Alvio Renzini
European Southern Observatory, Garching, D-85748, Germany
On leave from the University of Bologna, Department of Astronomy
, with a most likely value between 14 and 15Gyr; 3) the
determination of the age of metal rich clusters in the galactic bulge,
that together with NTT observations demonstrates that the galactic
bulge and halo formed at about the same time (i.e., 14--15Gyr ago),
and sets an upper limit of 
to any intermediate age
component; 4) the first results concerning the horizontal branch
luminosity-metallicity relation for globular clusters in M31; and 5)
the absence of hot horizontal branch stars in metal rich globulars
and the quest for the ultraviolet emission in elliptical galaxies at
low and moderate redshift.
Keywords: globular clusters,galactic bulge,elliptical galaxies
Globular clusters (GC) have been obvious HST targets since when HST was
first conceived. For given exposure time, the S/N ratio of faint
point-like sources scales as 
(the
telescope diameter over its resolving power), and therefore HST is
virtually equivalent to a 25m telescope operating with a 1 arcsec
seeing. This applies to isolated sources, but
HST can work in very crowded fields, where seeing would
further dramatically degrade the performance of ground-based telescopes.
So, HST is likely to remain the instrument par excellence for GC
imaging, even in the 8--10m telescope era.
Given the telescope performance, obvious GC targets for HST include:
Stars in the cluster central regions (
)
The bottom of the main sequence (MS)
White dwarfs (WD)
GCs in crowded fields
Giants in M31 GCs
and results pertaining to each of these categories have indeed been presented at this meeting. I have been asked by the organizers to summarize for the plenary session the highlights of the recent HST results on GCs that have been presented at the various parallel sessions. Since these proceedings will contain the corresponding papers, I will keep this review very succinct, while expanding somewhat on HST work on which I am directly involved.
The centers of GCs are among the most crowded fields one can imagine, yet they contain special objects that are produced only in this extreme environment. Moreover, knowing the dynamical state of GC cores is essential to understand their dynamical evolution. The high resolution of the HST, especially of the PC, allows accurate astrometry---hence proper motions---to be obtained for stars in the core of GCs, which gives important information on the cluster potential well and its dynamical status. In turn, this can be used in conjunction with other indicators (e.g., millisecond pulsars). In this mood, Meylan et al. (1996) have imaged the central regions of 47 Tuc, using for this purpose the ultraviolet F300W filter. At this wavelength, turnoff (TO) stars are as bright as stars at the tip of the red giant branch (RGB), and very clean cluster imaging is possible, without the annoying diffraction spikes and blooming columns that plague deep HST images of GCs at longer wavelengths. While still waiting for the second epoch observations of 47 Tuc, Meylan et al. have serendipitously discovered a very fast variable star (possibly some kind of cataclysmic): an object that has brightened over two magnitudes in less than one hour. Objects of this kind are likely to be binary, and most often inhabit the very crowded central regions of GCs, thus escaping detections by ground-based observations.
Determining the MS luminosity function (LF) to very faint limits, hence the
present-day mass function, is of great importance for our
understanding
of globular cluster formation and dynamical evolution, and to set
stringent constraints on the IMF. De Marchi has reported about the
extensive work in this direction by the group to which he belongs
(Paresce, De Marchi & Romaniello 1995, De Marchi & Paresce 1995a,1995b,
and these proceedings). Deep I-band LFs have been obtained for a
representative sample of clusters.
In spite of these clusters having had very different dynamical
histories, their LFs (as sampled near the half-light radius of the
clusters) look pretty similar, with a sharp peak at
, thereby declining towards fainter luminosities.
The peak corresponds to 
. At 
from the center of
NGC 7078
the LF is instead still rising, which suggests that low mass stars
have been expelled from the core by dynamical processes.
Quite similar results have been obtained by Cool, Piotto, & King
(1996, reported by Piotto at the meeting) for the clusters NGC 6397,
NGC 7078, and NGC 7099: the LF peak at 
is
confirmed, as well as the difference in LF between the core and the outer
parts of the clusters.
The authors emphasize that the snaky shape of the MS---with
at least three inflection points---should help constraining the
mass-luminosity relation, that is still uncertain for low mass stars.
Figure: (a): The HST 
, instrumental color-magnitude diagram of the
globular cluster NGC 6752. Reddening corrections have been applied.
(b): The absolute, instrumental color-magnitude diagram of the local,
calibrating WDs with known
trig parallaxes. The WD type is also indicated.
Figure: The instrumental color-magnitude diagram of the cluster
and local WDs, with the latter ones having been shifted in magnitude
to match the cluster sequence. This operation delivers the distance
modulus of the cluster: 
. The straight line is an
eye fit to the cluster WD sequence.
Figure: The luminosity difference between the HB and the turnoff
is plotted for a representative set of halo globular clusters
(filled circles) and for NGC 6528 and NGC 6553 (filled squares). For this
display [Fe/H]=0.0 and 0.1 has been adopted for the metallicity of the two
clusters. The lines correspond to ages of 18, 15, and 12Gyr (upper, middle,
and lower lines, respectively), with the following assumptions: 1)
helium abundance 
; 2) no enhancement of 
-elements
(i.e., solar proportions);
3) HB luminosity-metallicity relations 
= 1.17+0.39[Fe/H]
(solid lines), and 
=
0.73+0.15[Fe/H] (dotted line).
See Ortolani et al. (1995).
A quite successful attempt at reproducing the shape of the MS
with theoretical models was presented in a poster by Brocato et al.
(these proceedings). Models and cluster data are indistinguishable at
least down to 
, then the models are slightly bluer at
fainter magnitudes. These models very effectively illustrate the dominant
role played by the mass-luminosity relation in determining the shape
of the cluster LF. For 
the V-band luminosity drops
by about one magnitude as the mass decreases by 
. Instead,
Between 0.2 and 
the luminosity drops by 3 magnitudes, and
keeps dropping even more precipitously as the hydrogen burning limit
is approached. No doubt, such a big drop in luminosity (starting
indeed at
) must play a key role in producing
the rarefaction of the MS and the drop in the LF below this limit.
Of course, what is most interesting is the cluster mass function,
but enduring uncertainties in the M-L relation for 
still hamper a secure determination.
The detection of WDs in galactic GCs was listed among the top priorities of HST already in the Patras Book, back in 1982. The promise has been fully maintained indeed: as soon as the repaired HST has taken deep images of the nearby GCs the WDs have been found in the expected color-magnitude location and in the expected number (De Marchi, Paresce, & Romaniello 1995, Richer et al. 1995, Cool, Piotto, & King 1996). While this hardly came as a surprise, it is somewhat reassuring that indeed low mass stars die as theory predict they should.
To take full advantage of HST ability to detect GCWDs a special
experiment was designed to use the WD cooling sequence as a
standard candle, thus improving upon the determination of the distance
to nearby globular clusters (Renzini et al. 1996). Fig. 1a shows the
WDs detected with WFPC2 (W4 chip) at about 3 core radii from the
center of NGC 6752, one of the two nearest, low-reddening GCs.
Fig. 1b shows the absolute color-magnitude diagram (also
obtained with WFPC2) for a sample
of WDs in the solar neighborhood with well determined trig parallaxes,
and with mass close to that of GCWDs (
--
). A
comparison of the two panels shows that the two objects lying below
the main WD sequence are most likely WDs of the DB variety, as also
indicated by the U-band (F336W) photometry of the same objects.
The two
objects lying above the main WD sequence are most likely WDs in a
blend with a faint red
main sequence star, as suggested by the I-band (F814W) photometry.
Fig. 2 shows that a vertical shift by 13.05 mag is required to bring
the local WDs into coincidence with the WDs in NGC 6752, with the
formal accuracy of the fit being just a few 0.01 mag. This
determination improves considerably upon the accuracy with which we
know the distance modulus of a globular cluster, that
realistic estimates indicate to be 
--
mag when using
other standard candles (RR Lyrae and subdwarfs).
This WD method of distance determination is similar in approach to the traditional subdwarf method (fitting the cluster MS to local, trig parallax subdwarfs), but offers several advantages over it. Indeed, WDs have metal-free atmospheres and, therefore, the cluster and local WD progenitor metallicities do not need to be specified in order to determine the distance modulus. Other methods instead need both metallicities, which basically limits the accuracy of the distance determination. Moreover, WDs are locally much more abundant than subdwarfs, which allows further improvements in the future.
An accurate distance to the clusters is essential for determining
accurate GC ages. As a rule of thumb, the relative error in age is
equal to the the error in distance modulus, i.e., a 0.25 mag error in
the modulus generates a 
error in age (Renzini 1991). Other
observational uncertainties (e.g., the GC chemical composition) convey
substantially smaller age uncertainties. Thus, both
tightening the
Hubble constant (cf. Tammann's and Freedman's
contributions at this meeting) and GC ages
require an effort towards obtaining more accurate distances.
Armed with 
for NGC 6752, Renzini et al. (1996) determine
the age of
this cluster to be 15.3Gyr when helium diffusion is not taken into
account, and 14.0Gyr when such effect is included. This assumes an
-element enhancement [
/Fe]=0.6. The result is in very
good agreement with virtually any other determination of metal-poor GC
ages, but we claim to have substantially reduced the uncertainty in
this determination, from 
to below 
. The
implications one can derive from coupling this age to current
determination of 
are rather obvious, and will not be discussed further.
The case of bulge GCs offers an example of HST photometry for clusters
that lie in very crowded fields, for which even the outer parts are
difficult to study from the ground at TO luminosities. Ortolani et al.
(1995) have obtained sufficiently deep HST photometry of two metal rich
clusters of the bulge ([Fe/H]
), namely NGC 6528 and NGC
6553. The color-magnitude diagrams of the two clusters are remarkably
identical, from the MS all the way to the tip of the RGB.
Moreover, their luminosity difference between the horizontal branch (HB) and
the TO is the same, or even slightly larger, than that of the more
metal poor cluster 47 Tuc ([Fe/H]=-0.7). This implies that the two bulge
clusters are nearly as old as the inner halo cluster 47 Tuc, and
demonstrates that the bulge underwent rapid chemical enrichment to
solar abundance and beyond, very early in the evolution of our Galaxy.
Moreover, Ortolani et al. (1995) show that the MS luminosity function of the
two clusters is indistinguishable to that of the stars in Baade's
Window
(the low-reddening bulge field at 
from the Galaxy
center), that they have obtained with the ESO NTT with superb seeing
(
). From this Ortolani et al. (1995) infer that the whole bulge formed
quickly, some 15Gyr ago, and set an upper limit of
by number to any intermediate age population in the bulge.
Our Milky Way is a spiral galaxy in a very loose group that is
located rather away from major density peaks in the distribution of
galaxies. Nevertheless, her whole spheroidal component looks 
one
Hubble time old, from the outer halo globular clusters all the way to
the inner bulge. Along with the well known homogeneity of elliptical galaxies,
this appears to require a cosmological framework
allowing a very quick build up of spheroids---no
matter
whether in clusters or in the field---early in
the evolution of the universe. The
successful detection (finally!) of forming 
galaxies at
(cf. Giavalisco's contribution at this meeting) seems to
provide the direct evidence of such events.
Ortolani et al. (1995) study of bulge clusters leaves unanswered one
fundamental question: is the bulge younger or older than the halo?
i.e., did the galactic spheroid form outside-in or inside-out?
The answer to this question depends on
which calibration is adopted for the HB luminosity vs. metallicity
relation, i.e., on the slope (a) of the relation
[Fe/H]+b,
being the absolute magnitude of HB stars. If 
the inside-out scenario is favored, while the outside-in option is
favored if 
. The case is illustrated in Fig. 3.
A direct calibration of the the HB luminosity-metallicity relation can be accomplished for galactic globulars by applying to other clusters the WD method described in section 4, and precisely with this purpose in mind we have recently acquired---but not yet reduced---the corresponding HST data for the cluster 47 Tuc. A complementary approach consists in looking to a family of GCs all at the same distance, and get the relation in a very straightforward way. The ideal case is offered by GCs in M31.
Figure: The flux sampled by the F555W and F814W filters, and the 
upper limit to the flux trough the F218W filter for the brightest
elliptical galaxy in the WFPC2 field of view in the 
cluster
A895, as a function of the rest frame wavelength. The spectral energy
distribution of NGC 4649 (one of the local ellipticals with the
strongest UV rising branch) is also shown.
Rich et al. (1995) have demonstrated on the M31 globular G1 that
sufficiently accurate photometry can be obtained well
below the HB with WFPC2, the instrument of election for this kind of
purpose. Fusi Pecci et al. (these proceedings) have combined WFPC2
and FOC M31 data for 7 GCs to derive 
. Seemingly, Ahjar
et al. (1996) have obtained WFPC2 data for 4 M31 GCs, and derive
. These determinations appear to favor a flat
HB luminosity-metallicity relation, though the error bars are still
large,
and it may be premature to conclude that spheroids form outside-in.
But ongoing HST observations of GCs in both the Galaxy and M31 should
soon provide a firmer answer.
Globular clusters in the Magellanic Clouds have also been extensively observed
during Cycles 4 and 5, though few results have been published so far
(e.g., Gilmozzi et al. 1995).
Spanning a very wide range of ages, from few 
to 
yr,
and with many of them being very populous, MC globulars are of
fundamental importance for stellar population studies. Indeed,
MC clusters provide the best available stellar population
templates at ages
younger than galactic GCs, and allow the best possible
determination of the IMF in an extended range of masses above 
. The systematic HST study of a representative sample of the
most populous MC globulars should be completed in the near future.
IUE observations have shown that the UV output of nearby elliptical galaxies
increases with galaxy metallicity, as measured by the Mg
index
(Burstein et al. 1988). Purely energetic arguments, that are not prone
to the arbitrariness afflicting the UV spectral energy distribution
models, indicate hot HB stars and their progeny as the most likely
UV emitters in ellipticals (Greggio & Renzini 1990). Such hot HB
stars can be produced by old stellar populations provided either the mass
loss rate along the RGB or the helium abundance increase enough with
metallicity (or some combination thereof). Very metal-rich GCs may
offer a chance to check whether indeed the HB turns towards high
temperatures when high metallicities are reached. The Ortolani et al. (1995)
study of NGC 6528 and NGC 6553 shows
that, at least up to near solar metallicity, this is not the case: even
the most metal-rich GCs in the bulge have very red HBs.
Should we conclude that the UV of ellipticals is not produced by hot
HB stars? This may be a premature conclusion, as the bulge clusters
may not be as metal rich as the most metal-rich stars in ellipticals,
and their metallicity may be below the threshold for the transition
to the hot HB to take place.
An alternative check of the hot HB option is offered by stellar evolution theory. For decreasing age of a stellar population the HB is indeed predicted to move from high to low temperatures. Hence, if the UV of local elliptical is due to hot HB stars, the UV emission should rabidly decrease with lookback time, i.e., with redshift. At a redshift of 0.3--0.4 most hot HB stars should have already disappeared.
To check this prediction the cluster of galaxies A895 (z=0.37)
was observed with
WFPC2 during Cycle 4, spending a total of 9600 s integrating through
the F218W filter, and complementing this with short F555W and F814W
exposures. After troublesome cosmic ray removal and calibrations, the
preliminary result is shown in Fig. 4 for the brightest (hence
possibly most metal rich) elliptical in
the field of view (Viezzer et al. 1996).
Having obtained the fluxes at the three filters wavelengths, Viezzer et
al.
proceed to compare them to the spectral energy distribution
of the local metal-rich elliptical galaxy NGC 4649 (L. Buson, private
communication). Once the normalization is made at the rest frame
--6400Å\
range (sampled by the F814W filter), the rest frame 
Å\
flux of the galaxy in A895 appears appreciably in excess of the flux
of NGC 4649, that may partly be accounted by the passive evolution
effect, part may be due to the galaxy being perhaps an E+A object.
No flux at all is instead detected through the F218W filter, that
samples the rest frame 1500Å region, i.e., the UV rising branch of
nearby ellipticals. Unfortunately, the UV sensitivity of WFPC2 is
rather low, the background fairly high, and the corresponding
upper limit almost coincide with the 1550Å flux of NGC
4649.
One may say that indeed there is 
probability that the flux
is lower than in NGC 4649---thus supporting the theoretical
expectation of the disappearance of the UV rising branch---but deeper
UV exposures are needed to really prove the case.
The far better UV sensitivity of STIS
should easily allow to solve the problem.
I'm grateful to my HST PI's and Co-I's, with whom it was a real pleasure to collaborate. I'm also grateful to Lucio Buson for having kindly provided the file with the SED of NGC 4649.
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