The MUSE-Faint survey V. The binary fraction of Leo T

Abstract

The Leo T dwarf galaxy, the faintest and least massive galaxy known to have recent star formation ($\leq 1~Gyr$), exhibits a high dynamical mass-to-light ratio based on its stellar velocity dispersion ($7.07^{+1.29}_{-1.12}~\mathrm{km\ s^{-1}}$), indicating extreme dark matter dominance. We present the first measurement of the binary fraction of Leo T using MUSE-Faint multi-epoch spectroscopy. We also determine the binary fraction for both young and old stellar populations separately and gain insights into binary properties in more metal-poor environments than the Milky Way or Magellanic Clouds. Finally, we investigate the potential impact of binaries on the inferred stellar velocity dispersion. We employed a forward model methodology combining empirical scaling relations to predict stellar velocity variations and a constrained binary distribution from the literature. To estimate the close binary fraction, we limited the maximum semi-major axis ($a < 10~\mathrm{au}$) and repeated the analysis with a semi-amplitude threshold ($\geq~10~ \mathrm{km\ s^{-1}}$) to check the impact on the inferred stellar velocity dispersion.} The overall binary fraction of Leo T is estimated to be $55^{+40}_{-9} \%$, consistent with similar systems. The close binary fraction ($a < 10~\mathrm{au}$) is $30^{+34}_{-9} \%$, which is aligned with low-metallicity environments. We found a lower binary fraction for the older stellar population ($15^{+43}_{-15} \%$) when compared to the younger population ($35^{+40}_{-6} \%$). Finally, we found no significant inflation of the velocity dispersion estimate due to binary motions when compared to the dispersion inferred from the co-added spectra. This suggests that the co-added spectra effectively provide period-averaged velocities of the stars, thus mitigating the impact of binaries on the overall velocity dispersion measurement.

Figures (17)

• Figure 1: Distributions of the estimated masses using spexxy tool. The fit was made using the nearest-neighbour method. The full sample is shown, with identification of the young population in blue.
• Figure 2: Examples of spectra along with their corresponding spexxy best fit utilising three different epochs for a star likely to be in a binary system. The lower panel displays the combined spectrum from all five epochs, plotted in green. Given that this star is one of the most probable binaries in our sample, we provide a zoomed-in view of the $H\beta$ line, with a dotted black line indicating the expected line centre in the rest frame to illustrate the shift in velocity from epoch to epoch. This shift between epochs affects the co-added spectrum, where the line is strongly diminished as a result of multiple velocity components blending together.
• Figure 3: Colour-magnitude diagram of the 55 stars plotted against PARSEC isochrones drawn for constant $[\mathrm{Fe/H}] = -1.6$ and variable age. The magnitudes given are on the Vega Magnitude System. The region spanned by $0.1-1$ Gyr isochrones is represented by two isochrones shown for 0.2 and 0.8 Gyr. We also show a 9 Gyr isochrone for illustration of the old region spanned by $>5\,\mathrm{Gyr}$. The stars that were found to be consistent with the younger isochrones are shown as dark blue squares while the stars consistent with the older isochrones are shown as red diamonds.
• Figure 4: Distributions of intrinsic parameters depicted in joint distributions showcasing various regimes for the period $P$ on the left. We show the distribution alterations with respect to the mass ratio $q$ and the primary mass $M$. The upper-left plot illustrates the distribution for $M > 1.2~M_{\odot}$, while the lower left pertains to $M < 1.2~M_{\odot}$, each highlighting regimes for $q > 0.3$ and $q<0.3$. The top-right plot presents the distribution for the mass ratio q, while the bottom right illustrates the eccentricity distribution, notably demonstrating an abundance of circular orbits.
• Figure 5: Cumulative distribution functions of the $\chi^2$ statistic for Leo T. The solid black line shows the observed CDF. Shaded regions indicate the 95% confidence intervals of the mock CDFs for $f_{\mathrm{mock}} = 0$%, 50%, and 100%, derived from 10,000 realisations. Dashed curves show the corresponding median mock CDFs.