What is the shape of the dark matter halo?

Ph. D. Anna Dur­ka­lec from the NCBJ Astro­phy­sics Divi­sion belongs to the group of 12 asso­ciate pro­fes­sors awar­ded in the Insti­tute in 2020. Dr. Dur­ka­lec begins the descrip­tion of her rese­arch by asking a few questions:

What is the shape of the dark mat­ter halo?

At the moment we are not sure­…but of course, we would like to know.

But let’s start from the begin­ning and ask more gene­ral questions.

What is the dark mat­ter halo? and more impor­tan­tly – what is dark mat­ter?

The answer to this last question is not easy. We sim­ply don’t know the nature of dark mat­ter. What we do know is that there is a lot of it in the Uni­verse and it domi­na­tes over the visi­ble mat­ter, i. e. the one from which the gala­xies, stars, pla­nets, and humans are made (there is 5–6 times more dark mat­ter than visi­ble mat­ter!). Ano­ther pro­blem is that we are not able to direc­tly obse­rve dark mat­ter because it doesn’t emit or reflect any kind of elec­tro­ma­gne­tic radia­tion (light). We can only obse­rve the effects of its inte­rac­tion with visi­ble mat­ter because these two com­po­nents of the uni­verse are lin­ked by gra­vity. For exam­ple, dark mat­ter affects the rota­tio­nal velo­city of dust and stars in gala­xies and the velo­city of gala­xies in clu­sters. Some­ti­mes dark mat­ter acts like a giant lens – its enor­mous mass cau­ses the light of gala­xies to bend (phe­no­mena known as gra­vi­ta­tio­nal len­sing). More­over, without dark mat­ter, it would be extre­mely dif­fi­cult to explain the chan­ges that have occur­red in the lar­ge­–scale struc­ture of the uni­verse from its incep­tion to the pre­sent.

We know that dark mat­ter plays a huge role in the uni­verse. Without it, there would be no gala­xies, as they form and evo­lve in the local over­den­si­ties of dark mat­ter. This means that every galaxy and every clu­ster of gala­xies is embed­ded in a sur­ro­un­ding „halo” of dark mat­ter. In the­ory, gala­xies can the­re­fore be used to describe the distri­bu­tion of dark mat­ter in the uni­verse. To do that, one must cre­ate a phy­si­cal model, that com­bi­nes the mass, shape, and den­sity distri­bu­tion of dark mat­ter with the distri­bu­tion of gala­xies of dif­fe­rent mass, bri­ght­ness, color, or shape. Such models are cal­led HOD – Halo Occu­pa­tion Distri­bu­tion models.

In my work, I focu­sed on the study of the rela­tion­ship between dark mat­ter and gala­xies as seen in the uni­verse 10 bil­lion years ago. Using the data from VIMOS Ultra Deep Survey (VUDS) I found that shor­tly after the for­ma­tion of gala­xies, there Was a clear rela­tion­ship between the mass and the bri­ght­ness of gala­xies and the mass of dark mat­ter halo in which they were loca­ted – the more mas­sive and bri­gh­ter galaxy, the more mas­sive the halo occu­pied by it.

Howe­ver, to cor­rec­tly esti­mate the mass of the halo, it is neces­sary to deter­mine its shape. For many years it was assu­med, that the dark mat­ter halos were sphe­ri­cally sym­me­tri­cal. This assump­tion is, of course, a sim­pli­fi­ca­tion – suf­fi­cient or even neces­sary for stu­dies con­duc­ted based on obse­rva­tions of a rela­ti­vely small num­ber of gala­xies.

Howe­ver, in the era of cur­ren­tly plan­ned huge galaxy surveys, cove­ring seve­ral or even tens of mil­lions of gala­xies, this assump­tion may prove to be ina­de­qu­ate.

In our latest work, we pro­pose a solu­tion to this pro­blem. It turns out, that most dark mat­ter halos, espe­cially the most mas­sive ones, have an elon­ga­ted shape (simi­lar to an egg). This is mainly due to the pres­sure of dark mat­ter mass and/or gas flo­wing into the halo from a cer­tain direc­tion. The assump­tion of a sphe­ri­cal – sym­me­tri­cal halo shape is the­re­fore not only an oversim­pli­fi­ca­tion but also influ­en­ces the measu­re­ments of dark mat­ter mass in the halo signi­fi­can­tly ove­re­sti­ma­ting them. So we pro­posed a new model that takes into acco­unt possi­ble halo asym­me­tries. This model can be suc­cess­fully applied in new works based on plan­ned large sky surveys.

These fun­da­men­tal disco­ve­ries bring us clo­ser to solving key pro­blems: fir­stly, they allow us to more clo­sely exa­mine the chan­ges that the uni­verse has under­gone from its incep­tion to the pre­sent, and secon­dly, they bring us clo­ser to answe­ring questions about the nature of dark mat­ter.

[1] https://www.aanda.org/articles/aa/pdf/2018/04/aa30734-17.pdf