Dark and luminous matter in THINGS dwarf galaxies

TL;DR

This study confronts the cusp/core problem in dwarf galaxies by deriving high-resolution dark matter mass distributions for seven THINGS dwarfs using HI kinematics and careful baryonic modeling. It employs bulk velocity fields to minimize non-circular motions, alongside tilted-ring rotation curves and asymmetric-drift corrections, to reveal the underlying mass distribution. Baryons are modeled with Spitzer 3.6 μm and optical data to separate their dynamical influence, and two DM halo models (NFW and pseudo-isothermal) are fitted under various stellar mass-to-light assumptions. The results show that core-like pseudo-isothermal halos better describe the data, with an average inner slope of α ≈ -0.29, contrasting with cuspy LCDM predictions (α ≈ -1); this discrepancy emphasizes the role of baryonic feedback and resolution in shaping DM halos. The findings argue for incorporating baryonic physics in simulations to reconcile theory with observations and are complemented by a companion paper comparing to high-resolution N-body+SPH simulations that include feedback effects.

Abstract

We present mass models for the dark matter component of seven dwarf galaxies taken from "The HI Nearby Galaxy Survey" (THINGS) and compare these with those from numerical Lambda Cold Dark Matter (LCDM) simulations. The THINGS high-resolution data significantly reduce observational uncertainties and thus allow us to derive accurate dark matter distributions in these systems. We here use the bulk velocity fields when deriving the rotation curves of the galaxies. Compared to other types of velocity fields, the bulk velocity field minimizes the effect of small-scale random motions more effectively and traces the underlying kinematics of a galaxy more properly. The "Spitzer Infrared Nearby Galaxies Survey" (SINGS) 3.6 micron and ancillary optical data are used for separating the baryons from their total matter content in the galaxies. The sample dwarf galaxies are found to be dark matter dominated over most radii. We find discrepancies between the derived dark matter distributions of the galaxies and those of LCDM simulations, even after corrections for non-circular motions have been applied. The observed solid body-like rotation curves of the galaxies rise too slowly to reflect the cusp-like dark matter distribution in CDM halos. Instead, they are better described by core-like models such as pseudo-isothermal halo models dominated by a central constant-density core. The mean value of the logarithmic inner slopes of the mass density profiles is alpha = -0.29 +- 0.07. They are significantly different from the steep slope of ~ -1.0 inferred from previous dark-matter-only simulations, and are more consistent with shallower slopes found in recent LCDM simulations of dwarf galaxies in which the effects of baryonic feedback processes are included.

Figures (36)

• Figure 1: Schematic H i profiles with different asymmetries. The gray dashed lines represent the bulk motion and the light-gray dotted lines indicate an additional non-circular component. The black solid lines are the resulting profiles combining both the bulk and non-circular motions. As the non-circular component increases from the top to bottom panels the asymmetry of the resulting profile also increases. The long light-gray arrows in all panels indicate the central velocity of the bulk motion profile. The short black arrows in all panels indicate the derived velocity from the IWM, peak, single Gaussian fit, and hermite $h_{3}$ polynomial fit. The larger the asymmetry of a profile, the larger the velocity deviation from the bulk motion. In case the non-circular motion does not dominate the bulk motion (upper two rows), the derived velocity is close to that of the bulk motion and peak and hermite $h_{3}$ velocities give an equally good result. However, if the non-circular motion dominates the bulk motion (lower two rows), the derived velocity deviates significantly from the bulk velocity even if we use the hermite $h_{3}$ polynomial fit.
• Figure 2: (a): Comparison of the bulk rotation curve of IC 2574 with rotation curves derived using other types of velocity fields as denoted in the panel (i.e., IWM, hermite $h_{3}$, single Gaussian and peak velocity fields). The Martimbeau_1994 curve was derived using an IWM velocity field with a lower resolution. They adopted a large value for the inclination ($\sim$75$^{\circ}$). The IWM rotation curve which is most likely affected by random non-circular motions is the lowest among the others. See Section \ref{['Deriving_Rotcurs']} for more details. (b)(c): The radial mass surface density profiles of the stellar and gas components of IC 2574, respectively. (d)(e): The resulting rotation velocities of the stellar and gas components of IC 2574 derived from the surface density profiles given in the panels (b) and (c), respectively. More details can be found in Section \ref{['Baryons']}.
• Figure 3: The dark matter fraction $\gamma_{\rm dm}$ (as described in Eq. \ref{['gamma_dm']}) of the 7 THINGS dwarf galaxies. Most galaxies are dark matter dominated across all radii except the inner region of DDO 53 and the outer region of Ho II, respectively. This is discussed in detail in Section \ref{['DM_fraction_THINGS_properties']}
• Figure 4: Left: The relationship between the dark matter fraction and the absolute B magnitude of 19 THINGS dwarf and spiral galaxies. $<$$\gamma_{\rm dm}$$>$ is determined by radially averaging $\gamma_{\rm dm}$ values of each galaxy. For the spiral galaxies, $<$$\gamma_{\rm dm}$$>$ values are calculated over three regions, splitting a galaxy into three annuli (inner, middle and outer) as indicated by different symbols. Right: The relationship between the mean dark matter fraction $<$$\gamma_{\rm dm}$$>$ and the dynamical mass of the same galaxies. See Section \ref{['DM_fraction_THINGS_properties']} for more discussions.
• Figure 5: The baryonic Tully$\--$Fisher relation of the THINGS dwarf galaxies. The baryonic mass includes the stellar and gas components derived in Section \ref{['Baryons']}. The long dashed-line indicates the BTF relation calibrated using a sample of gas dominated galaxies in Stark_2009, and the short dashed-lines indicate the uncertainty in the relation. See Section \ref{['BTF_relation_THINGS']} for more details.