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Formation of Spiral and Elliptical Galaxies in a CDM Cosmogony

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Formation of Spiral and Elliptical Galaxies in a CDM Cosmogony Formation of spiral and elliptical galaxies in a CDM cosmogony Matthias Steinmetz and Ewald Muller MaxPlanckInstitut fur Astrophysik KarlSchwarzschildStrae Garching b Munc hen FRG Institut fur Theoretische Physik und Sternwarte der Universitat Kiel Olshausenstrae Kiel FRG astro-ph/9309004 1 Sep 93 Send oprint requests to M Steinmetz Thesaurus co des Submitted to Astronomy and Astrophysics Letters Abstract The formation of galaxies in a Cold Dark Matter cosmogony is investigated by following the evolution of dark and baryonic matter and of the frequencydep endent spatially averaged radiation eld The gas is allowed to form stars which are indep endently treated as a fourth collisionless comp onent By sup ernovae energy and metal enriched gas is returned to the interstellar medium Two extreme cases the formation of an isolated eld galaxy and the merging of two gas rich proto spirals are considered the latter b eing used as a mo del for galaxy formation in a cluster The isolated galaxy is assumed to interact with the surrounding matter via tidal elds only resulting in a rotation with a spin parameter The forming eld galaxy shows the main prop erties of spiral galaxies A metal rich bulge a metal p o or stellar halo and a disk of nearly solar comp osition The spiral is emb edded in a triaxial halo of dark matter The disk p ossesses an exp onential brightness prole while halo and bulge follow the de Vaucouleurs law The disk is rotationally supp orted with a at rotation curve The attening of the triaxial bulge is probably due to its rotation whereas the triaxial halo rotates only slowly The disk has a metallicity gradient of dlog Z dr kp c whereas the halo shows none The mo dels also exhibit a correlation b etween the disk to bulge ratio and the p ower of small scale uctuations The stars of the bulge form from gas which is initially lo cated in the largest maxima of the primordial density uctuations whereas the halo stars originate from gas accumulated in less pronounced maxima Merging proto spirals evolve to ob jects which can b e interpreted as luminous ellipticals p ossessing a de Vaucouleurs prole The metal rich Z Z elliptical rotates only slowly its attening b eing a result of an anisotropic velo city disp ersion The metallicity of the elliptical decreases with radius according to a p ower law Dep ending on the geometry of the merger event sometimes counter rotating gas is observed Key words Cosmogony Galaxies formation of kinematics and dynamics of stellar content of structure of Intro duction The explanation of the origin of the observed dierent morphologies of galaxies p oses one of the greatest challenges in the eld of cosmology and galactic dynamics In particular the origin of the two ma jor typ es of galaxies elliptical s and spirals has b een and still is a matter of controversial debates From observations one knows that spiral galaxies are rotationally supp orted and that their surface brightness follows an exp onential law Their rotation curves which are constant out to radii of kp c or even b eyond cannot b e explained by galactic mo dels which only consider visible matter Observations further show that many spirals p ossess a more or less pronounced central spheroidal bulge which like elliptical galaxies is stabilized by its velo city disp ersion and that the surface brightness of elliptical galaxies can well b e describ ed by a de Vaucouleurs prole log I r Observations by Bender imply that there exist two distinct classes of luminous elliptical galaxies having boxy and disky isophotes In general b oxy ellipticals seem to b e more massive M and b eing attened by an anisotropic velo city disp ersion B In contrast the ellipticity of generally less massive disky ellipticals M can B easily b e explained by their rotation Furthermore it is interesting that the bulges of spiral galaxies which also show a de Vaucouleurs prole and have masses comparable to disky ellipticals seem to b e rotationally attened to o During the last two decades several mo dels have b een prop osed to explain the morpho logical dierences b etween ellipticals and spirals However most of those mo dels ignored the inuence of the underlying cosmogony for the structure formation This drawback was the fo cus of the pioneering work of Katz who simulated the formation of spirals in a hierarchical cosmogony including gas dynamics and star formation In his simulations ob jects formed which have the main prop erties of observed spiral galaxies a disk a stellar spheroid and a dark matter halo On the other hand computer simulations of merging spiral galaxies see Barnes Hernquist show the formation of an ob ject which resembles the observed structure of high mass ellipticals except for their compactness In this letter we present the rst results of a comprehensive study which extends the work of Katz as well as that of Barnes Hernquist We have simulated the evolution of isolated spiral galaxies of dierent mass angular momentum and star formation history resulting from dierent primordial uctuation sp ectra Besides providing a more detailed analysis of the structure and the kinematics of the forming galaxy the simulations also incorp orate the metal enrichment by sup ernovae in a simple form which allows us to identify dierent stellar p opulations In order to study galaxy formation in a cluster environment we have also simulated the merging of two gas rich star forming proto spirals instead of two fully evolved spirals as in the work of Barnes Hernquist Metho d The simulations have b een p erformed with a combined smo othedparticlehydrodyna mics SPH and Nb o dy tree co de Steinmetz Muller Radiative co oling is describ ed by a timedep endent ionization network including the ions of H and He The evolution of the spatially averaged frequencydep endent radiation eld is treated as in Cen and star formation is included in a way similar to that describ ed by Katz Once stars are formed their further evolution is followed by a Nb o dy co de It is assumed that they are distributed according to a MillerScalo initial mass function Stars more massive than M explo de as Typ e II sup ernovae with an energy release of erg and return except for an M compact remnant all their mass to the gaseous comp onent It is further assumed that of the ejected mass are metals In a typical simulation dark matter and gas particles are used During a simulation typically to stellar particles are formed For numerical reasons the adaptive softening length of the gravitational p otential is restricted to b e larger than kp c stars kp c gas and kp c dark matter resp ectively A typical run requires b etween h spirals and h ellipticals CPU time on one pro cessor of a CRAY YMP For a more detailed discussion of the metho d and the results we refer to forthcoming publications Steinmetz Muller abc in preparation Spiral galaxies All calculations start at a redshift of z when baryons and dark matter are still equally distributed The initial mo del consists of a homogeneous sphere with a radius of kp c on which small scale uctuations are sup erimp osed according to the Zeldovich approximation This approach neglects all p ower on scales larger than the b ox volume However the analytical prop erties of the gravitational clustering pro cess allow one to derive some approximations for the inuence of the large scale p ower As long as no signicant amount of matter is transp orted into the simulated cosmic subvolume the large scale p ower can b e split into two main eects a compression of the volume and a tidal eld which causes the proto galaxy to rotate Both the amount of compression and rotation can b e predicted via p erturbation theory For a CDM scenario a density enhancement of a factor of ab out compared to the mean cosmic density is exp ected for z The rotation of the proto galaxy is usually measured by the dimensionless spin parameter p J jE jGM where J E G and M are the angular momentum the total energy the gravitational constant and the mass resp ectively Relatively indep endent on the sp ecial form of the primordial uctuation sp ectrum a spin parameter of can b e obtained Barnes Efstathiou Therefore realistic initial conditions for the formation of a eld galaxy are a rotating density enhanced sphere which expands according to the Hubble law However the expansion velo city of the sphere should b e reduced according to the density enhancement The expansion will end at z followed by a collapse phase As in the work of Katz simulations with these initial conditions lead to the formation of a spiral galaxy The evolution can b e separated into several phases For z the evolution of the dark and baryonic matter is identical Later the gas co ols and forms clumps which are denser than the dark matter Near the maxima of the primordial density uctuations the gas b egins to form stars Up to a redshift of z the most pronounced maxima merge and form an ob ject of ellipsoidal shap e which is surrounded by more diusely distributed stars The star formation rate during this phase is of the order of M yr Between a redshift of and the disk b egins to form by an accretion like pro cess and the star formation rate p eaks at ab out M yr At z the star formation rate b egins to drop the disk comp onent b ecomes more and more dominant and at z when the calculation was stopp ed a quasistationary state is reached The mass fraction of the gas in the disk has dropp ed to to and the star formation rate has leveled of at ab out
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