Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

oMEGACat. VII. Tracing Interstellar and Intracluster Medium of $ω$ Centauri using Sodium Absorptions

Z. Wang, A. C. Seth, M. Latour, J. Strader, M. Häberle, N. Neumayer, C. Clontz, S. Kamann, M. S. Nitschai, M. Alfaro-Cuello, A. Bellini, A. Feldmeier-Krause, M. Libralato, A. P. Milone, P. J. Smith, S. O. Souza, G. van de Ven

TL;DR

This work uses Na I D absorption measured with MUSE to disentangle foreground ISM from intracluster gas in ω Cen, exploiting a large LOS velocity separation to separate components. The foreground gas shows small-scale substructures that correlate with differential reddening, yielding a mean reddening of $E(B-V) = 0.153 \pm 0.003$ mag within the half-light radius, though with substantial scatter in the Na I D–reddening relation. No significant intracluster Na I D absorption is detected, placing an upper limit on atomic gas of $N_{atomic} \lesssim 2.17 \times 10^{18}\ \mathrm{cm}^{-2}$, while pulsar dispersion measures imply ionized gas that is likely predominantly foreground. The results suggest a short intracluster gas accumulation timescale and imply gas removal via internal processes rather than disk-stripping, with broader implications for the ISM/ICM content in globular clusters. The methodology demonstrates how high-resolution IFU data can separate ISM and ICM contributions and informs future studies of gas phases in dense stellar systems.

Abstract

We investigate the foreground interstellar medium along the line of sight and intracluster medium of $ω$ Centauri ($ω$ Cen) by measuring the equivalent width of Na I D absorptions from MUSE observations. The large line-of-sight velocity difference between $ω$ Cen and the foreground enables us to separate Na I D absorption contributed from atomic gas in the interstellar and intracluster medium. We find that small-scale substructures in the foreground Na I D distribution correlate with differential reddening derived from photometric methods. Using an empirical Na I D equivalent width-reddening relation, we determine an average reddening of $E(B-V)=0.153\pm0.003$ mag within the half-light radius of $ω$ Cen. However, the Na I D-inferred differential reddening is significantly larger than photometric estimates. This is likely due to scatter in the Na I D-reddening relation. We find no evidence for intracluster atomic gas from spectra of horizontal branch stars, as there is no significant Na I D absorption at $ω$ Cen's systemic velocity. Given this non-detection, we place the strongest upper limit to date on the intracluster atomic gas column density in $ω$ Cen of $\lesssim2.17 \times 10^{18}~\rm{cm^{-2}}$. We also estimate the ionized gas density from pulsar dispersion measure variations, which exceed the atomic gas limit by $\sim$50 times. Nevertheless, the strong correlation between dispersion measure and foreground Na I D suggests that much or all of this ionized gas resides in the foreground. Given ongoing mass loss from bright giant stars, our findings imply that the intracluster gas accumulation timescale is short, and gas removal in the cluster is likely not tied to stripping as $ω$ Cen passes through the Galactic disk.

oMEGACat. VII. Tracing Interstellar and Intracluster Medium of $ω$ Centauri using Sodium Absorptions

TL;DR

This work uses Na I D absorption measured with MUSE to disentangle foreground ISM from intracluster gas in ω Cen, exploiting a large LOS velocity separation to separate components. The foreground gas shows small-scale substructures that correlate with differential reddening, yielding a mean reddening of mag within the half-light radius, though with substantial scatter in the Na I D–reddening relation. No significant intracluster Na I D absorption is detected, placing an upper limit on atomic gas of , while pulsar dispersion measures imply ionized gas that is likely predominantly foreground. The results suggest a short intracluster gas accumulation timescale and imply gas removal via internal processes rather than disk-stripping, with broader implications for the ISM/ICM content in globular clusters. The methodology demonstrates how high-resolution IFU data can separate ISM and ICM contributions and informs future studies of gas phases in dense stellar systems.

Abstract

We investigate the foreground interstellar medium along the line of sight and intracluster medium of Centauri ( Cen) by measuring the equivalent width of Na I D absorptions from MUSE observations. The large line-of-sight velocity difference between Cen and the foreground enables us to separate Na I D absorption contributed from atomic gas in the interstellar and intracluster medium. We find that small-scale substructures in the foreground Na I D distribution correlate with differential reddening derived from photometric methods. Using an empirical Na I D equivalent width-reddening relation, we determine an average reddening of mag within the half-light radius of Cen. However, the Na I D-inferred differential reddening is significantly larger than photometric estimates. This is likely due to scatter in the Na I D-reddening relation. We find no evidence for intracluster atomic gas from spectra of horizontal branch stars, as there is no significant Na I D absorption at Cen's systemic velocity. Given this non-detection, we place the strongest upper limit to date on the intracluster atomic gas column density in Cen of . We also estimate the ionized gas density from pulsar dispersion measure variations, which exceed the atomic gas limit by 50 times. Nevertheless, the strong correlation between dispersion measure and foreground Na I D suggests that much or all of this ionized gas resides in the foreground. Given ongoing mass loss from bright giant stars, our findings imply that the intracluster gas accumulation timescale is short, and gas removal in the cluster is likely not tied to stripping as Cen passes through the Galactic disk.

Paper Structure

This paper contains 24 sections, 11 equations, 11 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Data
  3. oMEGACat with Other Datasets
  4. Combining the Datasets
  5. Target Selection
  6. Warm Horizontal Branch Stars
  7. All Other Stars
  8. Spectral Normalization and Re-estimation of S/N in Region
  9. Methods
  10. Measuring the Equivalent Width Using Markov Chain Monte Carlo
  11. Linking Equivalent Width to Extinction
  12. Mapping the Sodium Absorption from the Interstellar Medium
  13. The Spatial Distribution of the Equivalent Width
  14. Estimation of $E(B-V)$ and Differential Reddening
  15. Investigating the Sodium Absorption due to the Intracluster Medium
  16. ...and 9 more sections

Figures (11)

  • Figure 1: An illustration of the method used in this paper to probe the foreground interstellar medium (ISM) and intracluster medium (ICM) using the $\mathrm{Na\,\textsc{i}\,D}$ region of our MUSE spectra. The top portion shows the locations of the observer, the intervening interstellar medium, and the intracluster medium; each is labeled with its radial velocity. These differing radial velocities result in a Doppler offset between the interstellar and intracluster $\mathrm{Na\,\textsc{i}\,D}$ lines as shown in the bottom panel, which shows where we expect to see the lines from each component in our spectra.
  • Figure 2: Data sample used in this study. Panel (a): CMD of the data sample in Section \ref{['subsec:data-muse-hst']} using 2010ApJ...710.1032A photometry. The blue region defines the selection of warm HB stars. The black dotted line shows the ZAHB track from the BaSTI model (see Section \ref{['subsubsec:data-target-selection-hb']} for details). Panel (b):$V_{\rm LOS}$ distribution of the warm HB stars and all other stars. The gray histogram indicates stars excluded based on the selection criteria for the two samples. Panel (c): Proper motion distribution of the same samples, with colors corresponding to panel (b). Panel (d): Re-estimated S/N distribution in the $\mathrm{Na\,\textsc{i}\,D}$ region. Panel (e) and (f): Example MUSE spectra (black line) of warm HB and all other stars, with the S/N and $V_{\rm LOS}$ indicated in the top left corner. The green lines show the residuals between the observed spectra and the best-fit SPEXXY model from 2023ApJ...958....8N. Vertical lines are the positions of the $\mathrm{Na\,\textsc{i}\,D}$ absorptions from foreground (solid, located at 0 $\rm{km~s^{-1}}$) and intracluster (dashed, at 232.7 $\rm{km~s^{-1}}$) atomic gas of $\omega$ Cen. An additional contribution from the stellar photosphere, seen as the right-most absorption feature, is present in panel (f).
  • Figure 3: Demonstration of spectral fitting to measure the EWs of $\mathrm{Na\,\textsc{i}\,D}$, as described in Section \ref{['subsec:methods-ew']}. The top panel shows an example spectrum of a warm HB star, while the bottom panel shows a Voronoi-binned residual spectrum from all other stars. The equivalent widths (EWs) of the $\mathrm{Na\,\textsc{i}\,D}$ lines, in units of Å, are labeled in each panel. The red-shaded region indicates the wavelength range used for fitting. The gray-shaded regions are used to estimate the continuum ($f_{c}$) in Eq. \ref{['eqn:nad-fit']}. Fitting residuals are shown in green as a separate sub-panel. The warm HB spectrum is expected to have minimal photospheric contribution to $\mathrm{Na\,\textsc{i}\,D}$ and thus we name them as "ISM" and "ICM". For all other stars that are mostly cooler, contamination from the stellar photosphere could remain in the SPEXXY residuals. Therefore, we label the absorptions at $\omega$ Cen systemic velocity as "others". Note that the D2 line is at a shorter wavelength than the D1 line for both the ISM and ICM/others components.
  • Figure 4: Spatial distribution of $\rm{EW}_{\rm{D2, ISM}}$ from the foreground Na atomic gas. The left panel shows individual measurements from the original spectra of warm HB stars. The right panel shows measurements from Voronoi-binned SPEXXY residual spectra 2023ApJ...958....8N of all other stars. The number of stars used for each panel is noted in the subtitles. The x- and y-axes indicate coordinates in $(\alpha, \delta)$ relative to the center of $\omega$ Cen. The two blank regions near the center of $\omega$ Cen are due to the lack of non-AO observations. Both warm HB and all other stars demonstrate clear spatial substructures of foreground Na atomic gas, and they are consistent with the differential reddening distribution from previous studies (see Section \ref{['subsec:discuss-comparison']} for detailed discussion). The individual warm HB $\rm{EW}_{\rm{D2, ISM}}$ measurements and the Voronoi-binned $\rm{EW}_{\rm{D2, ISM}}$ map with uncertainties are available as a machine-readable table (columns listed in Table. \ref{['tab:ewd2ism_table']}) and a FITS image, respectively.
  • Figure 5: Comparison of $\rm{EW}_{\rm{D2, ISM}}$ measurements between warm HB stars and all other stars as shown in Fig. \ref{['fig:ew-ism-distribution']}. The gray dots represent individual warm HB stars. The y-axis represents $\rm{EW}_{\rm{D2, ISM}}$ measured from individual warm HB spectra, and the x-axis corresponds to $\rm{EW}_{\rm{D2, ISM}}$ values interpolated from the Voronoi-binned maps of all other stars, using the coordinates of the warm HB stars. The red line indicates a one-to-one relation for reference. The black line and gray-shaded region represent the median and the ($16^{th}$, $84^{th}$) percentiles of the comparison, respectively. This figure indicates that although warm HB stars and all other stars show a similar structure in their spatial distributions, their one-to-one comparison reveals a systematic offset.
  • ...and 6 more figures