Nearby Star-Forming Dwarf Galaxies
through HST Eyes
Alessandra Aloisi
(STScI)
HotSci@STScI Talk
25 August 2010
Nearby Star-Forming Dwarf Galaxies
through HST Eyes
In collaboration with:
F. Annibali (STScI)
L. Angeretti (INAF–OAB)
G. Clementini (INAF–OAB)
R. Contreras (INAF–OAB)
G. Fiorentino (INAF–OAB)
L. Greggio (INAF-OAP)
A. Grocholski (STScI)
E. Held (INAF-OAP)
C. Leitherer (STScI)
M. Marconi (INAF–OAN)
I. Musella (INAF-OAN)
J. Mack (STScI)
D. Romano (INAF-OAB)
A. Rys (IAC)
A. Saha (NOAO)
M. Sirianni (STScI/ESA)
M. Tosi (INAF–OAB)
R. van der Marel (STScI)
Alessandra Aloisi
(STScI)
HotSci@STScI Talk
25 August 2010
Hierarchical Galaxy Formation
• Dwarf galaxies are first
systems to collapse and
start forming stars
• Massive galaxies form
by merging/accretion of
these building blocks
• Starbursts usually
associated with
merging/accretion
phenomena
Mapping Galaxy Formation
High-Redshift Studies:
looking directly back in
time by observing distant
galaxies (e.g. HDFs,
GOODS, HUDF)
Mapping Galaxy Formation
High-Redshift Studies:
looking directly back in
time by observing distant
galaxies (e.g. HDFs,
GOODS, HUDF)
2
Mapping Galaxy Formation
High-Redshift Studies:
looking directly back in
time by observing distant
galaxies (e.g. HDFs,
GOODS, HUDF)
Stellar Archaeology:
studying nearby galaxies
by resolving their presentday stellar populations
and reconstructing their
star-formation history
2
Mapping Galaxy Formation
High-Redshift Studies:
looking directly back in
time by observing distant
galaxies (e.g. HDFs,
GOODS, HUDF)
Stellar Archaeology:
studying nearby galaxies
by resolving their presentday stellar populations
and reconstructing their
star-formation history
Aparicio & Gallart (2004)
Key Questions to be addressed
in the Nearby Universe
• Do primordial (< 1 Gyr old) galaxies exist in the local
universe?
• Are the oldest stars in dwarf galaxies as old as the oldest stars
in massive galaxies?
• Do present-day dwarf galaxies resemble the high-redshift
building blocks of massive galaxies or are there systematic
differences in the properties of “progenitors” and “survivors”?
• What is the observational evidence for the hierarchical buildup of dwarf galaxies ?
• Are starbursts in the local Universe always associated with
merging/accretion phenomena ?
Key Questions to be addressed
in the Nearby Universe
• Do primordial (< 1 Gyr old) galaxies exist in the local
universe?
• Are the oldest stars in dwarf galaxies as old as the oldest stars
in massive galaxies?
• Do present-day dwarf galaxies resemble the high-redshift
building blocks of massive galaxies or are there systematic
differences in the properties of “progenitors” and “survivors”?
• What is the observational evidence for the hierarchical buildup of dwarf galaxies ?
• Are starbursts in the local Universe always associated with
merging/accretion phenomena ?
Low-Metallicity
Dwarf Star-Forming Galaxies
• Z < 1/20 Z (as inferred from Oxygen in HII regions)
• chemically similar to primordial galaxies in the early Universe
• PI of 3 HST/ACS programs in Cycle 11, 14, and 17
• Targets chosen to be the most metal-poor BCDs in low-density
environments (e.g., voids)
SBS 1415+437
D = 13.6 Mpc
Z = 1/20 Z
HST/ACS
I Zw 18
D = 18.2 Mpc
Z = 1/50 Z
HST/ACS
DDO 68
D < 9 Mpc
Z = 1/50 Z
SAO 6 m
RGB Stars in Low-Metallicity Dwarf SFGs
I Zw 18
Aloisi et al. (2007)
SBS 1415+437
Aloisi et al. (2005)
RGB feature: oldest stars at least 1-2 Gyr old (z ~ 1)
Bulk (~ 80%) of the galaxy mass in this old stellar
population (as inferred by LF)
Is I Zw 18 Young or Old ?
Izotov & Thuan (2004)
+
Press Release
STScI-2005-35
“Hubble Uncovers a
Baby Galaxy in a
Grown-Up Universe”
Is I Zw 18 Young or Old ?
Izotov & Thuan (2004)
+
Press Release
STScI-2005-35
“Hubble Uncovers a
Baby Galaxy in a
Grown-Up Universe”
Momany et al. (2005)
Is I Zw 18 Young or Old ?
RGB
Aloisi et al. (2007)
+
Izotov & Thuan (2004)
+
Press Release
STScI-2007-35
Press Release
STScI-2005-35
“Hubble Finds
‘Dorian Gray’
Galaxy”
“Hubble Uncovers a
Baby Galaxy in a
Grown-Up Universe”
Momany et al. (2005)
Variable Stars
in I Zw 18
125 days
130 days
8.6 days
139 or 186 days
Lowest
metallicity
Cepheids
ever
observed !
Z = 1/50 Zo
Variable Stars
in I Zw 18
125 days
P = 8.6 days
130 days
8.6 days
139 or 186 days
Lowest
metallicity
Cepheids
ever
observed !
Z = 1/50 Zo
Aloisi et al. (2007)
Fiorentino et al. (2010)
12
Distance of I Zw 18
• Cepheids
– theoretical reddening-free Wesenheit relation
for the 3 confirmed Cepheids yields average
distance D = 19 ± 2 Mpc
• TRGB
– TRGB filtering technique gives D = 18 ± 2 Mpc
Distance larger than previously believed;
contributed to difficulty in detecting RGB
ULP Cepheids as New Stellar Distance
Indicators up to the Hubble Flow
IZw18
Closer metal-poor BCDs are
best places were to look for
ULP Cepheids due to low
metallicity and recent SFH
Several BCDs available within
the Local Volume !
A couple of ground-based
proposals (Gemini, TNG)
already approved in 2010 to
better characterize ULP
Cepheids in NGC 1705 &
IZw18 (PIs: Saha & Marconi)
Bird et al. (2009)
Key Questions to be addressed
in the Nearby Universe
• Do primordial (< 1 Gyr old) galaxies exist in the local
universe?
• Are the oldest stars in dwarf galaxies as old as the oldest stars
in massive galaxies?
• Do present-day dwarf galaxies resemble the high-redshift
building blocks of massive galaxies or are there systematic
differences in the properties of “progenitors” and “survivors”?
• What is the observational evidence for the hierarchical buildup of dwarf galaxies ?
• Are starbursts in the local Universe always associated with
merging/accretion phenomena ?
On the SFH of SBS 1415+437
Mass dominated by very old
(> 6 Gyr) more metal-poor
(Z < 0.0004) stars
SFR dominated by young
(< 10 Myr) more metal-rich
(Z > 0.001) stars
Number of stars in the CMD
dominated by stars with
intermediate age
(25 Myr – 0.4 Gyr) and
intermediate metallicity
(0.0004 < Z < 0.001)
SFH not inconsistent with SF started as early as 12.5 Gyr ago (z~7),
just around the epoch of re-ionization ?
Key Questions to be addressed
in the Nearby Universe
• Do primordial (< 1 Gyr old) galaxies exist in the local
universe?
• Are the oldest stars in dwarf galaxies as old as the oldest stars
in massive galaxies?
• Do present-day dwarf galaxies resemble the high-redshift
building blocks of massive galaxies or are there systematic
differences in the properties of “progenitors” and “survivors”?
• What is the observational evidence for the hierarchical buildup of dwarf galaxies ?
• Are starbursts in the local Universe always associated with
merging/accretion phenomena ?
Closest Starbursts in Dwarf Galaxies
• starburst properties similar to LBGs at z ~ 3
• place where to characterize processes related to galaxy
formation/evolution, i.e., merging and accretion
• PI of 2 HST/ACS programs in Cycle 14 and 15
• Targets chosen to be isolated starburst galaxies
NGC 4449
D = 3.8 Mpc
Z = 1/4 Z
HST/ACS
NGC 1569
D = 3.4 Mpc
Z = 1/5 Z
HST/ACS
Spatial Distribution of Stars in Starbursts
NGC 4449
Annibali et al. (2008)
Starbursts mainly located in the center of dwarf galaxies !
Stars in the Halo of Dwarf Starburst Galaxies
NGC 1569
[Fe/H] = – 1.0
1Gyr 3Gyr 10Gyr
I
V–I
Grocholski et al. (2008; 2010 in prep.)
Outer parts of dwarf starburst galaxies composed by intermediateage/old stars with Z slightly lower than in HII regions!
Outer Halos in Dwarf Starburst Galaxies
NGC 1569
NGC 4449
NGC 1569
NGC 4449
Rys et al. (2010, in prep.)
No evidence for stellar
population age/metallicity
gradients within the halos
Outer stellar envelopes in dwarf starburst galaxies are distinct
halos (not extensions of inner disks)
Key Questions to be addressed
in the Nearby Universe
• Do primordial (< 1 Gyr old) galaxies exist in the local
universe?
• Are the oldest stars in dwarf galaxies as old as the oldest stars
in massive galaxies?
• Do present-day dwarf galaxies resemble the high-redshift
building blocks of massive galaxies or are there systematic
differences in the properties of “progenitors” and “survivors”?
• What is the observational evidence for the hierarchical buildup of dwarf galaxies ?
• Are starbursts in the local Universe always associated with
merging/accretion phenomena ?
A New Distance to NGC 1569:
Definitive Evidence of Interaction
Karachentsev et al. (2003)
Foreground/background
M81 Group
IC 342 Group
Distance quoted in the literature
before our HST/ACS observations:
D = 2.2 Mpc
New distance inferred from RGBT
in our HST/ACS data is ~ 1 Mpc
larger than previously believed
D = 3.2 Mpc
Galaxy location is now just in front
of the IC 342 group
NGC 1569
UGCA 92
Grocholski et al. (2008)
Grocholski et al. (2010, in prep.)
Maffei 1 Group
Foreground/background
Galaxy has likely just
crossed the IC 342 group !
Evidence of Interaction in NGC 4449
from the Literature
Hunter et al. (2002)
NGC 4449
Theis & Kohle (2001)
models of the interaction
of NGC 4449 with DDO 125
(about 0.5 Gyr ago)
DDO 125
Hunter et al. (1998)
Evidence of Interaction in NGC 4449
from the Literature
Hunter et al. (2002)
NGC 4449
Theis & Kohle (2001)
models of the interaction
of NGC 4449 with DDO 125
(about 4-6 108 Gyr ago)
Hunter et al. (1999)
DDO 125
Hunter et al. (1998)
New Evidence of Interaction in NGC 4449
New Evidences of Interaction in NGC 4449
New Evidence of Interaction in NGC 4449
Courtesy of Martinez-Delgado (MPIA)
Image taken with a
15cm refractor with
a very large FOV
NGC 4449 is accreting a Fornax dSph-like satellite
Concluding Remarks
• Deep HST/ACS CMDs provide detailed new insights into the SFH and
properties of nearby dwarf star-forming galaxies
• Underlying old (> 1-2 Gyr) populations that dominate the mass are
present in even the most metal-poor systems
• Low-metallicity systems may have SF for look-back times comparable
to a Hubble time, so in this sense they would be similar to the building
blocks at high z
• Low metallicities in very metal-poor systems likely due to very low SF
activity prior to the current burst and low-density environment.
• NGC 4449 is first observational evidence of accretion in a dwarf
galaxy. Intense SF in NGC 1569 is now explained with the galaxy being
part of a group.
• Cepheids with the lowest metallicity and the longest periods ever
detected in I Zw 18
Other Ongoing Work: Spectroscopy in
Nearby Star-Forming Galaxies
Spectroscopy important to understand metal content of the
different galaxy components (i.e., stars and gas) and to provide
additional information on the chemical evolution of nearby starforming systems
• STIS spectra of A-F stars in the starburst dwarf galaxy NGC 1569 to infer
abundances in stars younger than ~ 50 Myr
• FUSE (900-1200 Å) spectra of a sample of starburst galaxies to infer metal
content in the neutral gas via absorption lines studies
• COS (1150-1700 Å) spectra of a subset of the FUSE sample to disentangle
observational biases in the FUSE spectral region (saturation, depletion,
contamination by ionized gas) and to study multiple sightlines within the
same galaxy
Other Ongoing Work: Spectroscopy in
Nearby Star-Forming Galaxies
Courtesy of Bethan James (STScI)