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Astron. Astrophys. 323, 363-373 (1997)

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6. Discussion

In order to quantitatively compare the results of the numerical simulations with the observational features, we have tentatively distinguished in our sample four main classes of late-type objects according to their [FORMULA], SFR/relative SFR (f), and [FORMULA]. We stress that only relative values of the last three parameters are reliable. The respective properties of the four classes have been summarized in Table 2. Clearly the galaxies belonging to these various classes occupy different regions of this 4D parameter space. As can be seen in the suggested classification of Table 1, some galaxies have either intermediate properties between two classes, or half the properties of one class and the other half from another class. Of course, some galaxies cannot be classified due to the lack of data. For the same reason, we decided not to include the abundance gradient as a fifth parameter in this classification.


Table 2. Characteristics of galaxy class

Class I. Galaxies with large [FORMULA] (0.65 - 0.95), weak SFR ([FORMULA]), [FORMULA], and very low values of [FORMULA] ([FORMULA]). Typical galaxies of this class are NGC 3344, NGC 3726 and NGC 5457.

Class II. Galaxies are here again characterized by large [FORMULA] (0.60 - 0.90), but much higher SFR ([FORMULA]), and f (4 - 10). The [FORMULA] is between -1.2 and -1.3. Representative galaxies of this class are NGC 4321, NGC 5248, and NGC 6946.

Class III. This class includes galaxies with strong bars [FORMULA] (0.25 - 0.45), very high SFR ([FORMULA]), [FORMULA], and [FORMULA]. Two typical objects are NGC 5236 and NGC 7479.

Class IV. As in the previous class, galaxies belonging to this last class have strong bars [FORMULA] (0.25 - 0.55), but much weaker SFR ([FORMULA]), [FORMULA], and [FORMULA]. Prototypes are NGC 3359, NGC 3887 and NGC 7741.

The four SBa-SBb objects initially selected (see Sect. 3) have a strong bar. The value of [FORMULA] for NGC 3351 ([FORMULA]) is -1.13. For NGC 7552 ([FORMULA]), it is -0.92. Both could be included in class III. NGC 4394 ([FORMULA]) and NGC 4725 ([FORMULA]) respectively have -1.35 and -1.42. They should belong to class IV. As already noted, they do not modify the discussion in this paper.

Of course, the transition from one class to the other is not instantaneous and some galaxies can populate regions outside the schematic and indicative zones drawn on Figs. 4 and 6. The confrontation with the results from numerical simulations of galaxies developing spontaneous bars clearly suggests possible evolutionary sequences as presented below.

i) Strong bar: When [FORMULA], the sequence should be [I [FORMULA] IV [FORMULA] III [FORMULA] IV]. In the case where the gas to star mass ratio is higher ([FORMULA]), the evolutionary sequence is [II [FORMULA] III [FORMULA] IV 1 ] instead. A subsequent evolution [IV [FORMULA] III] could only be considered if high amounts of fresh gas are provided to the bar over short timescales, i.e. most likely from outside.

ii) Weak bar: The evolution is essentially enclosed in zone [I] irrespective of the gas to star mass ratio. However, when [FORMULA], a mini-starburst allows the model to come very close to zone [II].

Class IV is hybrid. Indeed, galaxies belonging to this class are either in the "pre-starburst" phase or in the "post-starburst" one. The respective fraction of either type for the late-type galaxies is unknown. However, the "post-starburst" galaxies should certainly be more numerous since the "pre-starburst" phase appears quite short in numerical simulations. There are at least two observational possibilities to distinguish between these two phases. The first one relates to star formation and the second one to abundance gradient. In "pre-starburst" galaxies, star formation is increasing so that current star formation should already be higher than recent star formation. For instance [FORMULA] should be high (Tomita et al. 1996). According to numerical models (Sect. 5), these galaxies show widely different abundance gradients in the bar ([FORMULA] [FORMULA] steep) and disc ([FORMULA] [FORMULA] shallow) regions. On the contrary, in "post-starburst" galaxies, star formation is strongly declining so that current star formation should already be lower than recent star formation, i.e. [FORMULA] should be low. These galaxies present similar abundance gradients in the bar and disc regions ([FORMULA] [FORMULA] flat). For instance in our sample, NGC 3359 has two different radial abundance slopes; this galaxy seems to be in a "pre-starburst" phase and its bar should be young as already suggested by Martin & Roy (1995).

The formation of a spontaneous or induced strong bar in a gas-rich Sc-like disc appears to be a major dynamical event. It results in many secular alterations of the galaxy properties over typical timescales of one tenth of the Hubble time. In this paper, we have mainly focused on bar-induced changes of the star formation (both in spatial distribution and intensity) and the radial gaseous abundance gradient. However, bars are clearly able to deeply reshape the overall morphology, kinematics, and chemistry of disc galaxies on less than a Hubble time as well (see e.g. Martinet 1995).

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998