We compare optical and high quality x-ray data for three bright ellipticals in the Virgo cluster , NGC 4472 , 4649 , and 4636 .
The distribution of total mass in NGC 4472 and 4649 determined from x-ray data is sensitive to the stellar mass over a considerable range in galactic radius extending to r \approx r _ { e } , the effective radius .
The agreement of x-ray and optically-determined stellar masses provides a unique verification of the stellar mass to light ratio which is essentially constant over the range { { { { { { { { 0.1 \mathrel { \mathchoice { \lower 2.5 pt \vbox { \halign { \cr } $ \displaystyle \hfil < $ \cr% $ \displaystyle \hfil \sim$ } } } { \lower 2.5 pt \vbox { \halign { \cr } $ \textstyle \hfil < $% \cr$ \textstyle \hfil \sim$ } } } { \lower 2.5 pt \vbox { \halign { \cr } $ \scriptstyle \hfil < $% \cr$ \scriptstyle \hfil \sim$ } } } { \lower 2.5 pt \vbox { \halign { \cr } $% \scriptscriptstyle \hfil < $ \cr$ \scriptscriptstyle \hfil \sim$ } } } } r / r _ { e } \mathrel { % \mathchoice { \lower 2.5 pt \vbox { \halign { \cr } $ \displaystyle \hfil < $ \cr$% \displaystyle \hfil \sim$ } } } { \lower 2.5 pt \vbox { \halign { \cr } $ \textstyle \hfil < $ \cr% $ \textstyle \hfil \sim$ } } } { \lower 2.5 pt \vbox { \halign { \cr } $ \scriptstyle \hfil < $ \cr% $ \scriptstyle \hfil \sim$ } } } { \lower 2.5 pt \vbox { \halign { \cr } $ \scriptscriptstyle% \hfil < $ \cr$ \scriptscriptstyle \hfil \sim$ } } } } 1 .
However , for NGC 4636 the dark matter is important at all radii { { { { \mathrel { \mathchoice { \lower 3.0 pt \vbox { \halign { \cr } $ \displaystyle \hfil > $ \cr$% \displaystyle \hfil \sim$ } } } { \lower 3.0 pt \vbox { \halign { \cr } $ \textstyle \hfil > $ \cr% $ \textstyle \hfil \sim$ } } } { \lower 3.0 pt \vbox { \halign { \cr } $ \scriptstyle \hfil > $ \cr% $ \scriptstyle \hfil \sim$ } } } { \lower 3.0 pt \vbox { \halign { \cr } $ \scriptscriptstyle% \hfil > $ \cr$ \scriptscriptstyle \hfil \sim$ } } } } 0.35 r _ { e } .
Evidently the dark to stellar mass ratio varies in quite different ways in ellipticals of comparable optical luminosity , implying that the radial structure of dark halos may not be universal .
There is some evidence in NGC 4636 for additional support of the hot interstellar gas at { { { { r \mathrel { \mathchoice { \lower 2.5 pt \vbox { \halign { \cr } $ \displaystyle \hfil < $ \cr$% \displaystyle \hfil \sim$ } } } { \lower 2.5 pt \vbox { \halign { \cr } $ \textstyle \hfil < $ \cr% $ \textstyle \hfil \sim$ } } } { \lower 2.5 pt \vbox { \halign { \cr } $ \scriptstyle \hfil < $ \cr% $ \scriptstyle \hfil \sim$ } } } { \lower 2.5 pt \vbox { \halign { \cr } $ \scriptscriptstyle% \hfil < $ \cr$ \scriptscriptstyle \hfil \sim$ } } } } 0.35 r _ { e } ; either a field B \sim 10 ^ { -4 } G or a small ( mechanically unstable ) central region of high gas temperature ( T \sim 10 ^ { 7 } K ) is required .
The global temperature structure in the hot interstellar medium of many recently observed ellipticals is very similar , reaching a maximum near 3 - 4 r _ { e } .
This feature , which may suggest a new structural scale in these galaxies , is inconsistent with current theoretical gas dynamical models .