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Register a new userAndromeda XXI -- a dwarf galaxy in a low density dark matter halo
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Andromeda XXI (And XXI) has been proposed as a dwarf spheroidal galaxy with a central dark matter density that is lower than expected in the Standard $Lambda$ Cold Dark Matter ($Lambda$CDM) cosmology. In this work, we present dynamical observations for 77 member stars in this system, more than doubling previous studies to determine whether this galaxy is truly a low density outlier. We measure a systemic velocity of $v_r=-363.4pm1.0,{rm kms}^{-1}$ and a velocity dispersion of $sigma_v=6.1^{+1.0}_{-0.9},{rm kms}^{-1}$, consistent with previous work and within $1sigma$ of predictions made within the modified Newtonian dynamics framework. We also measure the metallicity of our member stars from their spectra, finding a mean value of ${rm [Fe/H]}=-1.7pm0.1$~dex. We model the dark matter density profile of And~XXI using an improved version of GravSphere, finding a central density of $rho_{rm DM}({rm 150 pc})=2.7_{-1.7}^{+2.7} times 10^7 ,{rm M_odot,kpc^{-3}}$ at 68% confidence, and a density at two half light radii of $rho_{rm DM}({rm 1.75 kpc})=0.9_{-0.2}^{+0.3} times 10^5 ,{rm M_odot,kpc^{-3}}$ at 68% confidence. These are both a factor ${sim}3-5$ lower than the densities expected from abundance matching in $Lambda$CDM. We show that this cannot be explained by `dark matter heating since And~XXI had too little star formation to significantly lower its inner dark matter density, while dark matter heating only acts on the profile inside the half light radius. However, And~XXIs low density can be accommodated within $Lambda$CDM if it experienced extreme tidal stripping (losing $>95%$ of its mass), or if it inhabits a low concentration halo on a plunging orbit that experienced repeated tidal shocks.
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Large-scale faint structure detected by the recent observations in the halo of the Andromeda galaxy (M31) provides an attractive window to explore the structure of outer cold dark matter (CDM) halo in M31. Using an N-body simulation of the interaction between an accreting satellite galaxy and M31, we investigate the mass density profile of the CDM halo. We find the sufficient condition of the outer density profile of CDM halo in M31 to reproduce the Andromeda giant stream and the shells at the east and west sides of M31. The result indicates that the density profile of the outer dark matter halo of M31 is a steeper than the prediction of the theory of the structure formation based on the CDM model.
The cold dark matter (CDM) cosmology, which is the standard theory of the structure formation in the universe, predicts that the outer density profile of dark matter halos decreases with the cube of distance from the center. However, so far not much effort has examined this hypothesis. In the halo of the Andromeda galaxy (M31), large-scale stellar structures detected by the recent observations provide a potentially suitable window to investigate the mass--density distribution of the dark matter halo. We explore the density structure of the dark matter halo in M31 using an N-body simulation of the interaction between an accreting satellite galaxy and M31. To reproduce the Andromeda Giant Southern Stream and the stellar shells at the east and west sides of M31, we find the sufficient condition for the power-law index $alpha$ of the outer density distribution of the dark matter halo. The best-fit parameter is $alpha=-3.7$, which is steeper than the CDM prediction.
The cusp-core problem is one of the main challenges of the cold dark matter paradigm on small scales: the density of a dark matter halo is predicted to rise rapidly toward the center as rho ~ r^alpha with alpha between -1 and -1.5, while such a cuspy profile has not been clearly observed. We have carried out the spatially-resolved mapping of gas dynamics toward a nearby ultra-diffuse galaxy (UDG), AGC 242019. The derived rotation curve of dark matter is well fitted by the cuspy profile as described by the Navarro-Frenk-White model, while the cored profiles including both the pseudo-isothermal and Burkert models are excluded. The halo has alpha=-(0.90+-0.08) at the innermost radius of 0.67 kpc, Mhalo=(3.5+-1.2)E10 Msun and a small concentration of 2.0+-0.36. AGC 242019 challenges alternatives of cold dark matter by constraining the particle mass of fuzzy dark matter to be < 0.11E-22 eV or > 3.3E-22 eV , the cross section of self-interacting dark matter to be < 1.63 cm2/g, and the particle mass of warm dark matter to be > 0.23 keV, all of which are in tension with other constraints. The modified Newtonian dynamics is also inconsistent with a shallow radial acceleration relationship of AGC 242019. For the feedback scenario that transforms a cusp to a core, AGC 242019 disagrees with the stellar-to-halo-mass-ratio dependent model, but agrees with the star-formation-threshold dependent model. As a UDG, AGC 242019 is in a dwarf-size halo with weak stellar feedback, late formation time, a normal baryonic spin and low star formation efficiency (SFR/gas).
B and V time-series photometry of the M31 dwarf spheroidal satellite Andromeda XXI (And XXI) was obtained with the Large Binocular Cameras at the Large Binocular Telescope. We have identified 50 variables in And XXI, of which 41 are RR Lyrae stars (37 fundamental-mode RRab, and 4 first-overtone RRc, pulsators) and 9 are Anomalous Cepheids (ACs). The average period of the RRab stars (<Pab> = 0.64 days) and the period-amplitude diagram place And~XXI in the class of Oosterhoff II - Oosterhoff-Intermediate objects. From the average luminosity of the RR Lyrae stars we derived the galaxy distance modulus of (m-M)$_0$=$24.40pm0.17$ mag, which is smaller than previous literature estimates, although still consistent with them within 1 $sigma$. The galaxy color-magnitude diagram shows evidence for the presence of three different stellar generations in And~XXI: 1) an old ($sim$ 12 Gyr) and metal poor ([Fe/H]=$-$1.7 dex) component traced by the RR Lyrae stars; 2) a slightly younger (10-6 Gyr) and more metal rich ([Fe/H]=$-$1.5 dex) component populating the red horizontal branch, and 3) a young age ($sim$ 1 Gyr) component with same metallicity, that produced the ACs. Finally, we provide hints that And~XXI could be the result of a minor merging event between two dwarf galaxies.
The similarity between the distributions of spins for galaxies ($lambda_{rm g}$) and for dark-matter haloes ($lambda_{rm h}$), indicated both by simulations and observations, is naively interpreted as a one-to-one correlation between the spins of a galaxy and its host halo. This is used to predict galaxy sizes in semi-analytic models via $R_{rm e}simeqlambda_{rm h} R_{rm v}$, with $R_{rm e}$ the half-mass radius of the galaxy and $R_{rm v}$ the halo radius. Utilizing two different suites of zoom-in cosmological simulations, we find that $lambda_{rm g}$ and $lambda_{rm h}$ are in fact only barely correlated, especially at $zgeq 1$. A general smearing of this correlation is expected based on the different spin histories, where the more recently accreted baryons through streams gain and then lose significant angular momentum compared to the gradually accumulated dark matter. Expecting the spins of baryons and dark matter to be correlated at accretion into $R_{rm v}$, the null correlation at the end reflects an anti-correlation between $lambda_{rm g}/lambda_{rm h}$ and $lambda_{rm h}$, which can partly arise from mergers and a compact star-forming phase that many galaxies undergo. On the other hand, the halo and galaxy spin vectors tend to be aligned, with a median $costheta=0.6$-0.7 between galaxy and halo, consistent with instreaming within a preferred plane. The galaxy spin is better correlated with the spin of the inner halo, but this largely reflects the effect of the baryons on the halo. Following the null spin correlation, $lambda_{rm h}$ is not a useful proxy for $R_{rm e}$. While our simulations reproduce a general relation of the sort $R_{rm e}=AR_{rm vir}$, in agreement with observational estimates, the relation becomes tighter with $A=0.02(c/10)^{-0.7}$, where $c$ is the halo concentration, which in turn introduces a dependence on mass and redshift.
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