DELVE-ing into the Milky Way's Globular Clusters: Assessing extra-tidal features in NGC 5897, NGC 7492, and testing detectability with deeper photometry

TL;DR

The study presents a DELVE DR2-based search for extra-tidal features around Milky Way globular clusters, uncovering a new envelope around NGC 5897 and tentative evidence around NGC 7492. By combining gala-based dynamical modeling with SPISEA synthetic populations and LSST foreground modeling, the authors interpret the envelopes as signatures of tidal debris and assess their detectability with deeper Rubin/LSST data. Their results imply that faint tidal tails may underlie observed envelopes, but foreground contamination substantially affects detectability, underscoring the need for additional membership discrimination such as proper motions or metallicities. Overall, the work expands the sample of GCs with extra-tidal features and provides a framework for interpreting such structures in current and upcoming surveys, with significant implications for tracing Galactic potentials and GC disruption histories.

Abstract

Extra-tidal features around globular clusters (GCs) are tracers of their disruption, stellar stream formation, and their host's gravitational potential. However, these features remain challenging to detect due to their low surface brightness. We conduct a systematic search for such features around 19 GCs in the DECam Local Volume Exploration (DELVE) survey Data Release 2, discovering a new extra-tidal envelope around NGC 5897 and find tentative evidence for an extended envelope surrounding NGC 7492. Through a combination of dynamical modeling and analyzing synthetic stellar populations, we demonstrate these envelopes may have formed through tidal disruption. We use these models to explore the detectability of these features in the upcoming Legacy Survey of Space and Time (LSST), finding that while LSST's deeper photometry will enhance detection significance, additional methods for foreground removal like proper motions or metallicities may be important for robust stream detection. Our results both add to the sample of globular clusters with extra-tidal features and provide insights on interpreting similar features in current and upcoming data.

Figures (12)

• Figure 1: The density of sky coverage, plotted in equatorial coordinates (RA and Dec), with the DECam camera and the location of 170 globular clusters (GCs) from vb+21 and the overlap with the DELVE footprint. The Dark Energy Survey (DES) covers the irregular polygon of higher star density in the lower center of the figure. The rest of the coverage is from the DELVE survey. GCs are denoted with a red cross, and those which had enough high quality data that we could search for extra-tidal envelopes have a blue square around the cross. NGC 5897 (the GC with a newly detected envelope) and NGC 7492 (which has conflicting reports in the literature and for which we detect an envelope), are shown as cyan stars with coordinates $(\alpha = 229.35\degree, \delta = -21.01\degree)$ and $(\alpha = 347.11\degree, \delta = -15.61\degree)$, respectively.
• Figure 2: Top left: A color-magnitude diagram of the 1000 closest stars to the center of Pal 5, after excluding those with bad photometry (Source Extractor parameter FLAGS $>$ 3; see Section \ref{['sec:delve']}). A cubic spline interpolation along the CMD of the cluster is shown as a dashed black line, and sources with photometry consistent with being within 0.05 mag of this fitted function are in blue. The rough depth of usable Gaia DR3 proper motions ($g\sim20.5$) is indicated with a dashed grey line. The MIST isochrone with the closest age and metallicity to the values in the h+96 catalog is shown as a light red line. Top middle: A density map of all sources within a 4$^\circ$ box around Pal 5, scaled by the standard deviation of the source density. Contours correspond to 2, 3, 5, 7, 10, 50, 100 sigma overdensities. The central King limiting radius $r_t$ of the GC is masked and circled in red, and the orange circle corresponds to the Jacobi radius calculated in this work (Section \ref{['sec:obs_analysis']}). The red arrow indicates the direction of the systemic proper motion of Pal 5. Top right: A map of the effective exposure time ($t_{\text{eff}} \times T_{exp}$) in DELVE DR2 for the $g$ band photometry over the same region as the middle panels. Middle left: A background model for the top middle panel, derived by fitting a second-order two-dimensional polynomial between $>$4 $r_t$ and the entire 4$^\circ \times 4^\circ$ field-of-view. Middle: Background-subtracted source density plot. Middle right: A map of the effective exposure time ($t_{\text{eff}} \times T_{exp}$) in DELVE DR2 for the $r$ band photometry over the same region as the middle panels. Bottom left: Angular-averaged source density plot of the top middle panel, with the limiting radius derived from h+96 marked in red, and the Jacobi radius calculated in Section \ref{['sec:obs_analysis']} in yellow. The error bars on the data points are the standard error in the spatial bins averaged (1.8$'$) over the annulus, in steps of 3.6$'$. The dashed line corresponds to the background value plus the standard error. Bottom right: Same as bottom left, but for the background-subtracted density plot.
• Figure 3: Same as Figure \ref{['fig:pal5example']}, but for NGC 5897. A clear photometric overdensity is seen extending from the GC beyond $r_J$ (yellow circle). A dashed blue circle is overplotted at $r=$ 28' to guide the eye, roughly at the radius out to which the photometric excess exists. Note that the excess does not preferentially align with the proper motion of NGC 5897 (red arrow); this is expected given the recent pericenter passage of NGC 5897 (see \ref{['sec:ngc5897_obs']} for more details).
• Figure 4: Top: A 3x3 postage stamp plot where the top left panel is the observed density plot NGC 5897, and the other panels are example noise maps generated to assess the significance of the excess in NGC 5897, as described in Section \ref{['sec:ngc5897_obs']}. Note that regions masked in white correspond to pixels with values equal to 0 in the raw data, corresponding to a lack of coverage (see Figure \ref{['fig:ngc5897']}). The red and yellow circles correspond to $r_t$ and $r_J$, respectively, and the colorscale corresponds to what is shown in Figure \ref{['fig:ngc5897']}. Bottom: An azimuthally averaged density profile of the background subtracted observed data is shown as blue points, with the mean, 1$\sigma$, and 2$\sigma$ of the distribution of density profiles from 1000 noise map samples shown in black/grey. The observed excess at the $r_J$ is at a 4.6 $\sigma$ significance relative to distribution of density profiles from the noise maps.
• Figure 5: Same as Figure \ref{['fig:pal5example']}, but for NGC 7492. A clear photometric excess is seen beyond the Jacobi radius of the GC (yellow circle). As with NGC 5897, this excess does not preferentially align with the proper motion of NGC 7492 (red arrow).