The case of NGC 5824, a cluster possibly embedded in a dark matter halo

Abstract

The globular cluster NGC 5824 exhibits a diffuse stellar envelope that extends beyond its nominal King tidal radius and symmetrically surrounds the cluster. The origin of these stars and whether they remain gravitationally bound to the cluster center is unclear. A possible explanation is that such clusters are embedded within dark matter halos, which influences their kinematic and photometric properties. Specifically, their outer volume density profile would be characterized by a power law with an index $γ> -3$. In this study we assessed this photometric prediction by comparing it to the cluster's observed profile through an analysis of deep $g$-band photometry from MegaCam and DECam, combined with Gaia DR3 proper motions and photometry. We determined star membership using color-magnitude diagrams and proper motion constraints in order to fit King and power-law profiles to the observed profile. Additionally, we analyzed NGC 5824's luminosity function to assess its spatial symmetry and extent. Our results show that NGC 5824 is symmetrically extended to at least $\sim20'$ with an outer surface density profile characterized by a power-law index of $γ\sim - 2.6\pm0.1$, which is consistent with the predicted values for a cluster embedded within a dark matter halo. Spectroscopic observations carried out to study the velocity dispersion profile will provide a more definitive answer regarding the dark matter content of NGC 5824.

Figures (14)

• Figure 1: sharp as a function of radius for NGC 5824 (top panel) and NGC 2419 (bottom panel). Gray dots represent all the detections within the MegaCam field. Blue dots indicate upward and downward exponentials applied as the sharp filter. Black dots denote all the detections that survived the sharp filtering. Purple dots highlight the stars selected for Test 1, which include MS, red giant, and BHB stars.
• Figure 2: Photometric errors as a function of MegaCam magnitudes for NGC 5824 (top panels) and NGC 2419 (bottom panels). Left panels show the $g$ band and right panels the $r$ band. The dashed vertical red line indicates magnitude $24$ for NGC 5824 and $25$ for NGC 2419 in both bands.
• Figure 3: Left: CMD of NGC 5824 with Clay photometry. Black dots correspond to the full sharp-cleaned data. Light blue dots are BHB candidates. Red dots are RGB candidates. Blue and yellow dots correspond to the isochrone. Right: NGC 5824 proper motion (PM) diagrams. Gray dots correspond to the full data with PM measurements. Light blue dots (upper panel) are BHB candidates. Red dots (lower panel) are RGB candidates.
• Figure 4: NGC 5824 results for Test 1.Top left: CMD and a star-count map of the cluster. Green, red, and blue dots correspond to the MS, RGB, and BHB star candidates, respectively, and the gray circle represents the best-fit $r_t$ parameter for this test. Note that MS candidates are omitted from the star-count map because they occupy the entire MegaCam field. Top right: Observed surface number density profile, constructed using MS, RGB, and BHB stars, along with data from Trager_1995 for the cluster's innermost region. Bottom left and bottom right: Corner plots obtained for the King profile and power-law fits, respectively. The dashed black lines correspond to percentiles $0.16$, $0.5$, and $0.83$, in that order, from left to right. $R_c$ and $R_t$ are expressed in arcminutes. $\Sigma_0$ is expressed in $\rm{arcmin}^{-2}$. $\gamma$ is dimensionless, and $\sigma$ is a normalization factor of the power-law model.
• Figure 5: CMDs of NGC 5824 based on MegaCam photometry (left) and DECam photometry (right). In the MegaCam CMD, black dots correspond to the full sharp-cleaned data, while in the DECam CMD, black dots correspond to stars within $1'\leq R \leq 35'$, where R is the angular distance from the cluster center. The yellow shaded regions indicate the color and magnitude selection used to construct the cumulative LFs.