CONFERENCE SUM1V1ARY

B.E.J. Pagel 1

1 NORD/TA, Copenhagen, Denmark.

Abstract

Some highlights of the conference are noted and commented on, especially abundances and chemical evolution and some problems relating to the overall cosmic abundance of heavy elements.

1 Introduction

A previous meeting on dwarf was held at OHP nearly 5 years ago.

At that meeting there were two concluding summaries, one from an observational point of view by Paul Hodge and the other from a theoretical point of view by Donald Lynden-Bell, who ended the conference with a disquisition on anti-tank weapons. Several of the issues discussed at that meeting have also come up at this one, e.g. dark matter, formation histories, chemical abundances, gas distribution and dynamics, clustering properties and environmental effects. Apart from 5 years' worth of advances in these and other relatively traditional fields , the most striking change at the present meeting is a dramatic advance in the cosmological con­ nection. resulting from the CF and AAT red-shift surveys, HST observations including the Hubble Deep Field and the detailed studies of interveningiabsorption-line systems in front of quasars. Red-shift surveys have shown that irregular star-forming galaxies are the ones having the strongest cosmological evolution at moderate red-shifts, but to what extent these can- be classed as dwarfs is controversial. The maintopics covered at this meetingwere: Statistical properties of local dwarfs, Intrinsic properties reievant to cosmoiogy, Properties at significant iook-back times, ivi odeis and theory and Prospects with new instrumentation. In this brief summary I can only comment briefly on what s�ruck me being some of the highlights. I thank Eva Grebel, Fran�ois Hammer and as Max Pettini for some additional comments and corrections to my spoken version.

481 2 Statistical properties of local dwarfs

The local luminosity function turns up at faint magnitudes (Jonathan Loveday and Neil Tren­ tham), and this is confirmedby morphological classification in the HDF (Simon Driver), leaving the possibility that dwarf galaxies make a significant contribution to cosmic matter density, lim­ ited by the resulting contribution to intra-cluster light. The current rate from theKISS H-a survey (John Salzer, Caryl Gronwall) is 0.013 hso M0 yr-1 Mpc-3, 1.7 times higher than the lower limit from the Madrid UCM survey (Jesus Gallego) that has been used as a benchmark in surveys of past SFRs. The new value is of order (1/2)h5l of the average SFR density over the history of the known universe. There could still be upward corrections for selection effects and dust-enshrouded starbursts. H I (Martin Zwaan, Steve Schneider, Lyle Hoffman) tracks visible galaxies very closely, most of it in large spirals, and has a cosmic abundance of about 1/6 of its value at red-shift 3 from DLA systems, consistent with cosmic evolution models like that of Pei & Fall in which the reduction in H I is ascribed to star formation. The H I mass function resembles the luminosity function. We were treated to some spectacular pictures (Hoffman, Eric Wilcots, Pierre-Alain Due). There are differences between the H I properties of BCGs on the one hand and dI and LSB galaxies on the other. H I envelopes of BCGs are denser and more regular, which makes it seem less likely that dls and LSBs are descendants of BCGs (Liese van Zee, Caroline Simpson, Claude Carignan) and the dark matter and surface brightness distributions are different (Gerhard Meurer). BCGs have bursting star formation,whereas dls have 'gasping' SF, i.e. periods of star formationseparated by quiescent intervals (Monica Tosi). The amount of fading of tidal dwarfs depends on the presence or absence of an old population; less than 50 per cent of tidal dwarfs are still around today (Uta Fritze von Alvensleben, Due). Molecules are present in dwarf galaxies, but CO is not a good tracer (Ulrich Klein, Chris Taylor).

3 Intrinsic properties related to cosmology

The IMF appears to be universal with a Salpeter slope above about l.5M0 or so, but somewhat more top-heavy than usually assumed (Claus Leitherer). Claus recommended that, for such purposes as deducing SFRs from UV or emission-line intensites, one should use the Salpeter function with a lower cutoffat 1M0. As noted by Piero Madau and others, this means a lower SFR by mass by about a factor of 2 relative to what is deduced from the widely used lower cutoff at 0.1M0. Daniel Kunth gave a very interesting discussion of Ly-a, which can appear as an emission line, a P Cygni profile or a damped absorption line according to the velocity field, which in turn may depend on the stage of development of superbubbles caused by star formationin an extended H I halo. One of the high points of this meeting was the investigation of star formation histories. Eline Tolstoy gave a superb description of HR diagrams obtained with HST for nearby systems such as Sex A, Peg, GR 8 and Leo A and their interpretation and Eva Grebel gave us also an excellent overview of population boxes in dSph, dE and dl members of the . Robbie Dohm-Palmer used HST to trace the star formationhistory in space and time over-t he last 600 Myr in Sex A and GR 8 and found star-forming clumps moving back and forth across concentrations in H I. Infra-red and spectral data show that most BCGs and star-forming galaxies have an older underlying (Georges Comte, Livia Origlia, Regina Schulte-Ladbeck, Armando Gil de Paz), which is of course needed to account for their metallicities if the H II regions are

482 not entirely self-enriched. I Zw 18 can be modelled either with a previous burst 100 to 400 Myr ago (Alessandra Aloisi) or with low-level continuous star formationup to the current burst (Franc;;ois Legrand). An apparent exception is SBS 0335, where there is no evidence for star formation older than 30 Myr (Thuan), but this system is so like I Zw 18 that one wonders. Thuan found zero metallicity in H I, but a similar claim for I Zw 18 has gone away as a result of work by Liese van Zee. So is SBS 0335 unique in having a pristine HIenve lope? Yuri Izotov discussed abundances in BCGs, mainly fromthe Second Byurakan Survey (SBS). His and Thuan's derived primordial helium abundance of about 0.24 is probably basically cor­ rect because previous lower values for I Zw 18 were affected by underlying stellar absorption, as was confirmedat this meeting (Jorge Iglesias/Pepe Vilchez). This removes any conflict with Tytler's 'low' deuterium abundance in the frameworkof Big Bang nucleosynthesis. Izotov re­ ported an 0 /Fe ratio similar to that found in Galactic halo , indicating a stellar population that is young relative to SNia lifetimes, maybe of the order of 1 Gyr. This is not unexpected, but one may ask whether it is clear that all the iron in the H II regions is in the gas phase, since iron is depleted in the Orion Nebula. A converse effect has frequently been claimed for the young population of the Magellanic Clouds, but in this case the Fe/O ratio now seems to be close to solar (Bernard Pagel).

From an apparent lack of scatter in N/O below 12 + log(O/H) = 7.9, Izotov deduces that primary nitrogen comes from massive stars that also produce oxygen. Francesca Matteucci argued for the same conclusion from one DLA system reported to have N/O > 1. However, we know from other DLA systems that N/O is generally low with a large scatter (Patrick Petitjean). From this I would rather suggest that primary nitrogen comes from intermediate­ mass stars with lifetimes of several hundred Myr, short enough to occupy periods between bursts in BCGs like I Zw 18, but long enough to lead to scatter in the DLA systems if these are very young. Izotov suggested corrections to C/O values reported in the literature leading to constant C / 0 at the lowest metallicites, but the various trends and corrections are the same within substantial error bars. A further interestingaspect of I Zw 18 is the presence of WR stars, discovered by Izotov and Thuan. Dan Schaerer discussed the implications for stellar mass loss rates at different metallicities, concluding that all existing estimates have to be more or less doubled, with implications for chemical yields and the ionization of the intergalactic medium. The metallicity-luminosity relation for irregular galaxies has a dispersion, possibly large (A. Maria Hidalgo Gamez), but there are uncertainties in the data, largely based on 'empirical' methods using strong lines (Grazyna Stasinska). Possibly tidal tail debris dwarfs could be recognized by excess abundance, above the norm, at a given luminosity (Uta Fritze). An important problem in the study of SFR history is the role of dust obscuration, discussed in a very nice presentation by Daniela Calzetti taking account of varying dust nature and geometry. Many starbursts are shrouded in dust and detectable only in the IR (Marc Sauvage), and some quite high values of the extragactic background at submm wavelengths have recently been reported fromDI RBE. Daniela concluded that currently adopted SFRsare underestimated by about a factor of 2. Dark matter continues with us (Taft Armandroff, Andi Burkert, Ky Lo, Gerhard Meurer, Rob Swaters) and the old results are largely confirmed. The new twists are a possible contribu­ tion from H2 (Lo) and the effect of tidal forces on dwarf spheroidals, especially the Sgr dwarf where the evidence for dark matter is greatly weakened (M.A. Gomez-Flechoso) . However, the Sgr dwarf is evidently a special case. Without dark matter, small dSph could not have survived (Doug Lin).

483 4 Properties at significant look-back times

Roberto Terlevich and Jorge Melnick have refined the scaling relationships between supersonic velocity dispersion and luminosity for giant H regions and Roberto showed how they might II be used as a distance indicator (perhaps the only one) out to 3, where some line profiles z = are already available. Fran<;ois Hammer described the Canada-France red-shift survey out to 1, combined z = with HST images which show that most of the evolution is in star-forming galaxies. These are highly compact, but luminous, so that their classification as dwarf galaxies is controversial, and other results from HDF seem slightly discrepant (Harry Ferguson). Rafael Guzman gave a fine description of his work with Dave Koo at Lick Observatory on BCGs with z � 1. Again the SFR is dominated by compact H II galaxies with similar specific SFRs to local ones, but with larger masses, a trend that continues to z = 3. His suggestion is that they lose mass and eventually evolve to bright dEs like NGC 205.

The collection of UV dropout galaxies with z � 3 is now over 400, which are definitelynot dwarfs but starburst galaxies with broad emission lines and virial masses of 1 to 5 1010 M0 x (Max Pettini). Rosa Gonzalez Delgado has computed evolutionary synthesis models for nearby objects of this type, e.g. NGC 7714, and finds excellent agreement between H diagnostics II and the UV stellar continuum. IRAS 0833+65. 7 is a good match to the high red-shift dropouts and its kinematics is dominated by outflow, with blue-shifted interstellar lines and a P Cygni profileof Ly-a. Piero Madau described his work on the cosmic SFR as a function of red-shift, favouring hierarchical clustering models and the Salpeter IMF; the model fits extragalactic background light as measured from deepgalaxy counts in bands from U to K, but falls short by the standard factorof 2 compared to the far infra-red. He discussed cosmic supernova rates and the ionization of the IGM; the combination of QSOs and known UV dropout galaxies leaves a

gap near z � 5. Patrick Petitjean discussed the properties of damped Ly-a systems. Line widths support the identificationwith rotating galactic proto-disks only in some cases, preferably at lower red­ shifts, while at higher red-shifts they are more consistent with separate clumps that may evolve into dwarf galaxies or merge to form larger systems. The metallicity, measured by undepleted zinc, shows no clear evolution with red-shift, but is anti-correlated with H I column density suggesting the possibility of bias. Abundance ratios are affected by depletion of iron and other metals on to dust grains, but there is some evidence in favour of an enhanced a-element/Fe ratio expected in young systems. N/Si and the few available N/O ratios are generally very low. Population stars, defined as those having ultra-low to zero metallicity, are significantly III underabundant in the Galactic halo for [Fe/HJ < -4, since none have yet been found compared with an expected number of about 9, and stars with [Fe/HJ < -3 have strange abundance ratios, notably within the iron group (Roger Cayrel). Does this imply supernova-driven low­ mass star formation accompanied by pollution? One scenario relates Population to the III formation of dwarf galaxies (Andrea Ferrara). Doug Lin discussed the theory of star formation in dwarf galaxies taking into account many physical processes and concluding that initially only high-mass stars are formed until [Fe/HJ -2.5. Application of chemo-dynamical modelling --+ to dwarf galaxies leads to the fairly surprising conclusion that N and 0 end up with the same spatial distribution although they were originally injected into the warm and the hot phases, respectively, of the ISM (Gerhard Hensler).

484 Inventory of cosmic baryons and 'metals' Densities expressed as n, in units of 7.7 1010h� Me Mpc-3 Pent = x 0 All baryons from BBNS (D/H 3.4 10-5 [?]) = x

Stars in spheroids .0015 [?] .0036 h501 [?] Stars in disks .0007 [?] .0013 h5J [?] Total stars .0022 [?] .0049 h5J [?] L Cluster hot gas .001 h5J·3 [?] .0043 h50 5 [?] L Group/field hot gas .0003 h5J·3 [?] .023 h50 3 [?] ° Total stars + gas .0036 h50 .4 [?] .033 h5J·5 [?] Machos ?? [?]

nz (stars, = 0.02 [?]) 4.4 10-5 [?] 9.8 10-5 h51 [?] z x X 0 nz (hot gas, .006) 7.8 10-6 hsri"·3 [?] 1.8 10-4 h5J·5 [?] z = x x Yield pz/p 2 [?] °·3 [?] * .024 h5�· .057 h50

Damped Ly-a .002 h501 [?, ?] Ly-a forest .074h5i [?, ?] Gals + DM halos (M/L 150 h ) 0.2 [?, ?] = 50 All matter .056 h-1.5) UB =

5 Inventory of cosmic baryons and 'metals'

By way of a conclusion, I should like to present in the accompanying table some estimates of cosmic densities of stars, gas and 'metals',and comment on the relationship between these quantities (which are admittedly only partially related to dwarf galaxies as such) and what we have heard at this meeting. The top entry gives the total cosmic density of baryonic matter based on Tytler's deuterium abundance, which is consistent with the new helium measurements and with 7Li [?]. Entries below give estimates of various components of baryonic matter according to Persic & Salucci [?] and according to more recent work by Fukugita, Peebles & Hogan [?], who give somewhat larger numbers. The most important difference is the vast amount of hot gas outside rich clusters of galaxies, based on the assumption of a universal ratio of hot gas to E galaxies, in the more recent reference, which brings the 'observed' baryonic matter within spitting distance of the BBNS total (and of the ionized gas associated with the Ly-a forest). Assuming the age of the universe to be 15 h5J Gyr, the mean past cosmic star formation rate is 0.011 h�0 [?] or 0.025 h�0 [?] Me yr-1 MpC3, the latter number being about twice the cmrent. rate from the KISS survey, modulo differences in the assumed IMF, and consistent with the SFR history derived by Madau and colleagues [?]. These authors noted a possible problem with the yield. If the hot gas has 1/3 solar metal­ licity, as assumed by Mushotzky & Loewenstein [?], and indeed is so very abundant, we come out with a very large yield as shown in the table, about 3 Ze , which is larger than one normally assumes in models of chemical evolution for our own Galaxy although naturally not out of line with models designed to explain the metallicity of intra-cluster gas. Is this really a universal

485 yield, or is there some flaw in the argument, e.g. how could young galaxies like SBS 0335 or I Zw 18 be formingin recent times with a much lower metallicity than 1/3 solar? Furthermore, there is an imf-independent relation derived by Songaila, Cowie & Lilly [?] between (unabsorbed) extragalactic background radiation and the cosmic abundance of metals:

I,,(lOOOA < Arest < 2000A) 5000 -(c p Z)o 4?T 1.8 10 2 h erg s-1 cm-2 Hz-1 -1 (1) x - 0 �; sr with the Mushotzky & Loewenstein parameters. Optical background light from faint galaxy counts as reported in [?] only reaches this level longward of 7000 A, so that the bulk of the metal production would have to take place at red-shifts 2.5 to 6, or be shrouded in dust so > that the radiation comes out in the far infra-red. With conventional yields, on the other hand, there is no difficulty about this argument. So the question as to whether the bulk of radiation from past star formation is accounted for by existing models is still open.

References

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