The M96 group of galaxies

This conspicuous group of galaxies contains 3 Messier objects (M95, M96, and M105), plus a larger number of fainter galaxies (including NGCs 3299, 3377, 3384, 3412 and 3489 as well as UGC 5889, which is also called NGC 3377A in the Sky Catalogue 2000.0). The Sc galaxy NGC 3389, which is in the same field and close to M105, is probably a background galaxy, as its redshift corresponds to a receding velocity of 1138 km/sec, whereas the members of the Leo I group have radial velocities of only 449..766 km/sec.

In our image on the right hand side, M95 is in the lower right corner, while M96 is in the lower middle, and M105 is the bright elliptical galaxy above and left of the picture's center, together with its fainter neighbors NGC 3384 and faint NGC 3889.

Nial R. Tanvir of the Cambridge Astronomy group has investigated this group of galaxies with the Hubble Space Telescope (Tanvir et.al. 1995, 1999). He and his coworkers found 8 Delta Cephei variable stars in the brightest galaxy of the group, M96, and were able to derive its distance from the Cepheid period-luminosity relation: M96 is 231+/-13 times more remote from us than the Large Magellanic Cloud, thus 12.7+/-0.8 Mpc (41+/-2 million light years).

An improvement of this value is hoped to be obtained from ongoing HST observations of Cepheid variables in the group galaxies M95 and UGC 5889 (both spirals). A first result was obtained for M95 in 1996-97 by the HST H0 Key Project Team (Graham et.al. 1997). They obtained a distance of 10.05+-0.88 Mpc (32.8+-2.9 million light years), which after correction for the new Cepheid distance scale becomes 10.90+-0.95 Mpc (35.5+-3.1 million light years). As a mean value of the two distances obtained in the work of Tanvir and the H0 key project team, we will use a distance of 38 million light years here.

From the distance derived here, it is possible to gauge a diameter-velocity dispersion-relation for elliptical galaxies, namely by using the galaxies M105 and NGC 3377. By the relation obtained for this, the distance of the Virgo and Coma clusters of galaxies could be estimated; one obtains a distance of 18.3+/-2.0 Mpc (60+/-6 million light years) for the Virgo cluster, and of 105+/-11 Mpc (~340 million light years) for the Coma cluster of galaxies. (The Virgo cluster distance may be estimated from that of the M96 group also by other methods, yielding about the same value).

The Coma distance then allows to estimate the value of the current Hubble constant (H0, "Hnaught") to be

H0 = 69+/-8 km/(s*Mpc) which is again (after the M100 result) intermediate between the rivaling values of H0=50 and H0=100 (the values given here are corrected for the new Hipparcos results of February, 1997).

The M96 group may be physically related to the M66 group which is roughly at the same distance and situated rather nearby in the sky.

The following table lists some data of the M96 or Leo I group members listed above (most data from the Sky Catalogue 2000.0):

Galaxy    Name II   RA (2000.0) Dec type   m_v       dim     RV

NGC 3299  H III.54  10:36.4 +12:42  SABdm  13.29p  2.1x1.7  465
M95       NGC 3351  10:44.0 +11:42  SBb     9.71   7.4x5.1  649
M96       NGC 3368  10:46.8 +11:49  Sbp     9.24   7.1x5.1  766
UGC 5889  NGC 3377A 10:47.4 +14:04  SABm   14.15p  2.0x1.9  449
NGC 3377  H II.99   10:47.7 +13:59  E5     10.21   4.4x2.7  595
M105      NGC 3379  10:47.8 +12:35  E1      9.26   4.5x4.0  752
NGC 3384  H I.18    10:48.3 +12:38  E7      9.96   5.9x2.6  641
NGC 3412  H I.27    10:50.9 +13:25  E5     10.55   3.6x2.0  737
NGC 3489  H II.101  11:00.3 +13:54  E6     10.33   3.7x2.1  576

Key:
Name: M/NGC/UGC number, RA/Dec: Right Ascension (h:m) and Declination (deg:m) for 2000.0, type: Hubble type, m_v: Visual magnitude, dim: Angular diameter in arc minutes, RV: Radial velocity wrt Galactic center in km/sec (from R. Brent Tully's Nearby Galaxy's Catalog).

References

John A. Graham, Randy L. Phelps, Wendy L. Freedman, , 1997. The Extragalactic Distance Scale Key Project VII. The Discovery of Cepheids in the Leo I Group Galaxy NGC 3351. Astrophysical Journal, Vol. 477, No. 2, pp. 535-559 (March 1997) [ADS: 1997ApJ...477..535G]. Results available online.
N.R. Tanvir, T. Shanks, H.C. Ferguson, and D.R.T. Robinson, 1995. Determination of the Hubble Constant from Observations of Cepheid Variables in the Galaxy M96. Nature, Vol. 377, No. 6544, p. 27 (September 1995) [ADS: 1995Natur.377...27T] - [Preprint: astro-ph/9509160]
N.R. Tanvir, H.C. Ferguson, and T. Shanks, 1999. The Cepheid distance to M96 and the Hubble constant. Monthly Notices of the Royal Astronomical Society, Vol. 310, No. 1, pp. 175-188 (November 1999) [ADS: 1999MNRAS.310..175T]

Hartmut Frommert
Christine Kronberg
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Last Modification: September 5, 2005