The fact that the mass range for the progenitors of SN II ( > 8 M_O) does not overlap with the mass range for the progenitors of SN Ia ( < 5 – 8 M_O ) has been used to investigate whether or not the relative likelihood of observing the two different types of supernova in spiral galaxies depends upon the recent star formation history of the underlying parent galaxy.

The L_IR/L_B ratio and the infrared surface brightness (SB_IR) of spiral galaxies were investigated to see if they could be used to determine the (relative) current to integrated star formation rates for these galaxies. Two factors were identified which might adversely affect the usefulness of the L_IR /L_B ratio and the infrared surface brightness (SB_IR ) as star formation rate indicators. These factor were:

b) the effects of HI gas depletion upon a galaxy’s (L_IR /L_B) ratio and SB _IR. In order to avoid these effects, all Sbc-Sdm
galaxies with HI deficiencies that were a factor of 3 below that of a “normal” galaxy with similar Hubbletype (T),
luminosity class (L) and optical diameter (D _L ) (i.e. those with a HI index greater than 2.9) were eliminated from
the final galaxy sample.

We have found that there is a good correlation between both log (L_IR/L_B ) (r = 0.81) and log (SB_IR) (r = 0.80) and log (H-a EQW) . However, these correlations are only true if Sbc-Sdm spirals are sub-divided into two fundamentally distinct galaxy groups, one consisting of the high blue luminosity, high IRE, H ₂ rich spirals (i.e. H₂ rich spirals) and the other of the low blue luminosity, low IRE, H₂ poorspirals (i.e. H₂ poor spirals).

We believe that reason why these correlations are only evident when the galaxy sample has been subdivide into these two distinct galaxy groups, is that the HII regions of the H₂ rich spirals are subject to a systematically higher extinction of
~ 0.7 magnitudes, compared to the HII regions of the H₂ poor spirals. The presence of this differential extinction means that:

Hence, both L_IR/L_B and SB _IR can be used to determine the (relative) current to integrated star formation rate for Sbc-Sdm galaxies that are not HI gas depleted, provided the following three important points are taken into account:

b) L_IR/L_B slightly overestimates the (relative) current to integrated star formation rate of H₂ rich spirals compared
to H₂ poor spirals because of the enhanced extinction associated with the HII regions of the H₂ rich galaxies.
The magnitude of this error depends on how much of the blue light of H₂ rich galaxies comes from star located
in regions of recent star formation.

c) L_IR/L_B overestimates the current to integrated star formation rates for galaxies with log(R₂₅)) > 0.45 i.e. those
with inclination angles above 70^o. The most likely reason for this is that the inclination corrections to L _B have
been over-estimated for these edge-on spirals.

Two main samples were constructed to investigate whether or not the relative likelihood of observing the two different types of supernova in spiral galaxies depends upon the recent star formation history of the underlying parent galaxy.

The first sample is the SN sample. This sample includes all Sbc-Sdm spirals in RC3 catalogue with Vo (3K) < 3000 km/sec that:

a) are members of the NGC and Index Catalogue.

b) are not HI gas deficient i.e. those with an HI index < 2.9.

c) that have had an historical supernova of type SN II, SN Ia/I*, or SNIb/Ic,which have been determined from
spectra rather than light curves.

d) that have had an historical supernova with designation between SN1885A and SN 2000DS.

Galaxies in the SN sample are divided into two groups. Those that have had one or more SN II/Ib/Ic (hereafter referred to as SN II galaxies) and those that have nothad SN II/Ib/Ic i.e. they have had SN Ia/I* only (hereafter referred to as SNI galaxies). A galaxy that has had both SN II/Ib/Ic and SN Ia/I* is classified as a SNII galaxy.

SN spectral types have been determined for supernovae in 103 sample galaxies. After removing the HI gas depleted spirals and those with no HI index, there were 62 SN sample galaxies left that had either SNII or SN Ib/Ic and 23 SN sample galaxies that had SN Ia.

The second sample is the Base sample. This sample includes the parentpopulation from which the SN galaxies are drawn i.e. consists of all of the Sbc-Sdm spirals in RC3 catalogue with Vo (3K) < 3000 km/sec that:

a) are members of the NGC and Index Catalogue.

Plots of SB_IR versus L_IR/L_B were constructed for galaxies in the Base, SNII/Ib/Ic, and the SNIa/I* samples. These plots clearly show that there is a population of Sbc-Sdm spiral galaxies with high current to integrated star formation rates (SB_IR > 35.0 in units of 10⁶ L _O Kpc^-2) that are preferentially producing SNII but not SNIa. The preference for SNII, but not SNIa, to be seen in galaxies with high current to integrated star formation rates, supports our original contention that the enhanced star formation rates in late type spirals are caused by bursts.

Hence, we conclude that Sbc-Sdm spiral galaxies experiencing short durationburst in their star formation rate, which causes them to pass through a distinct epoch in which they produce SN II at a much higher rate than they produce SN Ia. This epoch of enhanced SNII production last for the burst duration plus 3.5 x 10⁷ years i.e. 0.85 – 1.35 x 10⁸ years for burst lengths of 0.5 – 1.0 x 10 ⁸ years, respectively.

Having established this very important result, we have used the SN sample galaxies to study the general properties of the galaxies that are currently experiencing burst activity.

We find that while Non-Burst spirals can be found in galaxies with isophotal diameters ( D(Kpc)) ranging all the way up to 60 Kpc, Burst spirals are restricted to galaxies with D(Kpc) < 30 Kpc. The simplest model which explains both the observed distribution of Burst spirals with isophotal diameter, as well as the lack of Burst spirals with D(Kpc) > 30 Kpc, is one that requires that:

b)it becomes progressively more difficult to see the burst against the steady-state infrared emission of the
underlying parent galaxy as the galaxy becomes larger and more massive.

One consequence of this model is that there must be a population of spirals with diameters > 30Kpc that are undergoing bursts of star formation that are comparable in magnitude to those in spirals with diameters < 30 Kpc. In addition, the bursts in starformation in the larger galaxies must also be preferentially producing SNII but not SN Ia.

In order to get an idea of the level of enhancement in the star formation rates in the Burst spirals, a plot of H-alpha equivalent widths versus the (B-V)_To colour has been constructed for all the Sbc-Sdm galaxies in the sample of Kennicutt et al. (1987). Data points in this plot have be subdivide into Burst and Non-Burst spirals to see how they compare with interacting and non-interacting spirals. The resulting data distribution in this plot have been compared to models developed by Kennicutt et al. (1987), to show that the Burst spirals have enhancements in their current star formation rates that are comparable to those for interacting galaxies i.e. roughly two and ten times the steady-state star formation rate.

Plots of HI, H₂ , and (HI + H_2­) versus isophotal diameter D(Kpc) are used to show that Burst spirals have significantly higher H ₂ gas masses compared to Non-Burst spirals with similar diameters. The data shows that for Sbc-Sdm spirals in sample of Young et al. (1996) with an isophotal diameters of 20 Kpc, the average Burst spiral has an additional ~ 3.6 x 10⁹ M_O of H₂ gas, ~ 1.5 x 10⁹ M_O of HI gas, and ~ 5 x 10⁹ M_O of total gas mass, when compared to the average Non-Burst spiral.

This result would seem to put the Burst model into question, since this means that prior to the burst, the galaxies would have to experience an infusion of H­ ₂ gas mass from outside the galaxy that was comparable to the H ₂ gas mass of our own Galaxy. In addition, these galaxies would have to consume this additional gas in a time scale that is comparable to the length of the epoch in which Burst galaxies produce SN II at a much higher rate than they produce SN Ia i.e. 0.85 – 1.35 x 10⁸ years. In order accomplish this, the Burst spirals would have to absorb and consume H₂ gas at a rate that is 4 – 7 times higher than the observed rate of ~ 7 M _O per year (Young et al. 1996).

There are three ways in which the gas consumption rates implied by the Burst model could be reconciled with the observations:

The first is that only a fraction (f) of the infused molecular mass (i.e. f M_H2) is converted into stars by the burst, while the remainder is converted into atomic hydrogen (i.e. (1-f)M _H2 ) by of the energy released by the burst. This explainationis ruled out by fact that it would require such low “f” values that virtually all of the in-falling H₂ gas would be converted into HI.

The second is by having large regions of spiral galaxy’s gaseous disk spontaneously convert from HI to H₂, following an interaction or other event that triggers mass accretion or an increase in the gas mass density (Elmegreen 1993). This explanation is also ruled out as it would require that virtually all of theoriginal HI gas in the accreting spirals be converted into H ₂.

The third is that the significant enhancement in star formation rates in Burst spirals exposes the molecular clouds to much more intense radiation fields.The enhanced radiation fields increase the gas temperature in the molecular clouds, resulting in a decrease in the CO ¬ > H₂ conversion ratio X. As a result, it possible that the H₂ masses are overestimated for galaxies which are experiencing significant bursts in the level of their star formation.

It is only the last of these three explanations which provides a plausible solution to the H ₂ gas mass problem in Burst spirals, by reducing the amount of gas which must be consumed over the epoch of enhanced SN II production. Indeed, it is possible to completely eliminate the apparent excess of H₂ in Burst galaxies by reducing the CO ¬ > H₂ conversion ratio X by a factor of
~ 3.5, compared to the Galaxy.

However, if the CO ¬ > H₂ conversion ratio X for Burst spirals is less than ~ 3.5 times lower than that of non-Burst spirals then the Bursts spirals must absorb H₂ gas from their surrounding and convert a fraction (f) of this gas into stars. Unfortunately, there is no way to determine the amount of gas that is absorbed by the Burst spirals until we can better quantify
f, the decrease in X, and the amount of pre-existing gas within the galaxies that becomes involved in a burst.

The only constraints we can place upon the H₂ gas consumed by spirals during a burst is a lower limit upon the total H₂ gas (internal + infused) from current star formation rates determined from their H-alpha luminosities.

Plots of the absolute current star formation rates (CSFR) versus isophotal diameter (D₂₅ ) for all of the Sbc-Sdm NGC/IC galaxies in the sample of Young et al. (1996) show that while the absolute current star formation rate for Non-Burst spirals increases slowly with increasing galactic size, the star formation rates for Mergers, Burst spirals and Near Burst spirals show a completely different trend, increasing almost linearly with D₂₅(Kpc). These plots also show that the average absolute current star formation rate for the Burst spirals is ~ 7 M_O per year.

If the Burst galaxies maintain this rate over the life-time of the enhanced SNII production, lasting for ~ 0.85 – 1.35 x 10⁸ years, they will consume at least 6.0 – 9.5 x 10⁸ M_O of H₂ gas. Unfortunately, these values cannot be converted into a specific mass accretion rate until we know what fraction of the available H₂ gas is in fact turned into stars and what fraction of the pre-existing hydrogen gas within the galaxy is actually consumed during the burst.

However if we conservatively assume that only half H₂ gas is in turned into stars (with the remainder converted into HI), and that two thirds of the H ₂ consumed in the burst comes from the galaxy’s pre-existing reserves of hydrogen gas, then the Burst galaxies might absorb as much as ~ 4.0 – 7.0 x 10⁸ M _O of H₂ gas.

One possible source for such large amounts of H₂ gas could be gas–rich dwarf irregular galaxies that are in orbit about these galaxies. Of course, we cannot rule out the possibility that there are massive clouds of H ₂ in close proximity to latetype spirals that are as yet undetectable, and that these clouds are being dumped onto the main galaxy by the passage of a nearby companion.

We also investigated the possibility that the enhanced star formation seen in Burst spirals is a direct result of tidal interaction with close companions. In order to gauge how important this mechanism is for producing bursts, we have conducted a preliminary search around each of the SN sample galaxies to look for close companions.

A SN sample spiral was designated to have a close companion if there wasanother galaxy in the RC3 catalogue that had a projected separation from the SN galaxy of < 0.2 Mpc (H_o = 75 km/sec/Mpc ) and a recession velocity difference < 300 km/sec. If no such galaxy exists then the SN sample galaxy was designatedas not having a close companion.

We constructed plots of SB_IR against isophotal diameter (D₂₅) for the SNII sample galaxies with and without close companions. These plots clearly show that:

a) for galaxies with isophotal diameters < 25 – 30 Kpc, if a galaxy’s current star formation rate is enhanced (i.e. it has
an enhanced L_IR) then it has a close companion (< 0.20 Mpc). Note: The data does not support the converse,
hence, it is possible for a galaxy to have a close companion but not havean enhanced current star formation rate.

b) it is easier to distinguish bursts in smaller galaxies (by their enhanced SB_IR) because the bursts are seen against a
lower overall level of steady-state star formation rate.

In addition, we have plotted the L_IR of the SN sample galaxies against the distance of each from the companion galaxy with the smallest projected separation on the sky. From these plots we conclude that:

a) Bursts spirals are preferentially found in interacting galaxies (projected separation < 0.20 Mpc).

This leads us to conclude that group of galaxies that we have identified as Burst spirals are by and large synonymous with the group of post-interaction burst spirals identified by Barton et al. (2000).

Hence, based on the findings of Barton et al. (2000), as well as our own results,we believe that the following chain of events best describes what happens when a late type spiral tidally interacts with a companion galaxy:

a) There is close pass between two galaxies ( < 0.05 Mpc - Barton et al. 2000).

b) A burst of star formation is initiated in one or both of the galaxies at or near closest approach. There could be a
delay in the onset of the burst following closest approach of ~ a few by 10⁷ years. A comparison between the
data of Barton et al. (2000) and both the orbital and star formation models rules out delays that are longer than
~ 5 x 10⁷ years.

d) The burst in star formation lasts for at least ~ 10⁸ years, and ages as the two galaxies move further apart
(Barton et al. 2000).

e) Shortly after the start of the burst (~ 3 million years), there is a significantincrease in the SNII/Ib/Ic rate in
the burst galaxy. There is no corresponding increase in the SNIa rate.

f) The SNII/Ib/Ic rates do not return to their pre-burst levels until ~ 1.35 x 10⁸ years after the onset of the burst.

g) In contrast, the SNIa rate does not become enhanced until well after the end of the burst.

j) The significantly enhanced star formation rates expose the molecular clouds in the burst galaxy to a much
more intense radiation fields which increases the gas temperature in the molecular clouds, resulting in a
decrease in the CO ¬ > H₂ conversion ratio X. As a result, the inferred H ₂ masses of the burst galaxies
are overestimated.