A possible challenge for Cold and Warm Dark Matter

Abstract

Measuring the density profile and mass concentration of dark-matter haloes is a key test of the standard cold dark matter paradigm. Such objects are dark and thus challenging to characterise, but they can be studied via gravitational lensing. Recently, a million-solar-mass object was discovered superposed on an extended and extremely thin gravitational arc. Here we report on extensive tests of various assumptions for the mass density profile and redshift of this object. We find models that best describe the data have two components: an unresolved point-mass of radius $\leq10$ pc centred on an extended mass distribution with an almost constant surface density out to a truncation radius of 139 pc. These properties do not resemble any known astronomical object. However, if the object is dark matter-dominated, its structure is incompatible with cold dark matter models, but may be compatible with a self-interacting dark matter halo where the central region has collapsed to form a black hole. This detection could thus carry substantial implications for our current understanding of dark matter.

Figures (28)

• Figure 1: The cylindrical mass profile ($M_{\rm cyl}$) for the six best-fitting models at $z=z_{\rm lens}$. The vertical lines represent the 20 and 90 pc radii, which is also where the different models most closely agree with each other. The horizontal lines are the corresponding values of $M_{\rm cyl}$ for the uniform disk and point mass (UD+PM) model. In the legend, models appear in order of decreasing Bayes factor. Note that the top two models, a uniform disk with a point mass and a Sérsic profile with a point mass (denoted UD+PM and SER+PM, respectively), are plotted as a single curve and uncertainty band since the differences between them are too small to be easily distinguished at this plotting scale. Also shown are the cylindrical mass profiles for a broken power-law with a point mass (bPL+PM), a Plummer sphere with a point mass (PLU+PM), a broken power-law (bPL) and a pseudo-Jaffe with a point mass (PJ+PM). The uncertainty bands represent the 1-$\sigma$ confidence interval around the mean.
• Figure 1: Comparison with known stellar systems. Panel (a): Mass and concentration for the GCs in the Milky Way Baumgardt_2023 and for the KG$_{\rm{td}}$ and KG models of detection ${\@fontswitch\mathcal{V}}$ (coloured points, mean value and 1-$\sigma$ uncertainty). Panel (b): relation between stellar mass and effective radius from observations of different types of objects: dSph galaxies at $z = 0$norris_2014, strongly lensed star-forming clumps at $z = 2$ to 6 mestric_2022, eUCDs in the Virgo cluster wang_2023, dEs, dS0s, nuclear star clusters, GCs, UCDs, cEs and YMCs at $z = 0$norris_2014. The yellow diamonds represent simulated star-forming stellar clusters at $z\sim 6$. The green, red and black dots are detection ${\@fontswitch\mathcal{V}}$ when modelled with a Sérsic profile of index $n\sim7$, a King profile and a Sérsic profile of index $n\rightarrow 0$ with a point-mass, respectively.
• Figure 1: Cylindrical mass profiles. Panel (a): cylindrical mass profiles for the uniform disk plus point mass and Sérsic models. Panel (b): cylindrical mass profiles for the Pseudo-Jaffe models. Panel (c): cylindrical mass profiles for the NFW models with a redshift consistent with that of the main lens. Panel (d): cylindrical mass profiles for the power-law models. Panel (e): cylindrical mass profiles for the King models. We do not plot the KG model because it is indistinguishable from the KG$_{\rm \infty}$ one at this plotting scale. Panel (f): cylindrical mass profiles for the profiles with the redshift as a free parameter. In all panels, the vertical lines represent the 20 and 90 pc radii, which are also where the different models agree most closely. The horizontal lines are the corresponding values of $M_{\rm cyl}$ for the UD+PM model. In the legends, models appear in order of decreasing Bayes factor. The uncertainty bands represent the 1-$\sigma$ confidence interval around the mean.
• Figure 2: Comparison with CDM predictions. The contours represent the number density of CDM subhaloes with $M_{\rm{sub}}>10^5 h^{-1} M_\odot$. These were obtained using the SASHIMI-C semi-analytical subhalo model hiroshima_2018ando_2019 assuming a host redshift and mass of $z_{\rm h} = 0.881$ and $M_{\rm h} = 10^{12}M_\odot$. The coloured crosses show the $V_{\rm max}$ and $r_{\rm max}$ mean values and 1-$\sigma$ uncertainties inferred for different models of the detected object: an NFW profile with free concentration (NFW, $\Delta\ln{\@fontswitch\mathcal{E}} =-44$), an NFW halo with concentration drawn from the mass-concentration relation in duffy_2008 (NFW$_{\rm CDM, field}$, $\Delta\ln{\@fontswitch\mathcal{E}} =-75$) and an NFW subhalo with concentration drawn from moline_2023 (NFW$_{\rm CDM}$, $\Delta\ln{\@fontswitch\mathcal{E}} =-147$).
• Figure 2: Posterior distributions for the parameters of the uniform disk plus point-mass model. The contours represent the 1- and 2-$\sigma$ confidence regions.