Fricke, K.J.

Fritze, U.

Lindner, U.

Moller, C.S.

Astrophysics and Space Science

English

University of Hertfordshire Research Archive

arXiv:astro-ph/9904208v1  16 Apr 1999On the Nature and Redshift Evolution of DLA GalaxiesFritze - v. Alvensleben, U. (ufritze@uni-sw.gwdg.de), Lindner, U.,Mo¨ller, C.S. and Fricke, K.J.Universita¨ts – Sternwarte Go¨ttingenAbstract. We extend our spiral galaxy models that successfully describe nearbytemplate spectra as well as the redshift evolution of CFRS and HDF spirals toinclude – in a chemically consistent way – the redshift evolution of a series ofindividual elements. Comparison with observed DLA abundances shows that DLAsmight well be the progenitors of present-day spiral types Sa through Sd. Our modelsbridge the gap between high redshift DLA and nearby spiral HII region abundances.The slow redshift evolution of DLA abundances is a natural consequence of thelong SF timescales for disks, the scatter at any redshift reflects the range of SFtimescales from early to late spiral types. We claim that while at high redshift allspiral progenitor types seem to give rise to DLA absorption, towards low redshifts,the early type spirals seem to drop out of DLA samples due to low gas and/or highmetal and dust content. Model implications for the spectrophotometric propertiesof the DLA galaxy population are discussed in the context of campaigns for theoptical identifications of DLA galaxies both at low and high redshift.1. IntroductionThe basis for the work presented here is our set of chemically consistentspiral galaxy models with a range of Star Formation Histories (SFHs)specifying the spectral types Sa, Sb, Sc, Sd. Our models are verysimple 1-zone models without any dynamics meant to describe globalquantities of average galaxies of the respective types. Our approach isto use the simplest models possible with the smallest number of param-eters in order to see how far we can get and what kind of sophisticationsare required by a comparison with observed galaxy properties.Our unified spectrophotometric and chemical evolutionary synthesismodels allow to have a large number of observational constraints –spectrophotometric properties including gaseous emission and stellarabsorption features as well as ISM abundances – to restrict the smallnumber of model parameters, basically the IMF and the SFH. Whilein the local Universe a 1–1 - correspondance between spectral typesand morphological types is a long-standing matter of fact it is alsoclear that this correspondance has to break down at some yet unknownstage when going back towards the earliest phases of galaxy evolutionand formation.The choice of SFHs for the various spectral types is determinedby a number of observational constraints. These include type-averagedproceed.tex; 11/12/2007; 13:10; p.12luminosities, colours from UV through NIR for nearby galaxy samples,emission and absorption line properties, template spectra, and HII re-gion abundances that our model galaxies have to match after a Hubbletime as well as a comparison with observed galaxy luminosities andcolours over a large redshift range (cf. Mo¨ller et al. 1999).Here, we present the chemical evolution aspect of our spectroscop-ically successful spiral galaxy models and compare to abundances fora series of elements observed in Damped Lyα (DLA) Absorbers overthe redshift range from z ∼ 0.4 through z >∼ 4.4, which corresponds tolookback times of more than 90 % of the age of the Universe. Originally,DLA absorption was thought to arise in intervening (proto-)galacticdisks along the lines of sight to distant QSOs (Wolfe 1988). This viewis supported by arguments based on mass estimes from column den-sities and absorber sizes at z ∼ 2− 3 as well as by kinematic featuresconsistent with rotation at velocities of order 200 km s−1 (Prochaska &Wolfe 1997a, b, 1998). More recently, on the basis of [α/Fe] and [N/O]abundance ratios and their large scatter, an origin of DLA absorptionin dwarf or low surface brightness galaxies is also discussed (Matteucciet al. 1997, Vladilo 1998, Jimenez et al. 1999). Our aim is to see, if andin how far very simple spiral galaxy models are compatible with theobserved DLA abundance evolution over the large redshift range acces-sible. In this context it does not matter if DLA galaxies at the highestredshifts are not yet fully assembled massive disks but rather consistof galactic building blocks as suggested by Haehnelt et al. (1998). OurSFHs in this case are meant to describe SF in all the fragments boundto end up in one disk by z = 0.In a second step, we then present the spectrophotometric propertiesof those galaxy models that succesfully describe the DLA galaxy popu-lation and discuss them in the context of the large campaigns designedto optically identify DLA galaxies both at low and at high redshift.2. A Chemically Consistent Chemical Evolution ModelAs opposed to star clusters which basically form their stars “all at thesame time”, i.e. within ∼ 105 yr, any stellar system with a SFH moreextended than this is expected to feature finite distributions both inage and metallicity. Our spectrophotometric and chemical evolution-ary synthesis models are chemically consistent in the sense that weaccount for the increasing initial metallicity of successive generationsof stars. We keep track of the ISM abundance at birth of each starand use various sets of input physics for metallicities in the range−2.5 ≤ [Fe/H] ≤ +0.3. In particular, we use stellar evolutionary tracksproceed.tex; 11/12/2007; 13:10; p.2DLA Galaxies 3and lifetimes from the Padova group (Bressan et al. 1993, Fagotto et al.1994 a, b, c for 0.6 ≤ m∗ ≤ 120 M⊙, and from Chabrier & Baraffe 1997for m∗ ≤ 0.5 M⊙, stellar yields and remnant masses from v. d. Hoek& Groenewegen (1997) for m∗ ≤ 8 M⊙ and from Woosley & Weaver(1995) for stars 12 ≤ m∗ ≤ 40 M⊙, and model atmosphere spectra andcolour calibrations from Lejeune et al. (1998). SNIa contributions areincluded as described by Matteucci & Greggio (1986) and Matteucci &Tornambe` (1987) with SNIa yields from Nomoto et al. (1997).We use a Salpeter IMF and SFHs for the various spiral types as follows:Sa ... Sc Ψ(t) ∼ GM(t)Sd Ψ(t) ∼ const.(G: gas mass, M: total mass). Characteristic timescales for SF t∗ asdefined by∫t∗0Ψ · dt = 0.63 ·G(t = 0) aret∗ ∼ 2 Gyr Sat∗ ∼ 3 Gyr Sbt∗ ∼ 10 Gyr Sct∗ ∼ 16 Gyr SdOur models have a strong analytic power in the sense that theyallow to trace back the luminosity contributions to any wavelengthband as well as the enrichment contributions to any chemical elementof different stellar masses, spectral types, luminosity classes, metallicitysubpopulations, nucleosynthetic origins (PNe, SNI, SNII, single stars,binaries, ...) as a function of time or redshift.While for the spectro-cosmological evolution cosmological and evo-lutionary corrections as well as the effect of attenuation have to beconsidered, the chemo-cosmological evolution simply results from a 1-1-transformation of galaxy age into redshift (we use a standard cosmology(Ho, Ωo, Λo) = (75, 1, 0) and assume galaxy formation at zform = 5).The above SFHs were chosen as to give agreement after a Hubbletime of evolution with average colours for the galaxy types Sa, Sb, Sc,Sd from the RC3 and template spectra from Kennicutt (1992). Inmost nearby spirals HII region abundances show negative gradientswith galactocentric radius and even at a given radius within a galaxy,there may be considerable scatter among abundances of individual HIIregions. Geometrical considerations by Phillipps & Edmunds (1996)and Edmunds & Phillipps (1997) show that for an arbitrary sightlinefeaturing DLA absorption the most probable galactocentric distance topass through an intervening galaxy disk is around 1 Re. At the sametime, 1 Re seems to be a reasonable radius for HII region abundancesproceed.tex; 11/12/2007; 13:10; p.340.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5redshift z−3.5−3.0−2.5−2.0−1.5−1.0−0.50.00.5[Zn/H]a)0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5−3.5−3.0−2.5−2.0−1.5−1.0−0.50.00.5Sa  (model C)Sd  (model C)redshift z[Fe/H]Sa  (chem.cons.)Sa  (solar)Sd  (chem.cons.)Sd  (solar)Figure 1. Redshift evolution of [Zn/H] (1a) and [Fe/H] (1b) for models Sa and Sdcompared to observed DLA abundances.to compare to our global 1-zone models. The agreement of average HIIregion abundances for various galaxy types at 1 Re as measured byZaritsky et al. (1994), Oey & Kennicutt (1993), van Zee et al. (1998),and Ferguson (1998) with model ISM abundances after a Hubble timeconfirms our choice of SFHs.3. Abundance Evolution of Spiral Galaxy Models and DLAObservationsHigh resolution spectra are required to fully resolve the velocity struc-ture and derive precise heavy element abundances in DLA absorptionsystems. In recent years, a large number of precise abundances havebeen determined for elements C, N, O, Al, Si, S, Cr, Mn, Fe, Ni, Zn,... in a large number of DLAs over the redshift range z ∼ 0.4 throughz >∼ 4.4 (Boisse´ et al. 1998, Lu et al. 1993, 1996, Pettini et al. 1994,1999, Prochaska & Wolfe 1997, de la Varga & Reimers 1999). For thecomparison with the redshift evolution of our models we have carefullyreferred all published DLA abundances to one homogeneous set of os-cillator strengths and solar reference values (see Lindner et al. 1999 fordetails).Fig. 1 shows the redshift evolution of our “slowest evolution” Sdand our “fastest evolution” Sa models in comparison with the avail-able [Zn/H] (1a) and [Fe/H] (1b) abundances in DLAs. Sb and Scproceed.tex; 11/12/2007; 13:10; p.4DLA Galaxies 5models are omitted to avoid confusion, their abundance evolution runsin between those of the Sa and Sd models. Lindner et al. (1999) presenta more extensive comparison including all those 8 elements for whichabundances are measured in a reasonable number of DLAs.As compared to models using solar metallicity input physics only(thin lines), the chemically consistent models (heavy lines) show signifi-cant differences ranging up to 1.0 dex for some elements. Changes in theexplosion energy (Woosley & Weaver’s model C) are considerably lessimportant. It should be noted that no scaling or additional parametersare introduced. Using SFHs, IMF, and yields, as desribed above, resultsin absolute model abundances.The conclusions we draw from this comparison (including all ele-ments) are the following:− Sa – Sd models bracket the redshift evolution of DLA abundancesfrom z ≥ 4.4 to z ∼ 0.4,− models bridge the gap from high-z DLAs to nearby spiral HIIregion abundances,− the weak redshift evolution of DLA abundances is a natural resultof the long SF timescales for disks galaxies,− the range of SF timescales t∗ for near-by spirals from Sa throughSd fully explains the abundance scatter among DLAs at fixedredshift.This means that from the point of view of abundance evolution overmore than 90% of the age of the Universe normal spiral galaxies (ortheir progenitors) are perfectly consistent with DLA observations. Thisdoes not exclude the possibility that some starbursting dwarfs or LSBsmay also give rise to DLA absorption. Moreover, as indicated by theheavy line in Fig. 1b, the early type spirals that well seem to be presentin high-z DLA samples seem to drop out of low-z DLA samples. At thesame time as delimiting the region in abundance vs. redshift spacewhere DLA absorbers seem to occur, the heavy line in Fig. 1b. corre-sponds to a gas-to-total mass ratio of 0.5 in our galaxy models. Indeed,low-z DLAs are observed to generally have low N(HI) (e.g. Lanzetta etal. 1997).Our prediction from the comparison of models with data is thatwhile at high redshift (z >∼ 2) the DLA absorbing galaxy populationmay well comprise early as well as late type (proto-)spirals, the lowredshift (z <∼ 1.5) DLA absorption systems seem to have their origin inlate-type gas-rich and metal-poor spirals. A bias against high metal-licity and/or dust content in low-z DLA absorbing galaxies has beenproceed.tex; 11/12/2007; 13:10; p.56suspected e.g. by Steidel et al. (1997). In our view, the drop of theglobal gas content that clearly goes together with increasing metallicityin nearby spirals may add yet another reason: at low gas content theprobability is significantly reduced that an arbitrary sightline throughthe galaxy goes through a region with high enough HI column densityfor DLA absorption.4. The Spectrophotometric Aspect of DLA GalaxiesIf confirmed by a larger sample of low-z DLAs, this prediction hasimportant implications for the possibility of optical identifications. Lo-cally, early type spirals are brighter by ∼ 1.5 mag in B on average thanlate-type spirals despite the fact that any spiral type shows a rangein luminosity with considerable overlap between different types (e.g.Sandage et al. 1985).We can now use the spectro-cosmological aspect of our evolutionarymodels as e.g. presented by Mo¨ller et al. (this volume) to predict therange of apparent magnitudes and colours expected for DLA absorbinggalaxies at various redshifts. We find the intriguing result that in allthree bands B, R , and K the intrinsically brighter early-type spi-rals show almost the same apparent luminosities at z ∼ 2− 3 as theintrinsically fainter average late-type Sd galaxies at z ∼ 0.5.Sa Sdz ∼ 0.5 z ∼ 2 – 3 z ∼ 0.5 z ∼ 2 – 3B 22.5 24 – 25 25.5 29 – 30.5R 21 24 24.5 29K 18.5 21.4 – 22 22 26 – 27We thus would not expect low-z DLA galaxies to be more easilyidentified than at least the brighter ones among the high-z DLAs.On the basis of these luminosities we understand the non-detectionof DLA galaxies at z ∼ 3 down to R ∼ 25.5 (Steidel et al. 1998)and the small number of DLA candidates (2/10) at 1.5 ≤ z ≤ 2.5 toK ∼ 21 by Aragon - Salamanca et al. (1996). The galaxies identi-fied by Steidel et al. (1994, 1995) as candidates for DLA absorbersat zabs = 0.6922, 0.3950, 0.7568, and 0.8596 indeed have luminosities−19.5 <∼ MB<∼ −19.0 typical of late-type spirals. However, for their sam-ple of 7 DLA identifications in the range 0.4 <∼ z <∼ 1 Le Brun et al.(1997) find a variety of morphologies: spirals as well as compact andproceed.tex; 11/12/2007; 13:10; p.6DLA Galaxies 7LSB objects. At high redshift, Djorgovski et al. (1996) and Djorgovski(1998) report identifications of a bright disk galaxy (MB ∼ −20.4) withSFR >∼ 8M⊙yr−1 in agreement with our models at z ∼ 3.15, and of anMB ∼ −19.5 galaxy with low SFR at z ∼ 4.1. The two early-type DLAgalaxies (S0) put foreward by Lanzetta et al. (1997) at z = 0.16377 andby Miller et al. (1999) (NGC 4203 at z = 0) both show counterrotatinggas disks. HI mapping shows NGC 4203 to be abnormally gas-rich (vanDriel et al. 1988). These two cases raise the issue that recent accretionor merging may provide favourable conditions for DLA absorption.5. Conclusions and OutlookWe use very simple spiral galaxy models with standard IMF and SFHschosen as to agree with chemical and spectrophotmetric propertiesof nearby galaxies as well as with the observed redshift evolution ofluminosities and colours. When combined with a chemically consis-tent description of their chemical evolution and a standard cosmology,we find good agreement with the observed redshift evolution of DLAabundances over more than 90 % of the Hubble time. We claim that atz >∼ 1.5 all spiral types can give rise to DLA absorption while towardslower redshifts only the gas-rich metal-poor late spiral types do so. 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On the Nature and Redshift Evolution of DLA Galaxies

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