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Register a new userCluster 77 in NGC 4449: The nucleus of a satellite galaxy being transformed into a globular cluster?
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Authors
Francesca Annibali
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We report the discovery in our HST ACS B, V, and I images of NGC 4449 of a globular cluster (GC) which appears associated with two tails of blue stars. The cluster is massive (M~1.7x10^6 M_sun) and highly flattened (epsilon~0.24). From the color-magnitude diagrams of the resolved stars we infer active star formation in the tails over the past ~200 Myr. In a diagram of mean projected mass density inside r_e versus total mass the cluster lies at the upper end of the GC distribution, where galaxy nuclei are. The north-west tail is associated with a concentration of HI and infrared (dust/PAHs) emission which appears as part of a much longer stream wrapping around the galaxy. These properties suggest that the cluster may be the nucleus of a former gas-rich satellite galaxy undergoing tidal disruption by NGC 4449. If so, the cluster is seen in an earlier phase compared to other suggested nuclei of disrupted galaxies such as omega Cen (Milky Way) and G1 (M31).
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We present a kinematic analysis of the globular cluster(GC) system in the giant elliptical galaxy (gE) NGC 4636 in the Virgo cluster. Using the photometric and spectroscopic database of 238 GCs, we have investigated the kinematics of the GC system. The NGC 4636 GC system shows weak overall rotation, which is dominated by the red GCs. However, both the blue GCs and red GCs show some rotation in the inner region at R<4.3. The velocity dispersion for all the GCs is derived to be sigma_p = 225{+12-9} km/s. The velocity dispersion for the blue GCs (sig=251 km/s) is slightly larger than that for the red GCs (sig=205 km/s). The velocity dispersions for the blue GCs about the mean velocity and about the best fit rotation curve have a significant variation depending on the galactocentric radius. Comparison of observed stellar and GC velocity dispersion profiles with the velocity dispersion profiles calculated from the stellar mass profile shows that the mass-to-light ratio should increase as the galactocentric distance increases, indicating the existence of an extended dark matter halo. From the comparison of the observed GC velocity dispersion profiles and the velocity dispersion profiles calculated for the X-ray mass profiles in the literature, we find that the orbit of the GC system is tangential, and that the orbit of the red GCs is slightly more tangential than that of the blue GCs. We compare the GC kinematics of NGC 4636 with those of other six gEs, finding that the kinematic properties of the GCs are diverse among gEs. We find several correlations between the kinematics of the GCs and the global parameters of their host galaxies. We discuss the implication of the results for the formation models of the GC system in gEs, and suggest a mixture scenario for the origin of the GCs in gEs.
NGC 4449 is a nearby Magellanic irregular starburst galaxy with a B-band absolute magnitude of -18 and a prominent, massive, intermediate-age nucleus at a distance from Earth of 3.8 megaparsecs. It is wreathed in an extraordinary neutral hydrogen (H I) complex, which includes rings, shells and a counter-rotating core, spanning 90 kiloparsecs. NGC 4449 is relatively isolated, although an interaction with its nearest known companion-the galaxy DDO 125, some 40 kpc to the south-has been proposed as being responsible for the complexity of its HI structure. Here we report the presence of a dwarf galaxy companion to NGC 4449, namely NGC 4449B. This companion has a V-band absolute magnitude of -13.4 and a half-light radius of 2.7 kpc, with a full extent of around 8 kpc. It is in a transient stage of tidal disruption, similar to that of the Sagittarius dwarf near the Milky Way. NGC 4449B exhibits a striking S-shaped morphology that has been predicted for disrupting galaxies but has hitherto been seen only in a dissolving globular cluster. We also detect an additional arc or disk ripple embedded in a two-component stellar halo, including a component extending twice as far as previously known, to about 20 kpc from the galaxys centre.
We use deep images taken with the Advanced Camera for Surveys on board the Hubble Space Telescope of the disk galaxy NGC 891, to search for globular cluster candidates. This galaxy has long been considered to be a close analog in size and structure to the Milky Way and is nearly edge-on, facilitating studies of its halo population. These extraplanar ACS images, originally intended to study the halo field-star populations, reach deep enough to reveal even the faintest globular clusters that would be similar to those in the Milky Way. From the three pointings we have identified a total of 43 candidates after culling by object morphology, magnitude, and colour. We present (V,I) photometry for all of these, along with measurements of their effective radius and ellipticity. The 16 highest-rank candidates within the whole sample are found to fall in very much the same regions of parameter space occupied by the classic Milky Way globular clusters. Our provisional conclusion from this survey is that the total globular cluster population in NGC 891 as a whole may be almost as large as that of the Milky Way.
We present new radial velocities for 289 globular clusters around NGC 4636, the southernmost giant elliptical galaxy of the Virgo cluster. The data were obtained with FORS2/MXU at the Very Large Telescope. Together with data analysed in an earlier study (Schuberth et al. 2006), we now have a sample of 460 globular cluster velocities out to a radius of 12 arcmin (60 kpc) available - one of the largest of its kind. This new data set also provides a much more complete angular coverage. Moreover, we present new kinematical data of the inner stellar population of NGC 4636. We perform an updated Jeans analysis, using both stellar and GC data, to better constrain the dark halo properties. We find a stellar M/L-ratio of 5.8 in the R-band, higher than expected from single stellar population synthesis. We model the dark halo by cored and cuspy analytical halo profiles and consider different anisotropies for the tracer populations. Properties of NFW halos lie well within the expected range of cosmological simulations. Cored halos give central dark matter densities, which are typical for elliptical galaxies of NGC 4636s luminosity. The surface densities of the dark matter halos are higher than those of spiral galaxies. We compare the predictions of Modified Newtonian Dynamics with the derived halo properties and find satisfactory agreement. Therefore NGC 4636 therefore falls onto the baryonic Tully-Fisher relation for spiral galaxies. The comparison with the X-ray mass profile of Johnson et al. (2009) reveals satisfactory agreement only, if the abundance gradient of hot plasma has been taken into account. This might indicate a general bias towards higher masses for X-ray based mass profiles in all systems, including galaxy clusters, with strong abundance gradients.
Using a cosmological dark matter simulation of a galaxy-cluster halo, we follow the temporal evolution of its globular cluster population. To mimic the red and blue globular cluster populations, we select at high redshift $(zsim 1)$ two sets of particles from individual galactic halos constrained by the fact that, at redshift $z=0$, they have density profiles similar to observed ones. At redshift $z=0$, approximately 60% of our selected globular clusters were removed from their original halos building up the intra-cluster globular cluster population, while the remaining 40% are still gravitationally bound to their original galactic halos. Since the blue population is more extended than the red one, the intra-cluster globular cluster population is dominated by blue globular clusters, with a relative fraction that grows from 60% at redshift $z=0$ up to 83% for redshift $zsim 2$. In agreement with observational results for the Virgo galaxy cluster, the blue intra-cluster globular cluster population is more spatially extended than the red one, pointing to a tidally disrupted origin.
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