Mon. Not. R. Astron. Soc. 312, 194±206 (2000) Secular evolution of late-type disc galaxies: formation of bulges and the origin of bar dichotomy Masafumi Noguchiw Astronomical Institute, Tohoku University, Aoba, Sendai 980-8578, Japan Accepted 1999 September 21. Received 1999 September 21; in original form 1999 April 8 ABSTRACT Origins of galactic bulges and bars remain elusive, although they constitute fundamental components of disc galaxies. This paper proposes that the secular evolution process driven by the interstellar gas in galactic discs is closely associated with the formation of bulges and bars, and tries to explain the observed variation of these components along the Hubble morphological sequence. The ample interstellar medium serves as a coolant which makes the galactic disc gravitationally unstable and induces fragmentation of the disc. The resulting clumps spiral in to the disc centre because of dynamical friction. Efficiency of this process is governed by the gas-richness of the galactic disc, which is in turn controlled by two parameters: (i) the rapidity of the infall of the halo primordial gas to the disc plane and (ii) the amount of total accreted matter. When these two parameters are appropriately related to the mass and density of the galaxy and the effect of a star formation threshold in the disc is introduced, the gas-driven evolution through disc clumping leads to the appearance of two evolution regimes along the Hubble sequence, separated by the intermediate Hubble type for which the gas-driven mass concentration is minimal. Spiral galaxies of relatively early Hubble type (characterized by large mass and density) experience a rapid clump accumulation to the disc centre in their early evolution phase, which is identified with the formation of a bulge. On the other hand, the evolution of late-type spirals having small mass and density is strongly influenced by the existence of the star formation threshold. The disc in these galaxies becomes gas-rich in a relatively late epoch and experiences a prolonged clump-driven mass accumulation. I argue that this process leads to the formation of not a bulge but a short bar embedded in the galactic disc. The present scenario provides a natural explanation for the well-known bar dichotomy, namely that galactic bars are divided into two major groups on morphological grounds and each group is associated with different Hubble types. Key words: galaxies: evolution ± galaxies: formation ± galaxies: ISM ± galaxies: kinematics and dynamics ± galaxies: spiral ± galaxies: structure. accretion of the residual primordial gas has built the disc 1 INTRODUCTION components. This simple picture is, however, challenged by One of the most notable structural features of disc galaxies is that recent observations of the Milky Way bulge (McWilliam & Rich they are composed of two distinct components: discs and bulges. 1994) and bulges of other galaxies (Matteucci & Brocato 1990; The origin of this two-component structure remains unclear, Peletier & Balcells 1996; Kormendy 1993). McWilliam & Rich although it constitutes the backbone of disc galaxies. In the (1994) have found that the bulge stars of the Milky Way Galaxy classical picture of galaxy formation (Larson 1976; Gott & Thuan have a larger age spread than formerly considered and show a 1976), bulges have been formed within a relatively short period, as correlation between chemical and kinematical properties (Minniti a result of the collapse of a gaseous protogalaxy, and the later 1996). Peletier & Balcells (1996) have found for their sample of galaxies that the ages of the bulge and inner disc are correlated w E-mail: [email protected] and the difference between them is as small as 30 per cent. These q 2000 RAS Late-type disc galaxies 195 results suggest a slower build-up of the bulge components than has by mh, ms, mg and mb respectively. The total mass is denoted by M been hitherto considered, as well as a close relationship between ( mh ms mg mb. Each galactic component is treated as a bulges and discs. single zone so that any internal structures that might arise in it are There is no convincing theory for bulge formation at present, not treated. though several possibilities are considered. Cosmological simula- Under this simplification, the time evolution of masses in the tions for galaxy formation (Katz 1992) seem to indicate that the respective components is formulated as follows. primordial clumps arising from cold dark matter perturbations dm m MGt t merge to form a bulge when the host galaxy is assembled. Highly g 2SFR 2 g exp 2 ; 1 dt t b2 b contrasted to this idea is the secular formation by bar structures. fri Galactic bars induce inflow of interstellar gas, and the ample gas dms accumulated at the galactic centre provides raw material for the SFR; 2 dt bulge component (Friedli & Benz 1993). Noguchi (1998, 1999) has suggested the third possibility that the galactic disc in its early and evolution stage fragments into massive subgalactic clumps, which dm m fall to the galactic centre owing to dynamical friction and b g ; 3 constitute a bulge. dt tfri Bars are another prominent structure ubiquitous in disc where t is the time reckoned from the beginning of gas infall, galaxies. The relation between bulges and bars also remains which is here assumed to be 12 Gyr ago. unclear. Bending instability of galactic bars and the resulting SFR stands for the star formation rate for the whole galaxy, transformation into a box-shape or peanut configuration suggests represented as that bulges in some galaxies originate in bars [see the review by 2 Sellwood & Wilkinson (1993) and references therein]. It is also Sg R 21 SFR 355 2 M( yr ; 4 known that the morphological properties of bars change system- 103 M( pc 2 104 pc atically along the Hubble sequence and two major classes of bars are recognized (Elmegreen & Elmegreen 1985). where R is the galaxy radius and Sg denotes the gas surface 2 I develop here a scenario in a partial attempt to shed light on the density in the disc, which is approximated by mg/(pR ). The work origins of bulges and their relationship with galactic bars. This by Kennicutt (1989) suggests that a certain threshold exists for the paper is a sequel to previous papers (Noguchi 1998, 1999) and gas density, below which star formation is effectively inhibited. based on the simple model of disc galaxy evolution developed One plausible interpretation is that the star formation activity is there. A novel point here is that the role of a star formation associated closely with the gravitational instability of the gas disc, threshold is highly emphasized. Section 2 describes the evolution and the threshold corresponds to neutral stability. This interpreta- model briefly. The model results are compared with observations tion seems to be applicable not only to `classical spirals' from Sa and bulge formation in late-type spiral galaxies is discussed in to Sc, but also to later types including low-surface brightness Section 3. Origin of the bar dichotomy, i.e. the existence of the galaxies (van der Hulst et al. 1993). I introduce a threshold for star two classes of galactic bars, is discussed in Section 4, and formation by specifying a minimum velocity dispersion that the Section 5 summarizes the conclusions. gas component of the galactic disc can have. In the present model, the gas disc is assumed to stay at the marginal stability defined by Q 1, where Q denotes the stability parameter introduced by Toomre (1964). In this state, the surface density of the gas and the 2 THE CLUMP-DRIVEN EVOLUTION MODEL velocity dispersion (or the sound velocity), s, in the gas are FORDISCGALAXIES related by pGSg sk, where k is the epicyclic frequency The disc galaxy evolution model that serves as a basis for the approximated as k 2GM=R31=2, and G is the gravitational present study has been discussed extensively in Noguchi (1999). I constant. As Sg decreases in the late phase of evolution, s should repeat here those parts of the modelling that are indispensable for also decrease correspondingly to keep the condition that Q 1. the subsequent treatment. I followed the conventional picture of However, s cannot become less than s min, which is the specified galaxy formation: that the visible part of a galaxy has been formed lower limit of the velocity dispersion. Therefore, star formation from the primordial gas that has collapsed in the gravitational stops at the instant when s reaches s min. The threshold gas potential well of the already virialized dark matter halo. Only surface density is thus determined by Smin smink=pG. After this the late phase of the galaxy collapse is treated, in which the instant, the amount of gas consumed by star formation is balanced primordial gas accretes nearly perpendicularly to the disc plane by the amount of new gas added to the disc by accretion, so that and does not shrink in the radial direction, owing to sufficient the gas surface density is always kept nearly at the threshold value support from the centrifugal force. In this model, the gas-rich disc (i.e. Sg , Smin). On the other hand, when a sufficient gas supply of a young galaxy is subject to gravitational instability owing to is available, the star formation rate is determined by the current efficient radiative cooling. The resulting clumps drive the gas surface density, i.e. it is `density-dependent'. As a special dynamical evolution of the galaxy. Especially, they suffer from case, setting smin 0 allows star formation to proceed con- dynamical friction against ambient matter while orbiting in the tinuously depending on the current gas density. disc plane, and sink to the central region of the disc.