No Arabic abstract
We report on H${alpha}$ + [NII] imaging of the Arp 202 interacting pair and its tidal dwarf galaxy (TDG) candidate as well as a GMOS long slit spectrum from the TDG candidate, observed with the Gemini North telescope. Our H${alpha}$ + [NII] imaging reveals the TDG to have an elongated structure, $sim$ 1.9 kpc in length with the two principal star forming knots at either end. Our observations also show the TDG candidate has a recessional V$_{Halpha}$ $sim$ 3032 km s$^{-1}$, within 100 km s$^{-1}$ of the parent pairs mean velocity and an oxygen abundance of 12+log(O/H) = 8.10$pm$0.41. The TDGs oxygen abundance is in good agreement with that of a star forming region in NGC 2719A, one of the parent galaxies, which has an estimated oxygen abundance of 12+log(O/H) = 8.05$pm$0.41. The TDGs V$_{Halpha}$ and oxygen abundance confirm previous results validating the candidate as a TDG. The absence of detectable emission from the TDG in $textit{Spitzer}$ 3.6 $mu$m and 4.5 $mu$m images together with the lack of absorption lines and weak continuum in the spectrum is consistent with absence of an old population ($gtrsim$ 0.5 Gyr). The location of the TDG within the interaction debris and the absence of indicators of an old stellar population in the TDG are consistent with a scenario in which the TDG is formed from HI stripped from the parent galaxies and within the extended dark matter halo of one of the parents as proposed by (Bournaud et al. 2003; Duc et al. 2004).
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).
We present results from our Giant Metrewave Radio Telescope (GMRT) HI observations of the interacting pair Arp 202 (NGC 2719 and NGC 2719A). Earlier deep UV(GALEX) observations of this system revealed a tidal tail like extension with a diffuse object towards its end, proposed as a tidal dwarf galaxy (TDG) candidate. We detect HI emission from the Arp 202 system, including HI counterparts for the tidal tail and the TDG candidate. Our GMRT HI morphological and kinematic results clearly link the HI tidal tail and the HI TDG counterparts to the interaction between NGC 2719 and NGC 2719A, thus strengthening the case for the TDG. The Arp 202 TDG candidate belongs to a small group of TDG candidates with extremely blue colours. In order to gain a better understanding of this group we carried out a comparative study of their properties from the available data. We find that HI (and probably stellar) masses of this extremely blue group are similar to the lowest HI mass TDGs in the literature. However the number of such blue TDG candidates examined so far is too small to conclude whether or not their properties justify them to be considered as a subgroup of TDGs.
We simulate tidal streams in the presence and absence of substructures inside the zero redshift snapshot of the Via Lactea II (VL-2) simulation. A halo finder is used to remove and isolate the subhalos found inside the high resolution dark matter halo of VL-2, and the potentials for both the main halo and all the subhalos are constructed individually using the self-consistent field (SCF) method. This allows us to make direct comparison of tidal streams between a smooth halo and a lumpy halo without assuming idealized profiles or triaxial fits. We simulate the kinematics of a star cluster starting with the same orbital position but two different velocities. Although these two orbits are only moderately eccentric and have similar apo- and pericentric distances, we find that the two streams have very different morphologies. We conclude that our model of the potential of VL-2 can provide insights about tidal streams that have not been explored by previous studies using idealized or axisymmetric models.
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.
We extend the random-walk model of Vitvitska et al. for predicting the spins of dark matter halos from their merger histories. Using updated merger rates, orbital parameter distributions, and N-body constraints we show that this model can accurately reproduce the distribution of spin parameters measured in N-body simulations when we include a weak correlation between the spins of halos and the angular momenta of infalling subhalos. We further show that this model is in approximate agreement with the correlation of the spin magnitude over time as determined from N-body simulations, while it slightly underpredicts the correlation in the direction of the spin vector measured from the same simulations. This model is useful for predicting spins from merger histories derived from non-N-body sources, thereby circumventing the need for very high resolution simulations to permit accurate measurements of spins. It may be particularly relevant to modeling systems which accumulate angular momentum from halos over time (such as galactic discs) - we show that this model makes small but significant changes in the distribution of galactic disc sizes computed using the Galacticus semi-analytic galaxy formation model.