log (O/H) = 8.2±0.2 or about 1/3 solar

MB – 5logh70 = -16.8 ± 0.2

A “normal” dwarf• The galaxy is quite normal when compared with

other local dwarfs, e.g., in terms of:– Absolute magnitude, surface brightness, color, number

of HII regions, brightness of the first ranked HII region, metallicity-luminosity relation, neutral gas

• There is as yet very little known about the nature of galaxies causing DLAs. – This DLA has a logarithmic HI column density (20.35)

which is at the low end of the observed range. – It demonstrates a dwarf disk can cause a DLA locally.

Interesting testcase for understanding O/H in galaxies/DLAs

• From OI 1302.17 and assuming the line is optically thin, we get O/H > 1/135 of solar.

• This is about 40 times lower than the value derived in the HII region.

• The velocity paramter, b, of the gas must be several hundred km s-1.

• But if O/H in the neutral phase were the same as in the ionized one, then b⋲32 km s-1, in agree-ment with 21 cm profile, cf. Bowen et al. (2001).

• Resolve ==> new Cycle 12 data by Bowen et al.

Llog (N/O) = -1.4 (+0.2, -0.3)

SBS 1543+593

Models from

Twarog 1998Pei et al. 1999

Somerville et al. 1999

Cen & Ostriker 1999

Lilly et al. 2003

I Zw 18, the “most metal poor” galaxy

• In the ionized gas:– [O/H]NW = -1.52 ± 0.03 [O/H]SE = -1.51 ± 0.03– [N/H] = -2.36 ± 0.07[Fe/H] = -1.96 ± 0.09

• In the neutral gas:– Aloisi: [O/H] = -2.06±0.28 [N/H] = -2.88±0.11 [Fe/H] = -

1.76±0.12– Lecav: [O/H] = -1.39±0.08 [N/H] = -3.07±0.08

• Aloisi et al. (2003) find N/O in the neutral phase is about 2x higher than in the ionized phase.

• Lecavalier des Etangs (2003) derive N/O a factor of 10 lower than Aloisi et al. (2003).

• In the case of BCDs, where the ionizing cluster is used as a background source, the line of sight must include some ionized gas.

• The test is much cleaner in SBS1543+593, where there is a background QSO.

• Once the question of the validity of assumptions is resolved, we can again turn to the issue of understanding the differences in metallicities of disk galaxies and DLA systems.

• See Schulte-Ladbeck et al. 2004

Future Work (Cycle 13)