Using a semi-analytical model for galaxy formation , combined with a large N -body simulation , we investigate the origin of the dichotomy among early-type galaxies .
In qualitative agreement with previous studies and with numerical simulations , we find that boxy galaxies originate from mergers with a progenitor mass ratio n < 2 and with a combined cold gas mass fraction F _ { cold } < 0.1 .
Our model accurately reproduces the observed fraction of boxy systems as a function of luminosity and halo mass , for both central galaxies and satellites .
After correcting for the stellar mass dependence , the properties of the last major merger of early-type galaxies are independent of their halo mass .
This provides theoretical support for the conjecture of Pasquali et al .
( 2007 ) that the stellar mass ( or luminosity ) of an early-type galaxy is the main parameter that governs its isophotal shape .
If wet and dry mergers mainly produce disky and boxy early-types , respectively , the observed dichotomy of early-type galaxies has a natural explanation within the hierarchical framework of structure formation .
Contrary to naive expectations , the dichotomy is independent of AGN feedback .
Rather , we argue that it owes to the fact that more massive systems ( i ) have more massive progenitors , ( ii ) assemble later , and ( iii ) have a larger fraction of early-type progenitors .
Each of these three trends causes the cold gas mass fraction of the progenitors of more massive early-types to be lower , so that their last major merger involved less cold gas ( was more “ dry ” ) .
Finally , our model predicts that ( i ) less than 10 percent of all early-type galaxies form in major mergers that involve two early-type progenitors , ( ii ) more than 95 percent of all boxy early-type galaxies with M _ { * } \lower 3.01 pt \hbox { $ \buildrel < \over { \sim } $ } 2 \times 10 ^ { 10 } h ^ { -1 } \ > { % M _ { \odot } } are satellite galaxies , and ( iii ) about 70 percent of all low mass early-types do not form a supermassive black hole binary at their last major merger .
The latter may help to explain why low mass early-types have central cusps , while their massive counterparts have cores .