1. INTRODUCTION


We find that there is a strong preference for late type (Sbc-Sdm) spiral galaxies with historical supernovae (SN – 1885A to 1998BP) to be found in the nearby recession velocities < 3,000 km/sec) galaxy groups catalogued by Huchra and
Geller (1982) and Geller and Huchra (1983), hereafter referred to as HG/Cfa galaxy groups.


There are 567 Sbc-Sdm NGC galaxies in the Third Reference Catalogue of Bright Galaxies (RC3 – deVaucouleurs et. al. 1991) with recession velocities V(3K) < 3,000 km/sec). Of these, 567 late-type spiral galaxies, 491 are included in
survey limits of HG/Cfa studies. For the purposes of this study, we call these 491 galaxies the Base Sample .


103 out of the 491 galaxies (21.0 %) in the Base Sample have had historical supernova with designated SN types i.e.


            SN IISN                types are determined from spectra. (Branch (1986), and
            SN Ib/Ic                Kirshner (1988), for SN1885A to SN1988M, and the I.A.U.
            SN Ia/I*                circulars, for SN1988N to 1998BP.


            (SN II)                SN type designations are from the Asiago Catalogue
            (SN I)                  prior to July 26th 1988.
            (No Type)            SN detected but no designation given in the Asiago
                                        Catalogue prior to July 26th 1988.


(Note: six galaxies, NGC1084, NGC3184, NGC3294, NGC3938, NGC4321, and NGC 6946 have two SN types and so are counted twice. One galaxy, NGC5236 has two SN types and one SN with no type and so it is counted three times.)
Columns four through seven of Table 1 show the fraction and percentage fraction of the Base Sample spirals with one of the four main SN types, that are in HG/Cfa galaxy groups. Column three shows the corresponding data for all galaxies with typed SN, while column two shows the same for Base Sample spirals that have had NO historical SN.


                                                                             TABLE 1


SN Group

NO SN

TypedSN

II/(II)

Ib/Ic

Ia/I*/(I)

SN No Type

Fraction in HG/CFA groups

183/381

76/103

41/57

9/12

26/34

10/20

Percentage

in HG/CFA groups

48.0 %

73.8 %

71.9 %

75.0 %

76.5 %

50.0 %


                                                                  FIGURE 1



Figure1: shows the percentage of galaxies with given SN types that are located in HG/Cfa groups for each SN category a listed in Table 1.


Taking at face value, Figure 1 shows that galaxies with typed SN are much more likely to be found in HG/Cfa galaxy groups (70-75 %) than galaxies with no historical SN (48 %). (Note: The same is NOT true of galaxies with SN that have NOT been typed, suggesting that these SN events may be fundamentally different from the “normal” SN II and I). Before we can make this conclusion, however, we must rule out any possibility that the observed differences between SN and non-SN galaxies are not just a result of an underlying bias produced by the search methods used to find SN.


2. BIAS TOWARDS GALAXIES WITH BRIGHT BLUE APPARENT MAGNITUDES


The first, and most obvious observational bias, is the well-know fact that observers who are searching for supernovae preferentially look at spirals that have bright blue apparent magnitudes. If galaxies with bright blue apparent magnitudes are
preferentially found in HG/Cfa galaxy groups and not in field galaxies, then this would produce the observed preference for galaxies with SN to be found in HG/Cfa groups.


                                                                      TABLE 2



SN Group

NO SN

Typed SN

II/(II)

Ib/Ic

I/I*/(I)

SN No Type

Fraction with

m(B) < 12.5

193/373*

83/102

44/57

11/12

28/33

14/20

Percentage with

m(B) < 12.5

51.7 %

81.4 %

77.2 %

91.7 %

84.6 %

70.0 %


* 8 galaxies have no blue photographic magnitudes (m(B))


                                                                     FIGURE 2



Figure2: shows the percentage of galaxies which have blue photographic
            magnitudes (m(B) - RC3) brighter than 12.5, for each SN category
            listed in Table 2.


The strong observational bias for SN searchers to preferentially look at galaxies with bright blue apparent magnitudes is clearly evident in Table 2 and Figure 2. These show that 77 – 92 % of galaxies with typed SN have blue photographic magnitudes
(m(B)) brighter than 12.5, while only 51.7 % of galaxies with no historical SN are as bright.


However, when we compare the fraction of HG/Cfa group galaxies that are bright (61.3 %) with the fraction of field galaxies that are bright (55.4%), we find that there is very little difference between the two samples.


                                                                      TABLE 3


CATEGORY

FRACTION

WITH m(B) < 12.5

PERCENTAGE

WITH m(B)< 12.5

FIELD GALAXIES

124/224

55.4 %

HG/CFA GROUP
GALAXIES

166/271

61.3 %



There is, however, a noticeable difference in the absolute number of galaxies with m(B) < 12.5 belonging to HG/Cfa galaxy groups – 166 (or 57.2% of bright galaxies) compared to the number of galaxies with m(B) < 12.5 belonging to the field – 124
(or 42.7 % of bright galaxies). While this difference could partly explain why more SN have been found in galaxies which belong to HG/Cfa galaxy groups, it clearly does not fully account for the fact that, out of the 83 SN galaxies with m(B) < 12.5, – 64 (or 77.1 % of bright galaxies) are found in HG/Cfa galaxy groups but only 19 (or 22.9 % of bright galaxies) are found in the field.


3. BIAS TOWARDS GALAXIES WITH BRIGHT BLUE ABSOLUTE MAGNITUDES


There is a possibility that late type spirals with bright blue absolute magnitudes are preferentially found in HG/Cfa groups. In this case, if the likelihood of seeing a SN simply scales with the total number of stars in a galaxy, then you might expect to preferentially see SN in galaxies that belong to HG/Cfa groups, simply because they contain a higher proportion of galaxies with bright blue absolute magnitudes.


Figure 3 and Table 4 show the fraction of the 482 Sample Galaxies with blue absolute magnitudes (MB) that are in HG/Cfa galaxy groups, broken down by Hubble type. Galaxies have been divided into those that have MB brighter than the median MB) and those that have M B fainter than the median M B for their Hubble type (Low MB). Care has been taken to distinguish between Hubble types because of the effect of the varying contribution of the bulge has upon MB .


Figure 3 and Table 4 show that galaxies that have bright blue absolute magnitude are equally likely to be found in HG/Cfa galaxy groups as they are in the field. Hence, this rules out the possibility that SN will be preferentially found in HG/Cfa groups
simply because these galaxy groups might preferentially contain Sbc-Sdm spirals with bright blue absolute magnitudes.


                                                                 FIGURE 3




Figure 3 : The percentage fraction of Sample Galaxies in HG/Cfa
galaxy groups by Hubble Type. Galaxies have been divided
into those that have MB brighter than the median M B for
their Hubble type (High MB) and those that have M B fainter
than the median MB for their Hubble type (Low M B ).


                                                                   TABLE 4


Hubble Type

Fraction withLow MB

Percentage
with
Low MB

Fraction with
High MB

Percentage
with
High MB


Median
MB

Sbc

41/63

65.1

30/62

48.4

10.35

Sc

50/87

57.5

42/87

48.3

10.21

Scd-Sdm

57/92

62

41/91

45.1

9.84

Sbc-Sdm

148/242

61.2

113/240

47.1 %



4. BIAS CAUSED BY SAMPLING DIFFERENT REDSHIFTS



Most of the SN detected over the last 115 years have had peak apparent magnitudes brighter than 16. It’s only been in the last 10-15 years that CCD technology has allowed us to extend SN searches to significantly fainter magnitude limits. The net
effect of this magnitude cut-off, has been the detection of fewer and fewer SN at higher red shifts.


Hence, if field galaxies were systematically located at higher recession velocities than galaxies found in HG/Cfa groups, fewer SN would found in field galaxies compared to galaxies in HG/Cfa group simply because some of the SN located in the
field galaxies would be too faint to be detected by conventional SN searches. Such a bias, could result in SN being preferentially found in galaxies belonging to HG/Cfa groups. Figure 4 shows the first quartile, median, and third quartile for the RC3 Vo(3K) recession velocities for Base Sample galaxies, broken down by Hubble type.


The four data distributions on the left of Figure 4 refer to Base Sample, Sbc, Sc and Scd-Sdm field galaxies, respectively, while the four data distributions on the right refer to Base Sample, Sbc, Sc, and Scd-Sdm galaxies found in HG/Cfa galaxy groups.

Figure 4 shows that the median recession velocity for Base Sample field galaxies (1615 km/sec) is about 250 km/sec higher than that for Base Sample galaxies belonging to HG/Cfa galaxy groups (1350 km/sec).


                                                                           FIGURE 4

                                FIELD GALAXIES                                                                         HG/CFA GALAXIES


Figure 5 shows how the corresponding recession velocity distribution for galaxies with Typed SN, SNII/(II), SNIa/I*/(I), SNIb/Ic, and No SN Type, compared to recession velocity distributions for Base Sample field galaxies and Base Sample HG/Cfa galaxies.


This figure shows that galaxies with SNII/(II) have a recession velocity distribution similar to that of Base Sample HG/Cfa galaxies, while galaxies with SN Ia/I*/(I) have a recession velocity distribution intermediate between that of the Base Sample field galaxies and Base Sample HG/Cfa galaxies.


                                                               FIGURE 5



The question is, does this systematic differences in median recession velocity explain strong preference for Base Sample galaxies with typed SN to be found in HG/Cfa galaxy groups?


One way to answer this question is to redo our earlier analysis where we calculated the fraction of all Base Sample galaxies with SN of known type that are located within HG/Cfa groups. This time, however, we reduce the maximum recession velocity cut-off (Vo(3K)) of the Base Sample (3,000 km/sec) to a lower value to reduce the likelihood that we are including distance galaxies whose SN may have been too faint to be detected by conventional SN searches.


In Figure 6 we have replotted the percentage fraction of Base Sample galaxies that are in HG/Cfa galaxy groups, sorted by SN Type. Figure 6 differs from Figure 1 in that we now show the percentage fraction for each SN type, for three different recession velocity cut-offs to the Base Sample - 3,000 km/sec, 1890 km/sec, and 1500 km/sec. The additional recession velocity cut-offs have been chosen to limit the Base Sample to 1/4 th (1890 km/sec) and 1/8 th(1500 km/sec) of the original sample’s space volume.


                                                     FIGURE 6



Figure 6 clearly shows, that by reducing the cut-off recession velocity for the Base Sample of galaxies, there is almost no change in our original finding. Even if we limit the cut-off or maximum recession velocity of the Base Sample to 1500 km/sec, we still find that a much higher proportion of galaxies with typed SN are in HG/Cfa galaxy groups (81.8 %) than galaxies that have had no historical SN (56.8 %).


Note: All recessions velocities used are Vo(3K) values from the RC3 catalogue (de Vaucouleurs et. al. 1991) i.e. they are correction for motion of the local standard of rest with respect to the 3 K microwave background, except for those galaxies that are listed as either members or possible members of the Virgo Cluster by Binggelli et. al 1985, and those that appear in the Nearby GalaxyCatalog (Tully 1988). All Virgo cluster members are assigned a cosmological recession velocity of 1350 km/sec (Aaronson et. al. 1986), while weighted recession velocities, corrected for solar motion with respect to the centroid of the Local Group, have been used for galaxies belonging to the Nearby Galaxy Catalog . If a galaxy from the Nearby Galaxy Catalog belongs to a designated galaxian group, we use the corrected mean recession for that group.


5. CONCLUSION


Our preliminary investigations indicate that the likelihood of finding a SN in nearby (Vo(3K) ) < 3,000 km/sec) late type (Sbc-Sdm) spiral galaxies is dependent on the parent galaxy’s intergalactic environment. Our study shows that that there are 567 Sbc-Sdm galaxies in the NGC catalogue with recession velocities (Vo(3K) < 3,000 km/sec. 491 of these galaxies are
within the survey limits of the HG/Cfa galaxy group survey. Out of these 491 galaxies, 96 (19.6 %) have had historical SN (1885A to 1998BP) with designated types: SN II/(II), SNIb/Ic and SNIa/I*/(I). Note: Seven galaxies have had multiple typed
SN, so there have been a total 103 galaxies with typed SN if you count these galaxies twice. Using this sample, we find that 78/271 (28.8 % ) of all Sbc-Sdm galaxies in HG/Cfa galaxy groups have had typed SN, while only 25/224 (11.2%) of these type of spirals that are NOT in HG/Cfa galaxy groups have had typed SN. At face value, this means that’s are 2.6 times more likely to occur in Sbc-Sdm galaxies that are in HG/Cfa groups than Sbc-Sdm galaxies that lie outside these groups.


We also find that the likelihood of seeing SNIa/I* in Sbc-Sdm galaxies in HG/Cfa groups is 4.3 times higher than the likelihood of seeing them in galaxies outside HG/Cfa groups, while the corresponding enhancement for SN II is only 2.1. However, this result must treated with caution because of the small number statistics for the SNIa/I*/(I) type.


We have investigated a number of observational bias, which might affect our result e.g. the propensity for SN to target nearby face-on spirals that have bright blue magnitudes, however, we find that these biases do not affect our result. We speculate that the increased likelihood of tidal interactions between galaxies caused by the higher galaxian density within HG/Cfa groups, is the most likely explanation for the preference for SN to occur in these galaxy groups.


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Third Reference Catalogue of Bright Galaxies (Spinger-Verlag, New York, 1991).
4. Geller, M., and Huchra J. 1983,Ap. J. Suppl., 52, 61.
5. Huchra, J., and Geller, M. 1982, Ap. J., 257, 423.
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7. Kirshner, R. 1988, private communications.
8. Tully, R.B. Nearby Galaxy Catalog (Cambridge University Press, Cambridge, 1988).
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