What Makes Dark Matter Halos Dark

We investigate the factors that determine whether dark matter halos form galaxies using the Marvel-ous Dwarfs and DC Justice League zoom-in simulations. By analyzing halo occupation fractions, mass accretion histories, halo concentrations, and gas properties, we find that halo mass alone is insufficient to explain why some halos remain dark. Luminous halos grow faster, achieve higher concentrations, and retain dense, cool gas, enabling star formation. Dark halos grow more slowly, remain less concentrated, and have lower gas densities and higher gas temperatures, preventing efficient cooling after reionization and leaving them dark. These results highlight the interplay between dark matter dynamics and baryonic physics in shaping galaxy formation, demonstrating that mass growth, halo concentration, and gas content collectively determine whether a halo hosts a galaxy.​

17:37:02 Hello, everyone. Today, I’ll be talking to you about what makes some dark matter halos dark.
17:37:07 So, first of all, what is dark matter? Dark matter is like any other matter, but…
17:37:13 It’s invisible, it cannot interact with normal matter that we see and touch,
17:37:18 Like, it can’t bump into it, or you can’t feel it.
17:37:23 Um,
17:37:25 And it only interacts through gravity.
17:37:28 So, it can only pull things
17:37:30 using gravity.
17:37:32 And…
17:37:34 Dark Matter Halos is just a clump of dark matter that has collected together.
17:37:40 And from now on, I’ll just call it Halos.
17:37:46 And these halos is…
17:37:48 is the environment where galaxies are made.
17:37:52 But… sometimes galaxies aren’t made.
17:37:56 In those halos, and those are called dark halos. Dark Halos. And for the ones…
17:38:02 that galaxies do form in, those are called luminous halos.
17:38:06 So, the goal of my research is to figure out
17:38:09 what leads to these halos?
17:38:12 not forming any galaxies? What properties, what events?
17:38:17 created? Like, why…
17:38:19 Do these halos have galaxies, and these don’t?
17:38:23 And…
17:38:25 my method to look at… to do that is using, um…
17:38:30 large cosmological simulations.
17:38:34 simulations, uh, that are called
17:38:36 marvelous dwarfs and DCGL.
17:38:39 DCJL standing for DC Justice League.
17:38:46 These were made by…
17:38:49 Dr. Munshi and her colleagues.
17:38:52 Um…
17:38:54 using the simulation, it provided various datas about dark matter halos, their properties,
17:39:01 And I was able to utilize this data to track certain, um,
17:39:07 patterns of dark matter halos.
17:39:10 So, for my first plot,
17:39:12 I’ve calculated the halo occupation fraction versus the maximum
17:39:19 mass, a halo has ever had.
17:39:21 So the Halo occupation fraction is basically saying, okay, for this amount of halos,
17:39:26 Um, how many of them are luminous divided by the total
17:39:30 amount of, um, halos.
17:39:33 And from what you see is…
17:39:34 From the right end… from the right end, from the large end of the mass.
17:39:41 It’s 100%, one, at the top, and then…
17:39:46 Uh, some of… they… they begin to decrease.
17:39:50 And the decrease is gradual, but…
17:39:52 you’ll see that all these simulations have different…
17:39:56 point of mass where that halo occupation decreases.
17:40:00 Which tells us that because their masses
17:40:04 vary for that critical point. That means that halo mass, or halo peak mass, isn’t enough to tell us.
17:40:10 About why some dark… some…
17:40:13 halos don’t form galaxies.
17:40:17 So then, uh, I look at the mass assembly history of the halos, how fast they gain mass.
17:40:24 And what I find that luminous halos
17:40:27 were… or the one that made Galaxies gain mass really fast. Faster than dark halos.
17:40:33 And, of course, the Dark Hills.
17:40:35 didn’t gain mass as fast.
17:40:37 Which could explain why
17:40:40 Dark Halos are… don’t make any galaxies, because they couldn’t accumulate…