Warm Dark Matter and Large Scale Structure of the Universe

Home , Hot dark matter, Warm dark matter

Philipp Henkenjohann & Christian Reichelt

Universit¨atHeidelberg

September 29, 2017

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 1 / 19 Motivation

ΛCDM cosmology is consistent with a plethora of observations. In particular, the observed Large Scale Structure (LSS) of galaxies, clusters etc. can be explained by the growth of primordial perturbations in the dark matter (DM) density. The compatibility of theory and observation is remarkable, except for discrepancies at relatively small scales . 1Mpc. These problems may be solved by assuming DM to be warm instead of cold.

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 2 / 19 Outline

1) Small scale problems

2) Solution via warm dark matter

3) Warm dark matter candidates

4) Observational constraints

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 3 / 19 Core-cusp problem

Simulations of structure formation with cold dark matter (CDM) predict a steep density profile of the form ρ ∝ r−α, α ∼ 1, in galaxy cores. This seems to be inconsistent with the observed flat density distribution in galaxy cores. Various astrophysical processes have been proposed and improved simulations used to explain this difference but the situation remains inconclusive.

[Primack; 2009; arXiv:0909.2247]

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 4 / 19 Satellite abundance problem

∼ 1000 DM subhalos have been predicted for the Local Group while only ∼ 10 satellite galaxies have been detected. One explanation would be that some of the galaxies in the subhalos are too faint to be observable. The reason for this might be suppression of star formation.

[Primack; 2009; arXiv:0909.2247]

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 5 / 19 Galaxy abundance in mini-voids

Similarly to the satellite problem, the abundance of dwarf galaxies in mini-voids predicted by simulations is larger than observed. Most of the dwarf galaxies are located near the larger and brighter galaxies (see picture on next slide). This seems to be incompatible with hierarchical structure formation, i.e. small bound systems formed ﬁrst, as predicted by CDM.

[Primack; 2009; arXiv:0909.2247]

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 6 / 19 No. 1, 1997 GAS-RICH DWARFSGalaxy FROM abundance PSS II. in I. mini-voids 243

As inFigure 4, the dwarf samples are composed of the 12h 11h h h lowest width objects as compared to the total H I sample of 13 10 galaxies in the UGC. The PSS II dwarf sample has a slightly 14h 9h lower distribution of proÐle widths compared to the Schneideret al. (1990) dwarf sample, but this may be the 15h 8h result of a selection e†ect to detect face-on LSB objects in a visual survey that, in turn, emphasizes the nonrotational 16h 7h component to the velocity proÐle. Only six of the detected 10000 PSS II dwarfs displayed a double-horned H I proÐle indica- 8000 tive of a disk system. All six also displayed disk and bulge 6000 4000 morphology and spiral features upon inspection of deep ZCAT + PSS-II dwarfs CCD images. These six objects were latter classiÐed as a 2000 cz (km/s) new type of galaxy, dwarf spirals, and described in a pre- September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 7 / 19 vious paper(Schombert et al. 1995). [Schombert, Pildis, Eder; 1997; arXiv:astro-ph/9612130]

12h 11h 5. RESULTS 13h 10h

h h A sample of isolated dwarf galaxies has been selected 14 9 from PSS II plates. The candidates are selected by morpho- h h logical criteria and then conÐrmed with H I detection. We 15 8 show that a sample selected in this fashion yields an excel- lent set of isolated dwarf galaxies, (i.e., not companions to 16h 7h bright galaxies), and our plots and discussion in° 4 demon- 10000 8000 strate that morphologically selected candidates yield a 6000 sample dwarfs, as deÐned by optical or H I properties. 4000 UGC HI + PSS-II dwarfs The redshift distribution of the catalog lies between 500 2000 cz (km/s) and 10,000 km s~1, making it a more useful map of large- scale structure than previous dwarf catalogs. Our primary FIG. 6.ÈRedshift cone plot of our dwarf sample as compared to the distribution of galaxies from the CfA Redshift Survey (ZCAT, see Marzke result is shown inFigure 6, the redshift cone plot of our et al.1994) and the UGC H I sample. The ZCAT and UGC samples are dwarf sample as compared to the distribution of galaxies selected from the declination zones ]5¡ to ]25¡. The ZCAT sample was from the CfA Redshift Survey (ZCAT, seeMarzke, Huchra, selected for bright galaxies (likely to be higher mass objects). The UGC H I & Geller1994) and the UGC H I sample. The ZCAT and sample was selected to test for selection e†ects resulting from an H I UGC samples are selected from the declination zones 5¡ selected sample. Neither distribution is visually any di†erent from the ] redshift distribution of our dwarf galaxies, in conÑict with the predictions to ]25¡. of biased galaxy formation theory. The ZCAT sample was selected for bright galaxies (likely to be higher mass objects, a test of biased galaxy formation). The UGC H I sample was selected to compare our H I dwarf sampleÏs redshift distribution with another H I selected sample. Neither the ZCAT nor the UGC H I distribution is visually any di†erent from the redshift distribution of plates obtained at the Palomar Observatory 48 inch Oschin our dwarf galaxies, in conÑict with the predictions of biased Telescope for the Second Palomar Observatory Sky Survey, galaxy formation theory. A full analysis of the data will be which was funded by the Eastman Kodak Company, the presented in the latter papers of our series. National Geographic Society, the Samuel Oschin Founda- tion, the Alfred Sloan Foundation, the National Science We wish to thank the University of Michigan and the Foundation grants AST 84-08225 and AST 87-18465, and Michigan-Dartmouth-MIT Observatory for its generous the National Aeronautics and Space Administration grants support in carrying out the photometry portion of this NGL 05002140 and NAGW 1710. R. A. P. was partially program and Arecibo Observatory for the allocation of supported by a National Science Foundation Graduate time to search for H I emission from the candidate dwarf Fellowship. This research has made use of the NASA/IPAC galaxies. The research described herein was carried out by Extragalactic Database (NED), which is operated by the Jet the Jet Propulsion Laboratory, California Institute of Tech- Propulsion Laboratory, California Institute of Technology, nology, under a contract with the National Aeronautics and under contract with the National Aeronautics and Space Space Administration. This work is based on photographic Administration.

REFERENCES

Binggeli, B. 1993, Panchromatic View of Galaxies, ed. G. Hensler, C. Theis, Driver,S., Couch, W., Phillipps, S., & Windhorst, R. 1996, preprint & J. Gallagher (Kiel: Astron. Ges.) Eder,J., et al. 1997, in preparation Binggeli,B., Sandage, A., & Tammann, G. 1985, AJ, 90, 1681 Giovanelli, R., & Haynes, M. 1988, Galactic and Extragalactic Radio Caldwell,N., & Bothun, G. 1987, AJ, 94, 1126 Astronomy (New York: Springer), 522 Davis,M., & Djorgovski, S. 1985, 1985, ApJ, 299, 15 Giovanelli,R., Haynes, M., & Chincarini, G. 1986, ApJ, 300, 77 deJong, R. 1996, A&A, submitted Huchtmeier, W., & Richter, O. 1989, General Catalog of H I Observations Dekel,A., & Rees, M. 1987, Nature, 326, 455 of Galaxies (Berlin: Springer) Summary of small scale problems

All the above problems arise on scales . 1Mpc. It is therefore tempting to think of a common origin for these observation instead of a variety of astrophysical eﬀects. The idea: Find a mechanism that suppresses structure formation at small scales.

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 8 / 19 Brief review of structure formation

We start with an initial density distribution with small deviations from the average value in the radiation dominated era. (These initial conditions may be provided by primordial quantum fluctuations which get amplified during inflation.) During radiation domination perturbations on sub-horizon scales are essentially frozen. Only after matter-radiation-equality (EQ) can these perturbations grow effectively, first linearly and then non-linearly, to build stars, galaxies etc.

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 9 / 19 Free streaming

Consider only perturbations in the DM energy density. Since DM is weakly interacting it decouples from radiation already during radiation domination, i.e. before EQ. From this moment on the DM particles move freely on geodesics in the expanding spacetime. This free streaming suppresses perturbations on scales smaller than the free streaming length λFS as long as these do not grow, i.e. before EQ. This is exactly what we are looking for.

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 10 / 19 Free streaming length

The comoving free streaming length is

Z tEQ v(t) λFS = dt, 0 a(t) where v is the physical velocity of the particle.

We can split the integral in a part where the particle is relativistic and another where it is non-relativistic:

Z tNR 1 Z tEQ v(t) λFS ≈ dt + dt 0 a(t) tNR a(t)

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 11 / 19 Free streaming length

√ During radiation domination we have a(t) ∝ t and the velocity is redshifted according to v(t) ∝ a(t)−1.

p This implies a(t) = aNR t/tNR and hence

Z tEQ tNR aNR λFS ≈ 2 + 2 dt aNR tNR a(t) t t = NR 2 + ln EQ . aNR tNR

By increasing tNR we can make λFS larger!

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 12 / 19 Cold, warm, and hot dark matter

CDM corresponds to the particles being non-relativistic already at the time of decoupling, tNR = tdec. This DM seems to have the problems discussed in the beginning.

The other extreme, hot DM, has tNR = tEQ and leads to structure formation incompatible with observation (top-down).

Warm DM means to have tNR somewhere in between. This can be achieved by increasing the mass of DM compared to CDM while keeping interaction strengths constant. (Changing both parameters is of course also possible.)

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 13 / 19 Simulations of warm dark matter

Simulations of warm dark matter with masses 175eV, 350eV and 1.5keV have, among others, the following characteristics when compared to CDM: Halo core densities are lowered and smoothed. Overall number of low mass halos is reduced. Number of low mass satellite halos in high mass halos is suppressed. Voids are almost empty of small halos. Low mass halos are formed late in a top down process.

[Bode, Ostriker, Turok; 2001; arXiv:astro-ph/0010389]

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 14 / 19 Simulations of warm dark matter

[Credit: Ben Moore, University of Z¨urich]

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 15 / 19 Warm dark matter candidates

Sterile neutrinos: Only gravitational interaction - not charged under SM gauge groups (sterile) Usually right-handed chirality Yukawa interactions gives mixing with ordinary neutrinos (possible detection) For GUTs (e.g. SO(10)) they could in principle also have gauge interactions, but these gauge bosons are very heavy, i.e. interaction is heavily suppressed. Mass could be generated from a see-saw mechanism.

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 16 / 19 Warm dark matter candidates

Gravitino Spin 3/2 superpartner of graviton Must be stable - i.e. in models it should be the Lighest supersymmetric particle (LSP) (keV)

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 17 / 19 Current constraints

Measurements from the Lyman-α forest (Absorption spectra from quasars caused by hydrogen gas) gives 2σ lower limits:

mWDM > 4 keV

mνs > 28 keV

[Viel, Becker, Bolton, Haehnelt, Rauch, Sargent; 2007; arXiv:astro-ph/0709.0131]

The Euclid satellite can improve this by gravitational lensing because it not only relies on the visible baryons. From the weak lensing power spectrum a new constraint will be around:

mWDM > 2 keV

[Markovic, Bridle, Slosar, Weller; 2010; arXiv:astro-ph/1009.0218]

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 18 / 19 Thank you!

September 29, 2017Warm Dark Matter and Large Scale Structure of thePhilipp Universe Henkenjohann & Christian Reichelt 19 / 19