The ACS LCID project. VI. The SFH of the Tucana dSph and the relative ages of the isolated dSph galaxies

Abstract

We present a detailed study of the star formation history (SFH) of the Tucana dwarf spheroidal galaxy. High quality, deep HST/ACS data, allowed us to obtain the deepest color-magnitude diagram to date, reaching the old main sequence turnoff (F814 ~ 29) with good photometric accuracy. Our analysis, based on three different SFH codes, shows that Tucana is an old and metal-poor stellar system, which experienced a strong initial burst of star formation at a very early epoch (~ 13 Gyr ago) which lasted a maximum of 1 Gyr (sigma value). We are not able to unambiguously answer the question of whether most star formation in Tucana occurred before or after the end of the reionization era, and we analyze alternative scenarios that may explain the transformation of Tucana from a gas-rich galaxy into a dSph. Current measurements of its radial velocity do not preclude that Tucana may have crossed the inner regions of the Local Group once, and so gas stripping by ram pressure and tides due to a close interaction cannot be ruled out. On the other hand, the high star formation rate measured at early times may have injected enough energy into the interstellar medium to blow out a significant fraction of the initial gas content. Gas that is heated but not blown out would also be more easily stripped via ram pressure. We compare the SFH inferred for Tucana with that of Cetus, the other isolated LG dSph galaxy in the LCID sample. We show that the formation time of the bulk of star formation in Cetus is clearly delayed with respect to that of Tucana. This reinforces the conclusion of Monelli et al. (2010) that Cetus formed the vast majority of its stars after the end of the reionization era implying, therefore, that small dwarf galaxies are not necessarily strongly affected by reionization, in agreement with many state-of-the-art cosmological models. [abridged] Comment: Accepted for publication on ApJ, 19 pages, 10 figures, 2 tables. A version with full resolution figures is available at http://www.iac.es/project/LCID/?p=publications

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arXiv:1010.2982v1 [astro-ph.CO] 14 Oct 2010
Draft version October 15, 2010
Preprint typeset using LATEX style emulateapj v. 11/10/09
THE ACS LCID PROJECT. VI. THE SFH OF THE TUCANA DSPH
AND THE RELATIVE AGES OF THE ISOLATED DSPH GALAXIES1
M. Monelli2,3, C. Gallart2,3, S.L. Hidalgo2,3, A. Aparicio2,3, E.D. Skillman4, A.A. Cole5, D.R. Weisz4, L.
Mayer6,7, E.J. Bernard8, S. Cassisi9, A.E. Dolphin10, I. Drozdovsky2,3,12, P.B. Stetson11
Draft version October 15, 2010
ABSTRACT
We present a detailed study of the star formation history (SFH) of the Tucana dwarf spheroidal
galaxy. High quality, deep HST/ACS data, collected in the framework of the LCID project, allowed
us to obtain the deepest color-magnitude diagram to date, reaching the old main sequence turnoff
(F814 ∼ 29) with good photometric accuracy. Our analysis, based on three different SFH codes,
shows that Tucana is an old and metal-poor stellar system, which experienced a strong initial burst of
star formation at a very early epoch (≃ 13 Gyr ago) which lasted a maximum of 1 Gyr (sigma value).
We are not able to unambiguously answer the question of whether most star formation in Tucana
occurred before or after the end of the reionization era, and we analyze alternative scenarios that
may explain the transformation of Tucana from a gas-rich galaxy into a dSph. Current measurements
of its radial velocity do not preclude that Tucana may have crossed the inner regions of the Local
Group once, and so gas stripping by ram pressure and tides due to a close interaction cannot be
ruled out. A single pericenter passage would generate insufficient tidal heating to turn an originally
disky dwarf into a true dSph; however, this possibility would be consistent with the observed residual
rotation in Tucana. On the other hand, the high star formation rate measured at early times may
have injected enough energy into the interstellar medium to blow out a significant fraction of the
initial gas content. Gas that is heated but not blown out would also be more easily stripped via ram
pressure. We compare the SFH inferred for Tucana with that of Cetus, the other isolated LG dSph
galaxy in the LCID sample. We show that the formation time of the bulk of star formation in Cetus
is clearly delayed with respect to that of Tucana. This reinforces the conclusion of Monelli et al.
(2010) that Cetus formed the vast majority of its stars after the end of the reionization era implying,
therefore, that small dwarf galaxies are not necessarily strongly affected by reionization, in agreement
with many state-of-the-art cosmological models.
Subject headings: Local Group galaxies: individual (Tucana dSph) galaxies: evolution galaxies: pho-
tometry Galaxy: stellar content
1. INTRODUCTION
1Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science In-
stitute, which is operated by the Association of Universities for
Research in Astronomy, Inc., under NASA contract NAS5-26555.
These observations are associated with program 10505.
2Instituto de Astrof´ ısica de Canarias, La Laguna, Tener-
ife,Spain;ebernard@iac.es,
dio@iac.es, antapaj@iac.es, slhidalgo@iac.es.
3Departamento de Astrof´ ısica, Universidad de La Laguna,
Tenerife, Spain.
4Department of Astronomy, University of Minnesota, Min-
neapolis, USA; skillman@astro.umn.edu.
5School of Mathematics & Physics, University of Tasmania,
Hobart, Tasmania, Australia; andrew.cole@utas.edu.au
6Institut f¨ ur Theoretische Physik, University of Zurich,
Z¨ urich, Switzerland; lucio@physik.unizh.ch
7Department of Physics, Institut f¨ ur Astronomie, ETH
Z¨ urich, Z¨ urich, Switzerland; lucio@phys.ethz.ch.
8Institute for Astronomy, University of Edinburgh, Royal
Observatory,Blackford Hill,
ejb@roe.ac.uk
9INAF-Osservatorio Astronomico di Collurania, Teramo,
Italy; cassisi@oa-teramo.inaf.it.
10Raytheon; 1151 E. Hermans Rd., Tucson, AZ 85706, USA
11Dominion Astrophysical Observatory, Herzberg Institute
of Astrophysics, National Research Council, 5071 West Saan-
ish Road, Victoria,British Columbia V9E 2E7,
peter.stetson@nrc-cnrc.gc.ca.
12Astronomical Institute, St. Petersburg State University, St.
Petersburg, Russia
monelli@iac.es,carme@iac.es,
Edinburgh EH93HJ,UK;
Canada;
Nearby galaxies in the Local Group (LG) present a
variety of different properties in terms of stellar popu-
lations, gas and metal content, morphological type, and
mass. Understanding their differences and similarities
and how these evolved with time can bring important
insights into our knowledge of the formation and evolu-
tion of the LG, and of galaxies in general. The recovery
of their full star formation history (SFH) plays a key
role, being among the most powerful techniques to dig
into the mechanisms that drove the evolution of stellar
systems. Precise estimates of the epochs of star forma-
tion and their duration give direct information to com-
pare the observed properties with predictions. This is
particularly important for investigating the first stages
of the life of galaxies, because the properties of the stel-
lar populations during the first few Gyr can be used to
trace the impact of a variety of different physical mecha-
nisms. For example, a delay in the onset of the star for-
mation, or a truncation of the SFH at a very old epoch
can possibly be used to infer the role of cosmic reion-
ization (e.g., Ikeuchi 1986; Rees 1986; Efstathiou 1992;
Babul & Rees 1992; Chiba & Nath 1994; Quinn et al.
1996;Thoul & Weinberg
Barkana & Loeb 1999; Bullock et al. 2000; Tassis et al.
2003; Ricotti & Gnedin 2005; Gnedin & Kravtsov 2006;
Okamoto & Frenk 2009; Sawala et al. 2010; Mayer2010).
On the other hand, an episodic rather than continu-
1996; Kepner et al.1997;

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2 Monelli et al.
ous SFH might trace interactions of dwarf galaxies in
their orbit around bigger systems (Mayer et al. 2001),
or a possible cycle related to changes in the interstellar
medium linked to supernova explosions (Carraro et al.
2001; Stinson et al. 2007; Valcke et al. 2008). What is
crucial in these kinds of analysis is the capability to pre-
cisely and accurately recover how these systems formed
stars as a function of time, and to characterize their
chemical evolution.
Within this general context, we selected a sample of
isolated dwarf galaxy members of the LG, with the aim
of deriving quantitative SFHs over their entire lifetime.
The main objective of the LCID project (Local Cosmol-
ogy from Isolated Dwarfs14) is to study the details of the
evolution of nearby isolated galaxies, compare their prop-
erties and help constrain cosmological and galaxy forma-
tion and evolution models. A general description of the
project, and an overview of the results, are presented in
Gallart et al. (in prep). This paper is focused on the
study of one the most isolated dSph galaxies in the LG,
Tucana, with particular emphasis on the comparison its
properties with those of the Cetus dSph (Monelli et al.
2010b).
Understanding the currently isolated nature of Tu-
cana and Cetus is challenging in the context of galaxy-
formation theories. Models have been proposed sug-
gesting that dSph galaxies originate from small, initially
gas-rich galaxies that lost their gas through tidal and
ram-pressure interactions with their large host galax-
ies (see Mayer 2010, for a review). Before the LCID
project, however, the existing observations of Cetus and
Tucana did not allow us to draw definite conclusions on
the exact nature of these galaxies, particularly regard-
ing the question of how extended in time their SFHs
were. In Monelli et al. (2010b) and in this paper, we
show that Cetus and Tucana are similar to the oldest
Milky Way dSph satellites such as Draco, Ursa Minor,
Sculptor and Sextans, and therefore they do not follow
the morphology-density relation observed among Milky
Way dSph satellites (e.g. van den Bergh 1999). Their
mere existence, therefore, imposes new constraints on
dSph galaxy-formation models.
The first mention of the Tucana dSph appears in the
Southern Galaxy Catalogue published by Corwin et al.
(1985), but it was later ”rediscovered” by Lavery (1990).
The first CMD was presented in Lavery & Mighell
(1990), based on Anglo-Australian 3.9m V , I images.
Tucana appeared as an elongated spheroid (e=0.5), and
was classified as a dE5. Although their photometry
was quite shallow, Lavery & Mighell (1990) could give
the first estimates of the distance ((m − M) < 24.75),
metal content ([Fe/H] = −1.9) and luminosity (MV =
−9.5). Tucana was definitively recognized as a mem-
ber of the LG, with the peculiarity of being the first
isolated dSph, not linked with either the Milky Way or
M31. Lavery & Mighell (1990) also inferred that Tucana
is a predominantly old system, based on the lack of young
bright blue objects.
All these preliminary findings were later substantially
confirmed by deeper data. Castellani et al. (1996) pre-
sented V , I photometry reaching V ∼ 26 mag, which al-
lowed them to estimate the distance ((m−M)I= 24.72 ±
14http://www.iac.es/project/LCID
0.20) based on the tip of the red giant branch (RGB), and
a mean metallicity of [Fe/H] = −1.56, obtained compar-
ing the Tucana RGB with the ridge line of Galactic glob-
ular clusters (GCs) from Da Costa & Armandroff (1990).
Interestingly enough, they found that the color width of
the RGB was larger than expected on the basis of the
photometric errors. They interpreted this as a signature
of a ”consistent spread” in the metallicity of the Tucana
stars.
A different conclusion was reached by Saviane et al.
(1996), who presented V , I data collected with the
EFOSC camera mounted on the 2.2m ESO tele-
scope. A comparison with the same database from
Da Costa & Armandroff (1990) yielded a mean metal-
licity [Fe/H]∼ −1.80, and “no convincing indication of
an abundance spread.” An interesting point raised by
Saviane et al. (1996) concerns the absolute dimension of
Tucana. Using an exponential profile, they estimated a
core radius of 166 pc, similar to that of the small Milky
Way and M31 satellites (Mateo 1998).
Another very interesting characteristic of Tucana is the
strong horizontal branch morphology gradient first de-
tected by Harbeck et al. (2001), together with a double
peak in the color distribution of the RGB stars. Based
on these two hints, and on the likely absence of in-
termediate age stars (supported by the non-detection
of C stars, Battinelli & Demers 2000), Harbeck et al.
(2001) suggested that Tucana experienced a very fast
self-enrichment. This possibility was strongly supported
by Bernard et al. (2008) on the basis of the properties of
a sample of ∼400 RR Lyrae stars. They detected two dif-
ferent sub-populations of RR Lyraes having distinct spa-
tial distributions and different mean luminosities, and
following two period-amplitude relations with different
slopes. This supports a scenario with two different sub-
populations older than 10 Gyr, the more centrally con-
centrated one being slightly younger and slightly more
metal rich than the other.
The paper is organized as follows. §2 summarizes the
observations and data reduction strategy. In sect. §3
we present the CMD, while the derivation of the SFH
is extensively discussed in §4.
properties of the two isolated dSphs Cetus and Tucana.
Discussion and conclusions are presented in §6 and §7.
In §5 we compare the
2. OBSERVATIONS AND DATA REDUCTION
The data presented in this paper were collected with
the ACS camera aboard the HST (Ford et al. 1998), as
part of the project The onset of star formation in the
universe: constraints from nearby isolated dwarf galaxies
(PID 10505, PI C. Gallart).
The 32 orbits allocated to Tucana were executed be-
tween April 25 and 30, 2006.
split into visits, each of them including two orbits. One
F475W and one F814W image were collected during
each orbit, with exposure times slightly different among
the two orbits of the same visit: 1,070s and 957s in the
first orbit, 1,090s and 979s in the second one. Summa-
rizing, the total observing time on Tucana was 34,560s
in F475W and 30,976s in F814W. Note that, despite
the fact that Tucana is the most distant galaxy in the
LCID sample, the exposure times adopted are the short-
est. This choice was forced due to the constraints im-
The observations were

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The Tucana dSph galaxy3
Fig. 1.— Stacked, drizzled color image of the Tucana field. North is up and East is left. The field of view is ∼3′4×3′4. The image shows
a clear gradient in the number of stars when moving from the center to the outskirts. A sizable number of background galaxies is visible
as well.
posed by the South Atlantic Anomaly.
The pointing (α = 22h41m48s.43, δ = –64◦25′15′′.7)
was centered on the galaxy. Figure 1 shows a drizzled,
stacked image. The parallel WFPC2 field already pre-
sented in Bernard et al. (2009) was intended to sample
the halo of Tucana along its major axis, at the position
(22h40m52s.92; –64◦24′28′′.9). The CMD of this field,
located 6′from the Tucana center, does not show any con-
vincing evidence that stars of Tucana are still present at
this distance, since no RR Lyrae stars were detected.
2.1. Data reduction
The details of the data reduction strategy, com-
mon to all the LCID galaxies, have been presented in
Monelli et al. (2010b), so here we just recall the main
points. We performed two parallel and independent pho-
tometric reductions, using the DAOPHOT/ALLFRAME
(Stetson 1994) and DOLPHOT packages (Dolphin
2000b). We applied both codes to the original FLT im-
ages, working on the two chips separately. In the case of
DAOPHOT, we modeled individual PSFs for each frame,
using bright and isolated stars in the field. Typically, 400
stars and a Moffat function with index β = 1.5 were used.
The DOLPHOT code was applied using the ACS mod-
ule and following the recommended photometry recipe
provided in the manual for version 1.0.3.
Individual photometry catalogs for each frame were
calibrated. We applied the calibration provided by
Sirianni et al. (2005) identically to both sets of photom-
etry. The final CMD is presented in Fig. 2 and discussed
in section 3.
2.2. Completeness tests
The completeness tests were performed by adding syn-
thetic stars to the individual images, and repeating the
photometric analysis identically as for the real data.
In the case of the DAOPHOT photometry the list of
injected stars was created following the prescriptions in-
troduced by Gallart et al. (1996), using a synthetic CMD
created with IAC-star (Aparicio & Gallart 2004). This
was built adopting a constant star formation rate be-
tween 0 and 15 Gyr, and a metallicity distribution fol-
lowing a flat distribution in the range 0.0001 < Z <
0.005. Approximately 1,500,000 stars were simulated in
each of the two chips, in iterations of ∼ 50,000 stars.
In the case of DOLPHOT, we followed the method de-
scribed in Holtzman et al. (2006), simulating one star at
a time and covering the range −1 < MF475W−M814W<

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4 Monelli et al.
Fig. 2.— CMD of Tucana and Cetus obtained with DAOPHOT. Note that the color of the bluest HB stars is similar in both galaxies, but
the number of blue HB stars is significantly higher in Tucana than in Cetus. The RGB bumps are also obvious features in both galaxies as
indicated by the arrows on the right side of the RGBs. The arrow on the left side of the Tucana RGB mark the AGB clump. (Monelli et al.
2010a)

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The Tucana dSph galaxy5
5 mag, −7 < MF475W < 8 mag. The density of injected
stars is higher in the color-magnitude range where most
of the observed stars are found. A total of 140,000 stars
were simulated. We found that, at the level of the TO,
the completeness is of the order of 80%.
2.3. Comparison of the two photometry setss
We detect a small zero point offset for the brightest
stars in the mF475W (mF475W,DAO − mF475W,DOL =
+0.019 mag, for mF475W < 25) and in the mF814W
(mF814W,DAO − mF814W,DOL
mF814W < 24) bands, and a trend as a function of
magnitude. As already noticed in the case of Cetus
(Monelli et al. 2010b), moving toward fainter magni-
tudes, DAOPHOT tends to measure stars slightly fainter.
However, since the trend with magnitude is similar in
the two bands, the color difference shows a small resid-
ual zero point (∼ 0.032 mag), but no dependence on
the magnitude. As already noticed in the case of Cetus
(Monelli et al. 2010b) and LGS 3 (Hidalgo et al. 2010),
and discussed in Holtzman et al. (2006) in the case of
the WFPC2, small differences in the photometry are ex-
pected when using different packages. Most importantly,
we will show in section 4.5 that the impact of the differ-
ences in the photometry on the derived SFH is negligible.
=
−0.008 mag, for
3. THE CMD
Fig.2, left panel, presents the final CMD for Tu-
cana from the DAOPHOT photometry, calibrated to the
VEGAMAG system and shifted to the absolute plane.
The right panel shows the CMD of Cetus (Monelli et al.
2010b). The comparison of the two CMDs shows strik-
ing similarities in the different evolutionary features. An
in-depth comparison of the CMDs and the properties of
these two galaxies is presented in §5.
The Tucana CMD presented here is the deepest ob-
tained for this galaxy to date. It spans more than ∼8
mag, from the tip of the RGB down to ≈1.5 mag be-
low the oldest main sequence (MS) turn-off (TO), or
MF814W ∼4. The absence of a blue young MS and
the morphology of the old MSTO, at MF814W ∼ 2.5
mag, are typical features of an old stellar system, with
no strong recent or intermediate-age star formation. In
agreement with what is commonly found in stellar sys-
tems, the plume of relatively faint objects appearing at
1.5 < MF814W < 3, 0 < (MF475W− MF814W) < 0.5 is
most likely a population of blue stragglers. The possi-
bility of some residual star formation occurring in the
last 3-4 Gyr is discussed in §4.5. The RGB appears as a
dominant feature, between -4 < MF814W < 2 mag. The
width of the RGB suggests a limited spread in metal-
licity. The two over-densities located along the RGB at
MF814W ∼ −0.9,−0.7 and MF475W− MF814W ∼ +1.4
and highlighted by two arrows are the RGB bumps (see
Monelli et al. 2010a for a detailed study of the RGB
bump feature in several LCID galaxies).
clear over-density at MF814W ∼ −0.8 and MF475W −
MF814W∼ 1.2, and joining the RGB from the blue side,
can be identified with the AGB bump and the subsequent
AGB.
The Tucana HB, as previously noted by Harbeck et al.
(2001), shows a complex morphology, which can be ap-
preciated in detail in the present, much deeper data.
The other
Fig. 3.— Tucana CMD with superimposed ZAHBs of different
metallicities Z=0.0001 (dotted), 0.0003 (dashed), 0.0006 (solid)
and Y= 0.245 (red), 0.30 (green), 0.35 (blue) are shown. Only
the theoretical predictions with normal helium give a good repre-
sentation of the HB observed in Tucana.
It is well populated from the red to the blue side, to
MF475W−MF814W∼ 0. A few bluer and fainter objects
seem to continue the HB sequence, but they merge with
the BSs candidates and it is not possible to draw firm
conclusions on their nature on the basis of the present
data. A possible scenario to explain the strong blue com-
ponent of the Tucana HB is that this feature is populated
by a helium-enriched population. Helium-rich stars have
been invoked to explain the properties of the most mas-
sive Galactic globular clusters which have been proved
to host more than one population (Piotto et al. 2005;
Caloi & D’Antona 2005; Milone et al. 2008, 2010). The
effect of the helium enrichment is to produce, at fixed age
and for MF475W−MF814W> −0.2, a bluer and brighter
HB. Fig. 3 shows the Tucana CMD with selected ZA-
HBs from the BaSTI database superimposed, for metal-
licity Z=0.0001, 0.0003, 0.0006 and Y=0.245, 0.30, 0.35.
The figure shows that the HB is nicely bracketed by the
canonical helium ZAHBs, and a metallicity in the range
0.0003-0.001. Note that this is in excellent agreement
with the results of the SFH, suggesting a mean metal-
licity of Z = 0.0005 (see §4.5). The helium-rich ZAHBs
appear too bright to explain any significant number of
Tucana HB stars.
4. THE STAR FORMATION HISTORY OF TUCANA
The SFH of Tucana was derived,
the other galaxies of the LCID sample, using the
two photometry sets, two stellar evolution libraries
(BaSTI15, Pietrinferni et al. 2004 and Padova/Girardi16,
Girardi et al. 2000) and three different SFH codes:
IAC-pop (Aparicio & Hidalgo 2009), MATCH (Dolphin
2002), and COLE (Skillman et al. 2003). We performed
the same kind of analysis already presented for Cetus
(Monelli et al. 2010b) and LGS3 (Hidalgo et al. 2010).
In particular, we used the three SFH codes together
with the DOLPHOT photometry in combination with
similarly to