4. Global interpretation of the Mrk 86 star formation history
Summarizing, we have derived the properties of three well defined stellar populations;
4.1. What has been ocurring during the last 30 Myr?
Most of the HII regions in Mrk 86 are located in an bar oriented in north-south direction and placed 20" west of the galactic center (see Fig. 3 and Fig. 2 of Paper I) and in an arc going from south-east (#58 & #64 regions) to north (#13 & #16 regions) of the central starburst. There is no significant age gradients across these structures, suggesting the existence of a large-scale triggering mechanism that activated the star formation simultaneously in most of these regions about 10 Myr ago.
We propose that the large-scale triggering mechanism for the recent star formation was produced by the growth of a superbubble originated at the galaxy central starburst by the energy deposition of stellar winds and supernova explosions. In some cases, local triggering mechanisms are also present. This could be the case of the complexes formed by the #26-#27, #26-#18, #58-#64 and #16-#13 regions. These complexes are constituted by an evolved burst (age10 Myr) -the former-, and a very recent star-forming event, about 5 Myr old -the latter- (see Stewart et al. 2000)
Hereafter, we will mainly focus on the large-scale triggering mechanism described above. Following the models of Silich & Tenorio-Tagle (1998, STT hereafter; see also De Young & Heckman 1994), these collective supernova remnants grow at first elongated in the direction perpendicular to the galaxy disk. Due to this, the remnant blows out into the galaxy halo, leading to the formation of a secondary shell of swept out gas. About 20 Myr after the first supernova explosions (for the STT A100 model) the superbubble surrounds the inner densest part of the disk. Then, the leading shock, going through outer less dense regions, merges with its symmetrical counterpart. During this merging process a large fraction of swept out mass is strangled shaping a dense toroid. This region does not participate in the general outward motion, being strongly compressed towards the galaxy plane. As a result of this compression the activation of the star formation in the gas toroid could be produced if the mean surface density will be higher than 5-10 pc-2 (Skillman et al. 1987; Kennicutt 1989; see also Taylor et al. 1994for a sample of HII galaxies).
The predictions described above correspond to the A100 model of STT. This model assumes a 1010 massive galaxy with 10 per cent gas content, ISM central density of =20.2 cm-3, burst energy of =1056 erg and gas metallicity, Z=0.3 . For this model, adopting a toroid width of 500 pc and a strangled mass of 2108 , the surface density of the gas at the toroid will be about 64 pc-2. This value is intermediate between the gas surface densities measured in normal disks and in the infrared-seleted starbursts studied by Kennicutt (1998). The galactocentric distance and epoch predicted for the formation of the toroid in the A100 STT model are 1.5 kpc and 25-30 Myr, respectively.
In the case of Mrk 86 the epoch for the formation of this toroid should be about 20 Myr (the age difference between the starburst and currently star-forming regions) at a radius of 0.8 kpc. Although these values well agree with the properties deduced for the A100 STT model, the high dependence of the evolution of the superbubble on the ISM density profile and the distance uncertainty for Mrk 86 prevent us to carry out a more quantitative analysis. However, this scenario provides a reliable and attractive explanation for the evolution of the star formation activity in Mrk 86.
Finally, it should be noticed that the time since the formation of the toroid (10 Myr) is significantly lower than the galaxy rotation period (90 Myr), avoiding the disruption of this toroid by differential rotation. The galaxy rotation period was obtained using a projected angular velocity of 44 km s-1 kpc-1 and assuming an inclination of the rotation axis relative to the plane of the sky of 50o (GZG; see also Gil de Paz 2000).
The scenario described above is similar to that observed in other nearby dwarf star-forming galaxies. Thus, in the LMC the two arcs of stellar clusters place on the LMC4 superbubble rim round a population of 30 Myr old supergiants and Cepheid variables (Efremov & Elmegreen 1998). A similar study for the DEM192 superbubble also in the LMC was carried out by Oey & Smedley (1998).
4.2. What did happen 30 Myr ago?
Although we can accept that most of the recent star-forming activity in Mrk 86 was originated by a superbubble produced at the central starburst, we need to solve the question about the nature of the triggering mechanism of the star formation in the galaxy central starburst.
Under the evolutionary scenario described above, the observational properties of Mrk 86 about 30 Myr ago would be very similar to those of the nucleated Blue Compact Dwarfs (nE BCDs). It should show a very bright central starburst superimposed on an extended underlying component (see Papaderos et al. 1996a and references therein). This fact suggests the existence of an evolutionary connection between the different kinds of Blue Compact Dwarf galaxies. Therefore, the question about the star formation triggering in the central starburst of Mrk 86 is equivalent to the problem of the activation of the star formation in the BCD galaxy population as a whole.
Taylor et al. (1993) hypothesized that the close passage of a companion galaxy has triggered the present burst of star formation (the central starburst in the Mrk 86 case) in many (if not all) of the BCD galaxies. These kind of distant encounters are well acepted to lead the lost of angular momentum in the gas component, resulting in the fall of large amounts of gas to the galaxy central regions (Barnes & Hernquist 1992; Taylor et al. 1994; Mihos & Hernquist 1996). This radial inflow will be very efficient in Mrk 86, because of the low dark matter content expected for this object (GZG; see Taylor 1997).
The first candidate for such a tidal interaction is NGC 2537A ((2000)=08h13m40.9s (2000)=+45o 59´ 41"). However, the study of Heyd & Wiyckoff (1992) and the 21 cm line intensity map (see Fig. 6) show that this object is clearly a background galaxy. Then, the next candicate for the central starburst triggering is UGC 4278 ((2000)=08h13m58.8s (2000)=+45o 44´ 36"), a nearly edge-on spiral galaxy placed at a projected distance of about 33 kpc relative to Mrk 86 (see Fig. 6). Its heliocentric velocity is 553 km s-1 (Goad & Roberts 1981; see also Schneider & Salpeter 1992), and its radial velocity relative to the CMB is 715 km s-1.
The offset of this galaxy relative to the Tully-Fisher template obtained by Giovanelli et al. (1997) is in the I-band (R. Giovanelli, priv. comm.). Therefore, its peculiar radial velocity will be -253 km s-1, being the radial velocity corrected for peculiar motions 968 km s-1. Then, the distance to UGC 4278 could range between 13 and 19 Mpc, respectively for h=0.75 and 0.5. The large difference between this value and that derived by Sharina et al. (1999; see also Sect. 2.1 of Paper I) for Mrk 86 suggests that UGC 4278 is also a background galaxy.
Therefore, another triggering mechanism should be argued to explain the activation of the star formation in the Mrk 86 central starburst. This triggering mechanism could be related with the presence of previous massive star forming events. Our study has not revealed the existence of such an intermediate aged stellar population, probably excepting the #26 and #27 regions. As we indicated in Sect. 3.5 (see also GZG), the association constituted by the #26 and #27 regions shows a very steep velocity gradient that could be produced by the velocity field of an independent low mass system merged or in process of merging with Mrk 86. In that case, this merging process could be responsible for radial inflow of gas that led to the star formation activation in the central regions of Mrk 86. However, the large-scale distribution of neutral hydrogen in this galaxy (see Fig. 6; E. Wilcots, priv. comm.) indicates that, if this merging process took place, it should occur long time ago, probably several orbital periods ago.
© European Southern Observatory (ESO) 2000
Online publication: October 2, 2000