When I see this picture, I'm trying to understand how these two elements merged together and have two hypothesis :
But it appears that the collision speed was not particularly fast so I'm not sure in the end.
What process likely formed Ultima Thule?
Ultima Thule is a contact binary comprised of two formerly separate solid objects that gently1 (probably <5 km per hour) came together. If the objects had been molten when they came together they would have formed a spherical object. Since the lobes are distinct the objects must have been solid.
As for "understand[ing] how these two elements merged together", Ultima and Thule are believed to be have been two objects that formed separately through accretion. At some point the two (solid) objects collided at a very low velocity of "one or a few miles per hour"1. Since the escape velocity at point of impact is about 16 miles per hour2, gravity alone would be sufficient to keep the two lobes together1.
1 This topic was discussed in the 2019-01-02 New Horizons press conference. The presentation talking about formation mechanism starts here (the portion discussing approach speed is ~80 seconds later). A discussion of how strongly bound the lobes are starts here.
A note about the video links above: The official JHU APL video linked to above is missing the first few minutes. If the video is updated to include the complete press conference then the links above will point to the wrong times. The first two links are to Jeff Moore's presentation and the third is him fielding a question from Bill Horowitz (CBS News).
2 To quote the source:
Modeling it as two spheres, one 16 km across the other 12 km, with a density the same as water, ... [a]t a distance of 8 km from the center of mass, the escape velocity ... would be about ... 16 miles/hour.
It was a slow collision between two lumps of ices, but at these temperatures, ice is a kind of rock. Energy would be released in such a collision which resulted in the partial fusion of the two bodies, but not enough energy to significantly disrupt either one.
The resulting body does not have sufficient mass for its gravity to pull it into a spherical shape, and hence after the two bodies had joined, they remained in the "snowman" shape.
This graphic in Wired.com's Finally, New Horizons' First Photos of Ultima Thule suggest the contact process could have been extremely slow. Rather than two bodies on independent but similar heliocentric orbits approach slowly, touch, and stick, it proposes that they were gravitationally bound and very slowly approached each other as angular momentum was lost somehow.
NASA/JHUAPL/SWRI/JAMES TUTTLE KEANE
There are no fast processes for two bound bodies so far from the Sun to loose angular momentum quickly, so it would have had to be very slow.
In this step-by-step view we see how Ultima Thule probably formed when smaller chunks stuck together to form two objects. The twin-lobed structure of Ultima Thule vindicates accepted theory — called the accretion process — of how the solar system built itself up. NASA / JHUAPL / SwRI
The reason the two objects still appear so neatly intact is that they approached each other at about the same speed you back into a parking spot — 1 to 2 miles an hour — and slowly fused together. Take a look again at color photo and focus on the “neck” of the asteroid. The rest of Ultima Thule is reddish-brown, probably from methane or other organic compounds exposed to sunlight, but the neck is pale, almost white and indicates lots of fine-grained material. During today’s press conference, mission scientists suggested it could be material that rolled downhill from the steep slopes where the two bodies make contact. Much more detailed photos are on the way along with compositional measurements that will help answer that question in more detail.
Ultima Thule formed at the beginning of it all from small, icy chunks that coalesced into two planetesimals. The two spiraled into toward one another and joined to form what astronomers call a “contact binary.” If you want to know what things looked like shortly after the first solid materials coalesced from the solar nebula, Ultima Thule appears to be as close as we’ve come so far. Like taking a ride back in a time machine.
Here you can see the difference in the neck area vs. the rest of Ultima Thule. NASA / JHUAPL / SwRI
Let's calculate the gravitational attraction between a 10 km and a 20 km diameter snowball.
I'll assume the density is 1. The could have both voids (many "cosmic snowballs" do) which would lower it, but rocks as well which would raise it, but this is just an order of magnitude calculation.
The distance between centers is the sum of the radii, or 15,000 meters, and we can approximate them as spheres, then use Newton's shell theorem to treat each one as a point at their center.
$$ F=\frac{G m_1 m_2}{r^2}$$
$$m = \rho \frac{4 \pi}{3} r^3$$
With rho of 1 g/cm^3 or 1000 kg/m^3 I get m1, m2 = 5.2E+11, 4.2E+12 kilograms, the force pushing each body into the other (using G=6.7E-11 N m^2 kg^-2) is about 650,000 Newtons, or roughly 66 tons. That's 2/3 of the thrust of a Merlin-1D engine (~900 kN) on each asteroid pushing it towards the other 24/7!
After a while, the touching similar ices will start to fuse, adding some mechanical strength as well. That's over a long time, but things this far out have plenty of undisturbed time on their hands.
