The Star Formation History of Leo T from Hubble Space Telescope Imaging

2.1 MB·English

by  Daniel R. Weisz

Accpeted to ApJ January19,2012
PreprinttypesetusingLATEXstyleemulateapjv.2/16/10
THE STAR FORMATION HISTORY OF LEO T FROM HUBBLE SPACE TELESCOPE IMAGING
Daniel R. Weisz1, Daniel B. Zucker2,3, Andrew E. Dolphin4, Nicolas F. Martin5, Jelte T. A. de Jong6, Jon A.
Holtzman7, Julianne J. Dalcanton1, Karoline M. Gilbert1,10, Benjamin F. Williams1, Eric F. Bell8, Vasily
Belokurov9, N. Wyn Evans9
Accpeted to ApJ January 19, 2012
ABSTRACT
2 We present the star formation history (SFH) of the faintest known star-forming galaxy, Leo T,
1 based on deep imaging taken with the Hubble Space Telescope (HST) Wide Field Planetary Camera
0
2 (WFPC2). The HST/WFPC2 color-magnitude diagram (CMD) of Leo T is exquisitely deep, ex-
2
tending ∼ 2 magnitudes below the oldest main sequence turnoff, permitting excellent constraints on

n star formation at all ages. We use a maximum likelihood CMD fitting technique to measure the SFH
a of Leo T assuming three different sets of stellar evolution models: Padova (solar-scaled metallicity)
J andBaSTI (bothsolar-scaledandα-enhancedmetallicities). The resultingSFHs areremarkablycon-

3 sistent at all ages, indicating that our derived SFH is robust to the choice of stellar evolution model.
2 From the lifetime SFH of Leo T, we find that 50% of the total stellar mass formed prior to z ∼ 1
(7.6 Gyr ago). Subsequent to this epoch, the SFH of Leo T is roughly constant until the most recent

] ∼ 25 Myr, where the SFH shows an abrupt drop. This decrease could be due to a cessation of star
O
formation or stellar initial mass function sampling effects, but we are unable to distinguish between
C the two scenarios. Overall, our measured SFH is consistent with previously derived SFHs of Leo T.
. However, the HST-based solution provides improved age resolution and reduced uncertainties at all
h
epochs. The SFH, baryonic gas fraction, and location of Leo T are unlike any of the other recently
p
discovered faint dwarf galaxies in the Local Group, and instead bear strong resemblance to gas-rich
-
o dwarf galaxies (irregular or transition), suggesting that gas-rich dwarf galaxies may share common
r modes of star formation over a large range of stellar mass (∼ 105−109 M⊙).
t
s Subject headings: galaxies: individual (Leo T dIrr); galaxies: stellar content; Local Group
a
[

1 1. INTRODUCTION has a baryonic gas fraction (Mgas/(Mstar + Mgas) ∼
0.8;Ryan-Weber et al2008)comparabletoothernearby
v The discovery of dozens of faint dwarf galaxies in the
dwarf irregular galaxies (e.g., Weisz et al 2011a). The
9 Local Group has extended our studies of galaxy forma-
mixture of these unusual characteristicsprovides a truly
5 tion and evolution to the lowest end of the galaxy lu-
8 minosity function (e.g., Zucker et al 2004; Willman et al extremeandunusualenvironmentinwhichwecanstudy
4 2005; Belokurov et al 2007; Martin et al 2009). With the effects of star formation and stellar feedback in re-
1. extremely low masses and luminosities, the discovery lation to theories of low mass galaxy evolution (e.g.,
Dekel & Silk 1986; Orban et al 2008; Ricotti 2009).
0 of these galaxies has facilitated new insight into key
Inthispaper,wepresentthecolor-magnitudediagram
2 topics such as the nature of dark matter profiles, stel-
(CMD) and the star formation history (SFH) of Leo T
1 lar feedback and chemical evolution, and the so-called
based on imaging obtained with the Wide Field Plane-
: ‘missing satellites’ problem in the framework of CDM
v tary Camera 2 (WFPC2; Holtzman et al 1995) aboard
cosmologies (e.g., Moore et al 1999; Walker et al 2009;
i the Hubble Space Telescope (HST). The long integra-
X Governato et al 2010; Pen˜arrubia et al 2010).
tion times of HST/WFPC2 observations make for an
Leo T remains unique among the known extremely
r exquisitely deep CMD that extends below the ancient
a faint galaxy population. It has a low luminosity (MV ∼ main sequence turnoff (MSTO; & 10 Gyr), allowing for
−8;Irwin et al2007;de Jong et al2008b)andhighdark
well-constrainedmeasures of star formation at all ages.
matter content (M /L ∼ 60-160; Ryan-Weber et al
⊙ ⊙ This paper is organizedas follows. In §2 we detail the
2008; Simon & Geha 2007), yet, unlike other recent
WFPC2observationsandphotometricreductionsandwe
discoveries, Leo T is located in relative isolation (420
present the resultant CMD in §3. We then describe our
kpc from the Milky Way Irwin et al 2007; de Jong et al
methodformeasuringtheSFH,includingtheuseofmul-
2008b),exhibitsevidenceformultiplegenerationsofstar
tiple stellar evolution models, in §4. Finally, we present
formation (Irwin et al 2007; de Jong et al 2008a,b), and
and analyze the lifetime cumulative and absolute SFHs
ofLeo T in §5. The conversionbetween ageand redshift
1UniversityofWashington; [email protected] used in this paper assume a standard WMAP-7 cosmol-
2MacquarieUniversity
3AustralianAstronomicalObservatory ogy as detailed in Jarosik et al (2011).
4Raytheon
5Max-Planck-Institutfu¨rAstronomie 2. OBSERVATIONSANDPHOTOMETRY
6LeidenObservatory Observations of a single central field in Leo T were
7NewMexicoStateUniversity
8UniversityofMichigan taken with WFPC2/HST from 21 October 2007 to 29
9UniversityofCambridge October 2007 as part of HST program GO-11084 (PI:
10HubbleFellow D.Zucker). Thesetofobservationsconsistsof16F606W
2 Weisz et al.
(wideV)and26F814W(I)imageswithtotalintegration CMD is the most probable SFH of the observed popu-
times of 19200s and 31200s,respectively. The 2.5′× 2.5′ lation. A full description of MATCH can be found in
WFPC2 field encloses most of the area defined by the Dolphin (2002).
half-light radius (r = 1.5′; Martin et al 2008). To measure the SFH of Leo T, we selected a Kroupa
h
We performed PSF photometry on each of the images IMF(Kroupa2001)withamassrangeof0.15to120M
⊙
usingHSTPHOT,astellarphotometrypackagedesigned and a binary fraction of 0.35, where the mass of the sec-
foruse with WFPC2 (Dolphin 2000). We culledthe cat- ondary is drawn from a uniform mass distribution. We
alog of detected objects to only include well-measured selected the solar-scaled metallicity Padova set of stel-
stars by applying the following photometric criteria: lar evolution models with updated low mass AGB track
SNR > 4 and SNR > 4 and (sharp (e.g., Girardi et al 2010) for our primary SFH measure-
F606W F814W F606W
+ sharp )2 ≤ 0.075, yielding 3847 well-measured ment. WealsosolvedforSFHsofLeoTusingtheBaSTI
F814W
stars. Definitions ofthe photometric quality metrics can stellar evolution libraries with both solar-scaled and α-
be found in Dolphin (2000) and Dalcanton et al (2009). enhancedmodels (Pietrinferni et al2004). These partic-
The photometric catalog of well-measured stars is avail- ular models provide age sensitivity from . 25 Myr ago
ableasahighlevelscienceproductviatheHSTarchive.1 to 14 Gyr ago, which is needed to accurately model the
Tocharacterizecompleteness andobservationaluncer- mixed-agepopulationofLeoT.Withallmodels,wedes-
tainties we conducted 500,000 artificial star tests. After ignated a search range of metallicities of [M/H] = −2.3
applying the photometric quality criteria to the recov- to−1.2,withresolutionof0.1dex. Thelowermetallicity
ered artificial stars, we measured the 50% completeness limitissetbythe availabilityofthe models. We initially
limits to be m = 27.5 and m = 26.9. exploreda higher metallicity upper limit, but found this
F606W F814W
space to be largely unexplored by the code.
3. THECOLORMAGNITUDEDIAGRAM ForthePadovamodels,wedefined40logarithmictime
bins over the range log(t) = 6.6-10.15. The bins were
As highlighted in Figure 1, the CMD of Leo T has
spacedby 0.05dex for log(t)= 9.0-10.15and0.1 dex for
many interesting characteristics that suggest multiple
log(t) = 7.4-9.0. Given the lack of extremely luminous
episodesofstarformation. First,thereareseveralindica-
stars on the MS, we designated a single time bin for the
tors of ancient star formation, such as the oldest MSTO
youngest ages, log(t) = 6.6-7.4. For solutions using the
(F606W-F814W∼ 0.5; F814W ∼ 26), blue and red hor-
BaSTI models, we designated 39 identical time bins to
izontal branch (HB) populations (defined by the green
thePadovascheme,butexcludedthe youngesttime bins
box in Figure 1), and the red giant branch (RGB; ex-
as the BaSTI models only extend to log(t) = 7.4 and
tending vertically between F814W ∼ 19 and 25.5). The
7.45for the solar-scaledand α-enhancedmodels, respec-
broadness of this latter sequence suggests the presence
tively. We did not attempt solutions using other models
of multiple age and/or metallicity populations. Second,
such as Dartmouth (Dotter et al 2008) because they do
we see a sparse sampling of stars that are fainter than
notcoverthefullrangeinagesspannedbystarsinLeoT.
the HB (between F814W ∼ 24.5 and 25.3 and F606W-
To facilitate comparison between the models and obser-
F814W ∼ 0.2 to 0.4). These stars are consistent with
vations, the observed CMD and synthetic CMDs were
magnitudes and colors of intermediate age (∼ 2-10 Gyr
binned with a resolutionof0.1 in magnitude and0.05 in
ago)MSTOstars. Third,thepopulationofstarslocated
color.
near the bright limit of the RGB (F814W ∼ 20) may
Well-knowndifferencesbetweenthePadovaandBaSTI
be luminous intermediate age asymptotic giant branch
HBmodelscanintroducesignificantbiasesintothemea-
stars(AGBs). However,giventhelownumberofstarsin
suredSFHs(e.g.,Gallart et al2005). Wehavemitigated
the CMD, itis difficult to visually discernwhether these
these potential biases by placing the HB into a single
aretruly AGB starsorareassociatedwith the tip ofthe
Hess diagram bin, whose dimensions are indicated by
RGB population. Finally, Leo T has a small number of
thegreenboxinFigure1. Ineffect,this processrequires
luminous MS and blue helium burning stars (F814W ∼
that each model generate a HB, but the precise details
21-22, F606W-F814W ∼ 0-0.3), which both trace star
of the HB population (e.g., luminosity and morphology)
formation within the most recent ∼ 1 Gyr. The lack of
do not strongly affect the measured SFH. The most se-
extremely luminous MS stars suggests that Leo T has
cure leverage on the ancient SFH comes from the oldest
had little or no star formation at very recent times.
MSTO.
Additionally, to account for intervening Milky Way
4. MEASURINGTHESTARFORMATIONHISTORY
foreground populations, we used the ‘foreground’ util-
We have measured the SFH of Leo T using the
ity included in the MATCH package to construct model
CMD fitting package MATCH (Dolphin 2002). Briefly,
foregroundCMDsthatwereusedinthederivationofthe
MATCH builds sets of synthetic CMDs from user de-
SFH.ThisutilityproducesaCMD-basedontheresultsof
finedparametersincluding a stellar initialmass function
de Jong et al (2010), who measured thick disk and halo
(IMF), binary fraction, a searchable range of distance
structural parameters using MATCH. The model fore-
and extinction values, and fixed bin sizes in age, metal-
ground CMDs were made with the Dartmouth stellar
licity,colorandmagnitude,andthenconvolvesthemodel
evolution models (Dotter et al 2008) that include stars
CMD with observational biases as measured from artifi-
withmassesaslowas0.1M . Giventhesmallamountof
⊙
cialstar tests. MATCH then employs a maximum likeli-
expected foreground contamination in the Leo T CMD,
hoodstatistictocomparesyntheticandobservedCMDs.
wedonotanticipatethatthe particularchoiceofmodels
TheSFHthatcorrespondstothebestmatchedsynthetic
willsubstantially influence the measuredSFH. However,
the Dartmouth models provide a more complete census
1 http://archive.stsci.edu/hlsp of the intervening low mass Galactic stellar population,
3
(a) AGB (b) Padova
20 Young MS/
BHeB
22
W RGB
4
1
F824 HB
MSTO
26
(c) BaSTI (d) Model
20
22
W
4
1
824
F
26
−0.5 0.0 0.5 1.0 −0.5 0.0 0.5 1.0
F606W-F814W F606W-F814W
Figure 1. The deep HST/WFPC2 CMD of Leo T, corrected for foreground reddening using the values from Schlegel et al
(1998). Panel (a): The red-dashed line represents the 50% completeness limit and the blue error bars indicate photomet-
ric uncertainties, both determined by artificial star tests. We have also highlighted several age sensitive features, including
identification of the horizontal branch (green box) and oldest MSTO (magenta). Panel (b): The observed CMD of Leo T
with select Padova isochrones of 500 Myr (blue), 2 Gyr (green), 5 Gyr (magenta), and 10 Gyr (red) and a metallicity of Z =
0.0005, the value derived from the SFH measurement. Panel (c): Same as panel (b) only with the BaSTI isochrones, and
a metallicity of Z = 0.0003. The slight difference in metallicities is due to available values on the respective web interfaces
(Padova: http://stev.oapd.inaf.it/cgi-bin/cmd; BaSTI: http://albione.oa-teramo.inaf.it/). These two panels illustrate some of
the metallicity dependent differences in evolved star models, as discussed more extensively in Gallart et al (2005). Panel (d):
AmodelCMDfrom themost likely SFHbased onthePadovastellar libraries. Theredpointsareanexampleof thestars used
to model theinterveningforeground population.
resultinginamoreaccurateaccountingofanyforeground andextinction. For eachofthe three solutions,MATCH
contamination. found a best fit extinction corrected distance modu-
We quantify uncertainties in the SFHs using a set of lus of 23.05 and foreground extinction of A = 0.20.
V
Monte Carlo tests. The Monte Carlo tests are designed These values favorably compare with previously derived
to account for uncertainties due to the number of stars distances of 23.10±0.2 (Irwin et al 2007; de Jong et al
on the CMD (random uncertainties) and biases due to 2008b) and the foreground extinction value of A = 0.1
V
uncertaintiesinthestellarmodels(systematicuncertain- fromSchlegel et al(1998). Weshowthe simulatedCMD
ties). To quantify the random uncertainties we sample from the best fit SFH from the Padovasolution in panel
the best-fit CMD using a Poisson random noise gener- (d) of Figure 1.
ator. We then introduce additive errors in M and
bol
log(Teff) and resolve for the SFH. These additive val- 5. THELIFETIMESTARFORMATIONHISTORYOFLEOT
ues serve as a proxy for systematic uncertainties in the
In Table 1 and Figure 2, we present the cumulative
underlying stellar models by mimicking the scatter in
and absolute SFHs of Leo T. The cumulative SFH, i.e.,
SFH uncertainties obtained by using alternate isochrone
the fraction of the total stellar mass formed at a given
sets (e.g., BaSTI, Dartmouth; Pietrinferni et al 2004;
time, provides a normalized measure of the stellar mass
Dotter et al 2008) to fit the data. The SFH of the
accumulationinLeoT.ComparedtotheabsoluteSFHs,
new CMD is then measured identically to the original
cumulative SFHs are less affected by correlated SFRs in
solution, constituting a single Monte Carlo realization.
adjacent time bins, allowing us to plot the cumulative
We found that the uncertainties were stable after 50
SFHs at full time resolution. Regions where the cumu-
Monte Carlo tests, and thus conducted 50 realizations.
lative SFH exhibits zero growth for extended periods or
A more detailed descriptionof this processcan be found
has large uncertainties indicate intervals over which our
in Weisz et al (2011a).
knowledgeoftheshapeoftheabsoluteSFHisuncertain.
For each set of models, we allowed MATCH to deter-
WethereforeutilizethecumulativeSFHtoinformappro-
mine the best combinationof SFR, metallicity, distance,
priatetimebinningfortheabsoluteSFH.Foramorede-
4 Weisz et al.
absolute SFH derived with the Padova model as pur-
ple squares, and have over-plotted the SFH based on
imagingtakenwiththeLargeBinocularTelescope(LBT;
de Jong et al 2008b) as grey circles. Comparing the two
solutions,weseeonlysubtledifferences. First,duetothe
significantly deeper observations, the HST-based SFH
affords higher time resolution while maintaining SFR
uncertainties that are comparable to or smaller in am-
plitude than those presented in de Jong et al (2008b).
Specifically,wehavebinnedtheHST-basedsolutionwith
a time resolution of ∆log(t)= 0.2, compared to a value
0.3 for the LBT solution. Along with the higher time
resolution, we also see a decrease in SFR uncertainties.
Both of these improvements are particularly evident at
epochsmorerecentthanz∼0.5(∼5Gyrago),whereour
solutionconfirms that Leo T had a nearly constantSFH
toa highdegreeofconfidence. Within the mostrecent1
Gyr,thesmallnumberofluminousMSandbluecorehe-
liumburningstarsresultinlargefractionaluncertainties
on the SFH, restricting our ability to decipher precise
patterns of star formation.
Within the past 25 Myr, the precise SFR of Leo T
is difficult to quantify. On one hand, there are no lu-
minous MS or core helium burning stars, which can be
interpreted as a lack of recent star formation. How-
ever, the effects of stochastic IMF sampling also pro-
videanalternativeexplanation(e.g.,da Silva et al.2011;
Fumagalli et al. 2011). In this scenario, it is possible
that the recent SFR of Leo T is sufficiently low (. 10−5
M /yr) that star formation is continuous, but no mas-
⊙
sive stars are actually formed. Unfortunately, SFH and
Figure 2. Toppanel–ThelifetimecumulativeSFH,i.e.,thefrac- IMF effects are largely degenerate (e.g., Miller & Scalo
tion of total stellar mass formedprior to a given epoch, of Leo T
1979; Elmegreen & Scalo 2006), and we are not able to
measured using the Padova solar-scaled metallicity stellar evolu-
tion models (purple) and the BaSTI models (solar-scaled: green; distinguish between the two scenarios. As a result, we
α-enhanced: orange). Thelightlyshadedpurpleregionrepresents canonlyconcludethattheupperlimitontherecentSFR
the1-σuncertaintiesontheSFHmeasuredwiththePadovamod- inLeo T is ∼ 10−5 M –aSFR for which stochastic IMF
els. Bottom panel – The lifetimeabsolute SFH of Leo T, as mea- ⊙
sampling would not produce any luminous, young stars.
sured with the Padova models (purple squares). The plotted un-
certainties in the y-direction reflect the 1-σ uncertainties on the Integrated tracers of recent star formation in Leo T re-
SFR,whichthoseinthex-directionindicatethewidthofthetime veal only faint GALEX ultra-violet fluxes and no Hα
bin. For comparison, we have over-plotted the SFH measured by emission (Lee et al 2011; Kennicutt et al 2008), which
deJongetal(2008b)asgreycircles.
could be consistent with truncated star formation or a
stochastically sampled IMF scenario.
taileddiscussionofoptimizing time resolutionfor CMD- For epochs prior to z ∼ 2 (10 Gyr ago), we find
based SFHs, see Appendix A in Weisz et al (2011a). marginalimprovementovertheLBTsolution. TheHST-
ThebestfitSFHsfromthePadova(purple)andBaSTI based SFH confirms the relatively high SFR at ancient
models (α-enhanced: orange; solar-scaled: green) show times seen in the LBT solutions. On the whole, we con-
excellent consistency. Specifically, considering the mean cludethattheLBTandHSTSFHsareinexcellentagree-
cumulative SFHs for each model, we see that Leo T ment.
formed 50% of its total stellar mass prior to z ∼ 1 (7.6 TheCMDfittingprocessadditionallyprovidesarough
Gyr ago). For ages younger than this, Leo T exhibits estimate for the metallicity of Leo T. For solutions de-
a nearly constant SFH. For times prior to z ∼ 1 (7.6 rivedfromeachstellarlibrary,wefindameanisochronal
Gyr ago), the amplitude of the uncertainty envelope in- metallicity of [M/H] ∼ −1.6 – −1.8, which does not ex-
dicatesthatwecannotplacetightconstraintsonthepre- hibit significant variance over the lifetime of Leo T. Un-
ciseepochsofstarformation(i.e.,wecannotreliablydis- certainties on the mean metallicity are ∼ 0.3 dex. The
tinguish between separate bursts or extended constant meanisochronalmetallicitiesareslightlymoremetalrich
starformation). ThesimilarityintheSFHsderivedwith thanthemeanspectroscopicvalueof[Fe/H]∼−2.3±0.1,
different models suggests that the primary cause of the with a spread of 0.35 dex (Simon & Geha 2007). A
large uncertainties is the small number of truly ancient direct comparison between isochronal and spectroscop-
stars in the CMD. On the whole, we find that over the ically derived metallicities is challenging due to issues
WFPC2 field of view, the total stellar mass formed in such as RGB star selection effects and the conversion
LeoTis1.05+0.27×105M . Thisderivedvalueiscompa- from a canonical metallicity, i.e., [M/H], to [Fe/H] (e.g.,
−0.23 ⊙
rabletothepresentdaystellarmassestimateof1.2×105 Lianou et al. 2011). Due to these uncertainties, we are
M derived by Ryan-Weber et al (2008). only able to state that the isochronal metallicities from
⊙
In the bottom panel of Figure 2, we have plotted the the derived SFHs are coarsely comparable to the spec-
5
troscopically determined values. The SFH of Leo T provides additional insight into its
true morphological type. Although Leo T has a similar
6. SUMMARYANDCONCLUSIONS stellaranddarkmattermasstootherrecentlydiscovered
faint Local Group dwarf galaxies, its SFH, gas content,
We present the lifetime SFH of the Local Group
and location bear strong resemblance to nearby dwarf
gas-rich, faint dwarf galaxy Leo T based on deep
irregular galaxies (e.g., Mateo 1998; Dolphin et al 2005;
HST/WFPC2 imaging. The HST imaging covers nearly
Tolstoy et al 2009; Weisz et al 2011a,b, see Figure 3;).
the entire area defined by the half-light radius, and
TheaddedinformationfromtheSFHreinforcesprevious
the resulting CMD extends ∼ 2 mag below the ancient
suggestionsthat Leo T may be a Phoenix-like transition
MSTO.UsingtheMATCHCMDfittingroutine,wemea-
dwarf galaxy (e.g., Irwin et al 2007; Ryan-Weber et al
sured three SFHs of Leo T using the Padova (solar-
2008), i.e., gas-rich with current SF (e.g., Mateo 1998).
scaled) and BaSTI (solar-scaledand α-enhanced) stellar
However, extensive discussion of transition dwarfs in
evolution models. For all models considered, we found
Weisz et al(2011a),suggeststhatgalaxieswiththisdes-
virtually identical SFHs, confirming the robustness of
ignation are actually dwarf irregular galaxies that are
measurement. The SFH of Leo T shows that 50% of
eitherinbetweenepisodesofstarformationorinthepro-
its total stellar mass was formed prior to z ∼ 1 (7.6 Gyr
cessofpermanently losinggasdue to anexternaldistur-
ago)andthatthe SFH attimes youngerthanthis epoch
bance. Coupling this discussion with its empirical char-
isapproximatelyconstant. Thesparsesamplingofyoung
acteristicsLeoT appearsto bethe lowestmassdwarfir-
starsmakestheshapeoftheSFHwithinthemostrecent
regulargalaxyknowntodate. Consequently,thisfinding
1Gyruncertain. TheSFHofLeoTinthepast∼25Myr
suggests that the gas-rich dwarf galaxies share common
is uncertain due to the lack of luminous MS and BHeB
star formation processes across the entire known dwarf
stars. Their absence could either be due to a trunca-
galaxy mass spectrum (∼105−109 M ).
tion in star formation or stochastic sampling effects of ⊙
theIMF,butwecannotdistinguishbetweenthetwosce-
ACKNOWLEDGEMENTS
narios. We also found little evolution in the isochronal
DRWwouldliketothankJorgePenarrubiaforinsight-
metallicitiesofLeoT,suchthatoverthecoarseofitslife-
ful discussions on extremely low mass galaxies. NFM
time it is consistent with a constant value of [M/H] ∼
acknowledges funding by Sonderforschungsbereich SFB
−1.6.
881 ”The Milky Way System” (subproject A3) of the
GermanResearchFoundation(DFG).SupportforKMG
is provided by NASA through Hubble Fellowship grant
HST-HF-51273.01 awarded by the Space Telescope Sci-
ence Institute. This work is based on observationsmade
with the NASA/ESA Hubble Space Telescope, obtained
from the data archive at the Space Telescope Science
Institute. Partial support for this work was provided by
NASAthroughgrantsGO-11084andAR-10945fromthe
Space Telescope Science Institute, which is operated by
AURA, Inc., under NASA contract NAS5-26555. This
researchhasmade useofthe NASA/IPACExtragalactic
Database (NED), which is operated by the Jet Propul-
sion Laboratory, California Institute of Technology, un-
der contract with the National Aeronautics and Space
Administration. This research has made extensive use
ofNASA’s Astrophysics Data System BibliographicSer-
vices.
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Table 1
Youngest BinTime OldestBinTime AbsoluteSFR CumulativeSFFraction
log(t) log(t) (10−6 M⊙ yr−1)
(1) (2) (3) (4)
PadovaModel
6.60 9.00 5.02+−00..890300 1.00+−00..0000
9.00 9.20 5.87+−34..0955 0.95+−00..0011
9.20 9.40 8.85+−33..1363 0.92+−00..0032
9.40 9.60 4.46+−22..2024 0.84+−00..0055
9.60 9.80 5.39+−12..7243 0.78+−00..0066
9.80 10.00 8.83+−44..9687 0.66+−00..0096
10.00 10.15 9.15+−66..3977 0.36+−00..2230
BaSTISolar-Scaled
7.40 9.00 2.38+−11..9886 1.00+−00..0000
9.00 9.20 12.7+−54..0016 0.98+−00..0023
9.20 9.40 3.07+−22..3374 0.90+−00..0057
9.40 9.60 7.40+−21..3883 0.87+−00..0069
9.60 9.80 3.76+−53..7706 0.75+−00..0191
9.80 10.00 10.1+−23..5066 0.66+−00..1148
10.00 10.15 5.51+−95..2541 0.24+−00..3119
BaSTIα-enhanced
7.45 9.00 2.51+−11..4502 1.00+−00..0000
9.00 9.20 11.2+−44..0007 0.97+−00..0011
9.20 9.40 4.62+−22..3653 0.90+−00..0045
9.40 9.60 6.56+−11..7940 0.85+−00..0055
9.60 9.80 3.09+−63..3059 0.75+−00..0066
9.80 10.00 10.1+−32..1870 0.67+−00..1113
10.00 10.15 5.68+−65..8668 0.25+−00..2274
Note. — Theabsoluteand cumulativeSFHsof LeoTas derivedwith thePadovaandBaSTI stellarevolutionmodels. Columns(1) and (2)
indicatetheyoungestandoldestagesoftherespectivetimebins. TheabsoluteSFRincolumn(3)reflectstheSFRoverthedurationofthetimebin.
ThecumulativeSFvalueisthefractionoftotalstellarmassformedpriortothetimeindicatedincolumn(1). Thelisteduncertaintiesrepresentthe
16thand84thpercentilesmeasuredfromthesetofMonteCarlorealizations(§4). ByconstructionthecumulativeSFHiszeroatlog(t)=10.15.
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