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Register a new userKinematical asymmetry in the dwarf irregular galaxy WLM and a perturbed halo potential
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WLM is a dwarf irregular that is seen almost edge-on that has prompted a number of kinematical studies investigating its rotation curve and its dark matter content. In this paper, we investigate the origin of the strong asymmetry of the rotation curve, which shows a significant discrepancy between the approaching and the receding side. We first examine whether an $m = 1$ perturbation (lopsidedness) in the halo potential could be a mechanism creating such kinematical asymmetry. To do so, we fit a theoretical rotational velocity associated with an $m = 1$ perturbation in the halo potential model to the observed data via a $chi-$squared minimization method. We show that a lopsided halo potential model can explain the asymmetry in the kinematic data reasonably well. We then verify that the kinematical classification of WLM shows that its velocity field is significantly perturbed due to both its asymmetrical rotation curve and also its peculiar velocity dispersion map. In addition, based on a kinemetry analysis, we find that it is possible for WLM to lie in the transition region, where the disk and merger coexist. In conclusion, it appears that the rotation curve of WLM diverges significantly from that of an ideal rotating disk, which may significantly affect investigations of its dark matter content.
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We present 12CO J = 1-0 and J = 2-1 observations of the low metallicity (12 + log(O/H) = 7.74) Local Group dwarf irregular galaxy WLM made with the 15 m SEST and 14 m FCRAO telescopes. Despite the presence a number of HII regions, we find no CO emission. We obtain low upper limits on the integrated intensity (I(CO) >= 0.18 K km/s for CO (1-0)). The non-detection is consistent with the result of Taylor, Kobulnicky and Skillman (1998), that dwarf galaxies below a metallicity of ~ 7.9 are not detected in CO emission. WLM shows that this trend continues for low metallicity galaxies even as their metallicities approach 7.9. These results are consistent with the models of the metal poor ISM by Norman and Spaans (1997). By comparing our CO data with observations of star formation in WLM, we find evidence for a high CO to H$_2$ conversion factor.
In the indirect dark matter (DM) detection framework, the DM particles would produce some signals by self-annihilating and creating standard model products such as gamma rays, which might be detected by ground-based telescopes. Dwarf irregular galaxies represent promising targets for the search for DM as they are assumed to be dark matter dominated systems at all radii. These dwarf irregular galaxies are rotationally supported with relatively simple kinematics which lead to small uncertainties on their dark matter distribution profiles. In 2018, the H.E.S.S. telescopes observed the irregular dwarf galaxy Wolf-Lundmark-Melotte (WLM) for a live time of 19 hours. These observations are the very first ones made by an imaging atmospheric Cherenkov telescope toward this kind of object. We search for a DM signal looking for an excess of gamma rays over the background in the direction of the WLM galaxy. We present the first results obtained on the velocity weighted cross section for DM self-annihilation as a function of DM particle mass.
We identify the rich Carbon star population of the Magellanic-type dwarf irregular galaxy WLM (Wolf-Lundmark-Melotte) and study its photometric properties from deep near-IR observations. The galaxy exhibits also a clear presence of Oxygen rich population. We derive a Carbon to M-star ratio of C/M=0.56(+/-0.12), relatively high in comparison with many galaxies. The spatial distribution of the AGB stars in WLM hints at the presence of two stellar complexes with a size of a few hundred parsecs. Using the HI map of WLM and the derived gas-to-dust ratio for this galaxy we re-determined the distance modulus of WLM from the IR photometry of four known Cepheids, obtaining (m-M)o=24.84(+/-0.14) mag. In addition, we determine the scale length of 0.75(+/-)0.14 kpc of WLM disk in J-band.
We search for an indirect signal of dark matter through very high-energy gamma rays from the Wolf-Lundmark-Melotte (WLM) dwarf irregular galaxy. The pair annihilation of dark matter particles would produce Standard Model particles in the final state such as gamma rays, which might be detected by ground-based Cherenkov telescopes. Dwarf irregular galaxies represent promising targets as they are dark matter dominated objects with well measured kinematics and small uncertainties on their dark matter distribution profiles. In 2018, the H.E.S.S. five-telescope array observed the dwarf irregular galaxy WLM for 18 hours. We present the first analysis based on data obtained from an imaging atmospheric Cherenkov telescope for this subclass of dwarf galaxy. As we do not observe any significant excess in the direction of WLM, we interpret the result in terms of constraints on the velocity-weighted cross section for dark matter pair annihilation as a function of the dark matter particle mass for various continuum channels as well as the prompt gamma-gamma emission. For the $tau^+tau^-$ channel the limits reach a $langle sigma v rangle$ value of about $4times 10^{-22}$ cm3s-1 for a dark matter particle mass of 1 TeV. For the prompt gamma-gamma channel, the upper limit reaches a $langle sigma v rangle$ value of about $5 times10^{-24}$ cm3s-1 for a mass of 370 GeV. These limits represent an improvement of up to a factor 200 with respect to previous results for the dwarf irregular galaxies for TeV dark matter search.
We present multi-tracer dynamical models of the low mass ($M_{*} sim 10^{7}$), isolated dwarf irregular galaxy WLM in order to simultaneously constrain the inner slope of the dark matter (DM) halo density profile ($gamma$) and flattening ($q_mathrm{DM}$), and the stellar orbital anisotropy ($beta_{z}, beta_{r}$). For the first time, we show how jointly constraining the mass distribution from the HI gas rotation curve and solving the Jeans equations with discrete stellar kinematics leads to a factor of $sim2$ reduction in the uncertainties on $gamma$. The mass-anisotropy degeneracy is also partially broken, leading to reductions on uncertainty by $sim 30%$ on $M_mathrm{vir}$ (and $sim 70%$ at the half-light radius) and $sim 25%$ on anisotropy. Our inferred value of $gamma = 0.3 pm 0.1$ is robust to the halo geometry, and in excellent agreement with predictions of stellar feedback driven DM core creation. The derived prolate geometry of the DM halo with $q_mathrm{DM} = 2 pm 1$ is consistent with $Lambda$CDM simulations of dwarf galaxy halos. While self-interacting DM (SIDM) models with $sigma/m_{X} sim 0.6$ can reproduce this cored DM profile, the interaction events may sphericalise the halo. The simultaneously cored and prolate DM halo may therefore present a challenge for SIDM. Finally we find that the radial profile of stellar anisotropy in WLM ($beta_{r}$) follows a nearly identical trend of increasing tangential anisotropy to the classical dSphs, Fornax and Sculptor. Given WLMs orbital history, this result may call into question whether such anisotropy is a consequence of tidal stripping in only one pericentric passage or if it instead is a feature of the largely self-similar formation and evolutionary pathways for some dwarf galaxies.
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