This is an electronic version of an article published in The Astrophysical Journal. Clavel, J. et al. Damped Lyα systems as spiral galaxies. The Astrophysical Journal 487 (1997): 29-3

Díaz, Angeles I.

Ferrini, Federico

Mollá, Mercedes

The Astrophysical Journal

Díaz Beltrán, Ángeles Isabel

Biblos-e Archivo

DAMPED Lya SYSTEMS AS SPIRAL GALAXIESFEDERICO FERRINI,1 MERCEDES MOLLA´, AND ANGELES I. DI´AZDepartamento de Fı´sica Teo´rica, Universidad Auto´noma de Madrid, 28049 Madrid, SpainReceived 1996 December 10; accepted 1997 July 11ABSTRACTIt is shown that the distribution of element abundances with redshift in damped Lya (DLA) systems can beadequately reproduced by the same evolutionary models reproducing the halo and disk components of the MilkyWay Galaxy at different Galactocentric distances. In this sense, the DLA systems are well represented by normalspiral galaxies in their early evolutionary stages.Subject heading: early universe — galaxies: abundances — galaxies: evolution — quasars: absorption lines1. INTRODUCTIONThe evolutionary history of galaxies is a subject that hasdeserved great attention since the early development of theo-retical astrophysics. A variety of cosmic phenomena take partin it: large-scale dynamics, star formation and evolution,interactions between stars and interstellar medium (ISM), andso on. In this context, the history of element abundances is ofparticular importance, since it relates to the history of starformation, the mass distribution in stars, the element nucleo-synthesis in stellar interiors, and their ejection and mixing withthe ISM.The solar neighborhood constitutes our vicinity both inspace and time, and its observational study can be carried outwith a great deal of accuracy. The building of consistentmodels of its chemical evolution has set the foundations of ourpresent ideas about Galactic evolution. The tracing of thewhole Galaxy structure via globular clusters and other Galaxypopulations (bulge, halo, and thin and thick disks) has madeit possible to study its evolution up to a Galactocentricdistance of 115–25 kpc and in the past 111–16 Gyr.Although data on external galaxies are fewer and much lessdetailed, they have still allowed their study with the use ofsimilar methods.However, the number of parameters involved in any chem-ical evolution model is large, and a unique solution to theproblem does not exist. Even the evolutionary picture for theMilky Way Galaxy (MWG), which provides the most completeset of observational data to constrain models, is not yet wellestablished.In many cases, models that give similar results for thepresent time diverge significantly at different epochs. New dataon high-redshift objects—i.e., objects far away in space andtime—can now be used to test model-predicted past Galaxyproperties that can then constitute very important constraintsfor evolution models.The recent paper by Lu et al. (1996; hereafter LSBCV)presenting a large set of new observations on element abun-dances of damped Lya (DLA) systems, together with acompilation of similar measurements from the literature, givesan excellent opportunity to verify the consistence of modelsand contribute to the understanding of early structures of theuniverse.In recent years we have computed self-consistent chemicalevolution models for spiral galaxies (Molla´, Ferrini, & Dı´az1996), which are a natural extension of previously developedones for the solar neighborhood (Ferrini et al. 1992) and theMWG (Ferrini et al. 1994). In the present Letter we havetaken the numerical outputs calculated for the MWG and usedthem in conjunction with the LSBCV data in order to testdamped Lya systems having abundances consistent with thoseexpected for high-redshift spiral galaxies. Arguments in favorof this hypothesis include the suggestion from Wolfe (1995)that the low-ionization metal absorption lines in damped Lyasystems constitute direct evidence for disks and the directimage identification made in several cases (see LSBCV forreferences).To compare directly with data by LSBCV, we have assumedvalues of q0 and H0 of 0.1 and 50 km s21 Mpc21, respectively.The redshift z corresponds to the redshift of damped Lyasystems.2. OBSERVATIONS AND MODELLSBCV derived element abundances for 14 Lya systemsfrom observations with the 10 m Keck telescope. Data for nineother galaxies were compiled from the literature. The objectshave redshifts between 0.7 and 4.4, and hydrogen columndensities have been derived for 20 of them.The abundances of three of the elements analyzed byLSBCV are predicted by our models: Fe, Si, and S. A total of17 objects have Fe abundances derived directly from iron linemeasurements, and the distribution of [FeyH] as a function ofredshift can be represented by two bands roughly centered at[FeyH] 1 21 and [FeyH] 1 22 (Fig. 1). The more metallicsystems seem to be limited to z , 3, while the less metallicones extend over the whole redshift range. A similar situationis found for Si (also for Cr and Ni). Observational errors in theabundance determinations are of the order of 0.1 dex, accord-ing to LSBCV, and therefore do not modify our qualitativeinterpretation.The observed distribution suggests the possibility that theobserved gas belongs to early Galactic systems with essentiallytwo levels of enrichment, with the highest being limited to z ,3. This could be explained by assuming the existence of twodifferent episodes of star formation: an initial one leading to acertain level of enrichment of the ambient gas and correspond-ing to the lower metallicity level and a second episode, startingfrom an already enriched gas and corresponding to the higher1 Dipartimento di Fisica, Universita` di Pisa, Piazza Torricelli 2, 56100 Pisa,Italy.THE ASTROPHYSICAL JOURNAL, 487 : L29–L32, 1997 September 20© 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A.L29metallicity level observed in some systems. This naturallyexplains the observed distribution with redshift as a time delay.The simplest structure that we may imagine being able tofollow the outlined evolution is a spiral galaxy. Indeed, asalready suggested by Larson & Tinsley (1978) and evaluatedby Ferrini & Galli (1988), in the initial spheroidal structure ofa spiral galaxy, star formation may begin in the halo region.Then, after some delay, a disk is formed, and when the criticalconditions for an efficient star formation there are met, asecond, and probably more important episode of star forma-tion, will take place. The observed two-level distribution couldthen be interpreted as the signature of the middle of the earlyphase and the beginning of the second one. These argumentswill be verified a posteriori by the comparison of model resultswith present observations.In what follows we describe some of the model featuresrelevant to the present problem. We assume that the primor-dial gas of the spheroidal protogalaxy collapses to form thehalo and that the gas recycled by star formation then collapsesonto the Galactic plane to form the disk at a rate inverselyproportional to the collapse time, which is an input parameterof the model. The early Galactic sphere is divided intoconcentric annular cylindrical regions 1 kpc wide, perpendic-ular to the disk. The numerical code calculates the timeevolution of the halo and disk components of every cylindricalregion, considering that the diffuse gas forms molecular cloudsand stars are formed either by cloud-cloud collisions or by theinteraction of massive stars with clouds (induced star forma-tion). The rates for the occurrence of these processes arefunctions of the Galactocentric radius (see Ferrini et al. 1994)and are proportional to the corresponding efficiencies repre-sentative of the probabilities of cloud formation, cloud-cloudcollisions, and the interaction of massive stars, and clouds areassumed to vary with galaxy morphological type (see Ferrini &Galli 1988) since the spiral density wave changes the condi-tions for star formation in each galactic region: a strongerspiral density wave will compress the diffuse gas, favoring theformation of clouds, and will increase the frequency of cloud-cloud collisions. On the other hand, the induced star forma-tion, because of the interaction of molecular clouds withnearby massive stars, seems to be a local process, and there-fore its corresponding efficiency can be taken as constant forevery zone and galaxy.We have assumed the effects of dust to be negligible. Thesituation regarding the possible presence of dust in DLAsystems is not clear. LSBCV do not find any strong evidencefor element depletion by dust grains. The recent work byPettini et al. 1997 claims that dust may be present in somehigh-redshift galaxies, but always in lower concentrations thanin the Milky Way ISM. Galaxies at the highest redshifts (lowmetallicity) are, most probably, dust free. Therefore, it seemsthat the presence of dust would affect only the abundancesderived for high-metallicity systems, which would have beenunderestimated.3. MODEL RESULTS AND DAMPED Lya COMPARISONThe simplest piece of comparison we may start with is theevolutionary history of the Milky Way, which is assumed to bea typical spiral galaxy. Therefore, we have taken the modelresults from Ferrini et al. 1994, where we computed theelement distributions in the halo and in the disk of our Galaxyby varying the radial Galactocentric distance of every consid-ered region. As a first attempt we have extracted the outputsfor the solar region (1 kpc wide centered on 8 kpc) and tworegions centered on 5 kpc and 14 kpc from the Galactic center(hereafter called inner and outer zones). These may beconsidered representative of three distinct and coherent zonesof the Galaxy. We have extracted the abundances of elementsin the gas phase in both the halo and the disk components,which we plot separately.The plot of the halo enrichment history for iron (Fig. 1a) inthe three regions, solar (solid curve), outer (dotted curve), andinner (dashed curve), immediately reveals a few features: theevolution has been faster near the center; at the same z thereis a difference of 10.2 dex in [FeyH] between differentregions. The evolutionary tracks of the three consideredregions well cover the group of data with the lower [FeyH]values even at the highest redshifts but do not fit the larger[FeyH] data. The corresponding disk evolution (Fig. 1b)obviously follows the same principle of a more rapid evolutionin more central regions, but now the efficiency of matterconversion, because of a higher star formation rate (SFR), ishigher, and the differences in the evolution of the variousregions are more important: at z 5 3, D[FeyH] 3 0.5 betweenthe solar and inner regions, and D[FeyH] 3 1.7 between thesolar and outer regions. Two important aspects should beFIG. 1.—[FeyH] vs. redshift according to the Milky Way evolutionarymodel, for the solar neighborhood (solid curve), an inner radius (dashed curve),an outer radius (dotted curve), for (a) the halo and (b) the disk. The circles arethe Lu et al. 1996 data.L30 FERRINI, MOLLA´, & DI´AZ Vol. 487noted: (i) the evolution of disk regions well contain the higher[FeyH] data, with the inner region showing abundances evenhigher than observed, and (ii) the evolutionary history of thedifferent regions can well reproduce the spread in the obser-vations. In considering the other elements for which data andmodels can be compared, i.e., S and Si, the same qualitativeresults as for iron are found: the lower [SiyH] data fall on theevolutionary tracks for halo regions, while the higher [SiyH]values correspond to disk regions. For sulfur the situation isless clear since only four data points are available. Their valuesare well covered by the disk region evolution.Assuming that the absorbing gas in the line of sight to thedamped Lya systems may belong to both halo and diskcomponents, we can evaluate its corresponding theoreticalabundance by averaging the abundances derived for the twopopulations with their corresponding weights. In doing so wefind that the haloydisk average in the outer region wellreproduces the low-abundance data, while the solar regionevolution better fits the higher abundance points. We shouldalso take into account that the observations possibly averagenot only on the two populations at a certain radius but alsoover a great part of the whole Galactic structure.The column density of atomic hydrogen, N(H I), is alsoderived by LSBCV. Data for a few more objects can be foundin Pettini et al. (1997). We can estimate N(H I) as due to thecontribution of the gas present both in the spheroidal and diskcomponents of the galaxy. The column density is clearlysensitive to the inclination angle of the disk with respect to theline of sight. In Figure 2 the two solid lines represent theevolution of the total N(H I) for a face-on galaxy (lower curve)and for an edge-on galaxy (upper curve). The distribution ofobserved values is bound by these simple estimates.The possibility that DLA systems are indeed normal spiralgalaxies has been considered by LSBCV. The main objectionto this hypothesis seems to be the low Fe content of DLAsystems when compared with that of old Galactic disk stars(Edvardsson et al. 1993). However, this fact results from theimplicit identification of the disk with the solar neighborhood.This is evident from our evolutionary tracks, since outerGalactic regions show an evolution perfectly compatible withobservations. Moreover, the comparison with Galactic diskstars may not be appropriate; indeed, damped Lya observa-tions refer to gas probably belonging to different components,and therefore the observed abundances are averaged over thehalo and disk gas, if not over the whole Galactic content. Atearly times, in the collapse scenario, it is reasonable to expectthat there may be enough gas at low metallicity still present,thus reducing the observed integrated [FeyH]. This means thatthe value of [FeyH] for damped Lya systems can be smallerthan the corresponding Galactic disk star values at the sameage. This argument also weakens the need for a significantdepletion of iron onto dust grains.The enrichment level [FeyH] 1 22.2 at z 3 4.4 is indicativeof an early nucleosynthesis. On the other hand, the end of theupper level ([FeyH] 1 21) at z # 3 may imply that there aredifferent regions involved in the enrichment process whoseconsequences we are observing and that they begin theiractivity at different times, perhaps with a delay of a fewgigayears between the star formation events responsible forthe [FeyH] 1 22 and [FeyH] 1 21 enrichment levels. In thecollapse model, this can be interpreted as the delay betweenthe onset of star formation in the halo and the disk. In ourmodels this is naturally understood, since the process of starformation in the disk is not immediate as in the case ofadopting a Schmidt law: i.e., it is not sufficient to have gas toproduce stars; this would force an immediate star formationeven with a limited amount of gas (110 MJ), and therefore nodelay between the star formation in the halo and the diskwould be observed. In the multiphase model, the system needsto form clouds as a step prior to star formation, and thisproduces a natural delay. Furthermore, since the process ofstar formation is nonlinear—quadratic in the cloud massfraction in the case of cloud-cloud collisions and proportionalto the product of massive star fraction times the cloud fractionin the case of the massive star–cloud interactions—it isinefficient until the cloud fraction is large and enough massivestars are formed. This may be interpreted as a thresholdcondition. In fact, the consequences of the multiphase ap-proach and the threshold condition are indistinguishable.These physical mechanisms may be responsible for the delayof a few gigayears between the halo and disk populations, aswe have shown in previous works. The presence of a thick diskwould obviously enforce this delay.As a final comment concerning the SFR present in the DLAsystems, according to LSBCV the absence of Lya emission inthe systems they consider implies a low rate of massive starformation (#20 MJ yr21). The SFR from our models in theinterval 0.7 , z , 4.4 ranges between 1 and 14 MJ yr21.Roughly speaking, we may say that the halo component ischaracterized by a star formation burst taking place in the firsthalf gigayear, while the disk evolves with an almost constantstar formation, amounting to a few MJ yr21.The aim of the present Letter was to verify with detailedevolutionary models the hypothesis put forward by Wolfe andLSBCV that the DLA systems are normal spiral galaxies. Weare not concerned here with the cosmological implications ofthis fact, and therefore we do not compare the consequencesof our models with the results of other authors concerningcosmic chemical evolution (Lanzetta, Wolfe, & Turnshek1995; Pei & Fall 1995; Malaney & Chaboyer 1996; Kauffmann1996). That is deferred to a forthcoming paper. Using thequantitative results from our models, we can confirm theFIG. 2.—Column density for face-on and edge-on Milky Way twins; thecircles are Lu et al. 1996 and Pettini et al. 1997 data.No. 1, 1997 DAMPED Lya EVOLUTION L31conjecture of LSBCV that the large metallicity spread ob-served in DLA systems can be explained by the heterogeneousmixture of Galactic regions and galaxy types present in thesample. The observed values are a consequence not only of thenucleosynthesis and stellar lifetimes but also of the differenttimescales for the main star formation episodes in the variousregions and of their different star formation histories.This work has been partially supported by Integrated Actionwith Italy A-28 and Direccio´n General de Investigacio´n Cien-tı´fica y Technica project PB-93-052. Contribution from theMinistero dell’Universita` e della Ricerca Scientifica and Agen-zia Spaziale Italiana are acknowledged. F. F. thanks theUniversidad Autonoma de Madrid for an invitation for asabbatical period, during which this Letter has been written.REFERENCESEdvardsson, B., Andersen, J., Gustafsson, B., Lambert, D. L., Nissen, P. E., &Tomkin, J. 1993, A&A, 275, 101Ferrini, F., & Galli, D. 1988, A&A, 195, 27Ferrini, F., Matteucci, F., Pardi, C., & Penco, U. 1992, ApJ, 387, 138Ferrini, F., Molla´, M., Pardi, M. C., & Dı´az, A. I. 1994, ApJ, 427, 745Kauffmann, G. 1996, MNRAS, 281, 475Lanzetta, K. M., Wolfe, A. M., & Turnshek, D. A. 1995, ApJ, 440, 435Larson, R. B., & Tinsley, B. M. 1978, ApJ, 219, 46Lu, L., Sargent, W. L. W., Barlow, T. A., Churchill, C. W., & Vogt, S. S. 1996,ApJS, 107, 475Malaney, R. A., & Chaboyer, B. 1996, ApJ, 462, 57Molla´, M., Ferrini, F., & Dı´az, A. I. 1996, ApJ, 466, 668Pei, Y. C., & Fall, S. M. 1995, 454, 69Pettini, M., King, D. L., Smith, L. J., & Hunstead, R. W. 1997, ApJ, 478, 536Wolfe, A. M. 1995, in QSO Absorption Lines, ed. G. Meylan (Berlin: Springer),13L32 FERRINI, MOLLA´, & DI´AZ

Damped Lyα systems as spiral galaxies

https://repositorio.uam.es/bitstream/handle/10486/13459/63325_1997ApJ_487L_29F.pdf?sequence=1

Damped Lyα systems as spiral galaxies

Abstract

Similar works

Full text

Available Versions

Biblos-e Archivo