ACS Observations of a Strongly Lensed Arc in a Field Elliptical

We report the discovery of a strongly lensed arc system around a field elliptical galaxy in Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) images of a parallel field observed during NICMOS observations of the HST Ultra-Deep Field. The ACS parallel data comprise deep imaging in the F435W, F606W, F775W, and F850LP bandpasses. The main arc is at a radius of 1.6 arcsec from the galaxy center and subtends about 120 deg. Spectroscopic follow-up at Magellan Observatory yields a redshift z=0.6174 for the lensing galaxy, and we photometrically estimate z_phot = 2.4\pm0.3 for the arc. We also identify a likely counter-arc at a radius of 0.6 arcsec, which shows structure similar to that seen in the main arc. We model this system and find a good fit to an elliptical isothermal potential of velocity dispersion $\sigma \approx 300$ \kms, the value expected from the fundamental plane, and some external shear. Several other galaxies in the field have colors similar to the lensing galaxy and likely make up a small group.Comment: Accepted for publication in ApJ Letters. 10 pages, 3 figures. Figures have been degraded to meet size limit; a higher resolution version and addtional pictures available at http://acs.pha.jhu.edu/~jpb/UDFparc

Discrepant Mass Estimates in the Cluster of Galaxies Abell 1689

We present a new mass estimate of a well-studied gravitational lensing cluster, Abell 1689, from deep Chandra observations with a total exposure of 200 ks. Within r=200 h-1 kpc, the X-ray mass estimate is systematically lower than that of lensing by 30-50%. At r>200 h-1 kpc, the mass density profiles from X-ray and weak lensing methods give consistent results. The most recent weak lensing work suggest a steeper profile than what is found from the X-ray analysis, while still in agreement with the mass at large radii. Previous studies have suggested that cooler small-scale structures can bias X-ray temperature measurements or that the northern part of the cluster is disturbed. We find these scenarios unlikely to resolve the central mass discrepancy since the former requires 70-90% of the space to be occupied by these cool structures and excluding the northern substructure does not significantly affect the total mass profiles. A more plausible explanation is a projection effect. We also find that the previously reported high hard-band to broad-band temperature ratio in A1689, and many other clusters observed with Chandra, may be resulting from the instrumental absorption that decreases 10-15% of the effective area at ~1.75 keV.Comment: 18 pages, 15 figures. ApJ accepte

Discovery of two M32 twins in Abell 1689

The M31 satellite galaxy M32 has long been considered an object of unique properties, being the most extreme example of the very rare compact elliptical galaxy class. Here we present the spectroscopic discovery of two M32 twins in the massive galaxy cluster Abell 1689. As these objects are so rare, this is an important step towards a better understanding of the nature of compact galaxies. The two M32 twins had first been detected within our photometric search for ultra compact dwarf galaxy (UCDs) candidates in A1689 (Mieske et al. 2004) with the Advanced Camera for Surveys (ACS). Their luminosities (M_V ~ -17 mag) are very similar to M32 and their surface brightness profiles are consistent with that of M32 projected to A1689's distance. From our ACS imaging we detect several fainter compact galaxy candidates with luminosities intermediate between M32 and the Fornax UCDs. If spectroscopically confirmed as cluster members, this would almost close the gap in the magnitude-surface brightness plane between the region of UCDs and the compact ellipticals, implying a sequence of faint compact galaxies well separated from that of dwarf ellipticals.Comment: 5 pages, 5 figures, accepted for publication in A&A letter

Halo mass - concentration relation from weak lensing

We perform a statistical weak lensing analysis of dark matter profiles around tracers of halo mass from galactic- to cluster-size halos. In this analysis we use 170,640 isolated ~L* galaxies split into ellipticals and spirals, 38,236 groups traced by isolated spectroscopic Luminous Red Galaxies (LRGs) and 13,823 MaxBCG clusters from the Sloan Digital Sky Survey (SDSS) covering a wide range of richness. Together these three samples allow a determination of the density profiles of dark matter halos over three orders of magnitude in mass, from 10^{12} M_{sun} to 10^{15} M_{sun}. The resulting lensing signal is consistent with an NFW or Einasto profile on scales outside the central region. We find that the NFW concentration parameter c_{200b} decreases with halo mass, from around 10 for galactic halos to 4 for cluster halos. Assuming its dependence on halo mass in the form of c_{200b} = c_0 [M/(10^{14}M_{sun}/h)]^{\beta}, we find c_0=4.6 +/- 0.7 (at z=0.22) and \beta=0.13 +/- 0.07, with very similar results for the Einasto profile. The slope (\beta) is in agreement with theoretical predictions, while the amplitude is about two standard deviations below the predictions for this mass and redshift, but we note that the published values in the literature differ at a level of 10-20% and that for a proper comparison our analysis should be repeated in simulations. We discuss the implications of our results for the baryonic effects on the shear power spectrum: since these are expected to increase the halo concentration, the fact that we see no evidence of high concentrations on scales above 20% of the virial radius suggests that baryonic effects are limited to small scales, and are not a significant source of uncertainty for the current weak lensing measurements of the dark matter power spectrum. [ABRIDGED]Comment: 17 pages, 5 figures, accepted to JCAP pending minor revisions that are included in v2 here on arXi

Strong Lensing Analysis of A1689 from Deep Advanced Camera Images

We analyse deep multi-colour Advanced Camera images of the largest known gravitational lens, A1689. Radial and tangential arcs delineate the critical curves in unprecedented detail and many small counter-images are found near the center of mass. We construct a flexible light deflection field to predict the appearance and positions of counter-images. The model is refined as new counter-images are identified and incorporated to improve the model, yielding a total of 106 images of 30 multiply lensed background galaxies, spanning a wide redshift range, 1.0$<$z$<$5.5. The resulting mass map is more circular in projection than the clumpy distribution of cluster galaxies and the light is more concentrated than the mass within $r<50kpc/h$. The projected mass profile flattens steadily towards the center with a shallow mean slope of $d\log\Sigma/d\log r \simeq -0.55\pm0.1$, over the observed range, r$<250kpc/h$, matching well an NFW profile, but with a relatively high concentration, $C_{vir}=8.2^{+2.1}_{-1.8}$. A softened isothermal profile ($r_{core}=20\pm2$\arcs) is not conclusively excluded, illustrating that lensing constrains only projected quantities. Regarding cosmology, we clearly detect the purely geometric increase of bend-angles with redshift. The dependence on the cosmological parameters is weak due to the proximity of A1689, $z=0.18$, constraining the locus, $\Omega_M+\Omega_{\Lambda} \leq 1.2$. This consistency with standard cosmology provides independent support for our model, because the redshift information is not required to derive an accurate mass map. Similarly, the relative fluxes of the multiple images are reproduced well by our best fitting lens model.Comment: Accepted by ApJ. For high quality figures see http://wise-obs.tau.ac.il/~kerens/A168

Ultracompact dwarf galaxies in Abell 1689: A photometric study with the advanced camera for surveys

The properties of ultracompact dwarf (UCD) galaxy candidates in Abell 1689 (z = 0.183) are investigated, based on deep high-resolution images from the Hubble Space Telescope Advanced Camera for Surveys. A UCD candidate has to be unresolved, have i < 28 mag (M-V < -11.5 mag), and satisfy color limits derived from Bayesian photometric redshifts. We find 160 UCD candidates with 22 mag < i < 28 mag. We estimate that about 100 of these are cluster members, based on their spatial distribution and photometric redshifts. For i greater than or similar to 26.8 mag, the radial and luminosity distribution of the UCD candidates can be explained well by Abell 1689's globular cluster ( GC) system. For i less than or similar to 26.8 mag, there is an overpopulation of 15 +/- 5 UCD candidates with respect to the GC luminosity function. For i less than or similar to 26 mag, the radial distribution of UCD candidates is more consistent with the dwarf galaxy population than with the GC system of Abell 1689. The UCD candidates follow a color-magnitude trend with a slope similar to that of Abell 1689's genuine dwarf galaxy population, but shifted fainter by about 2-3 mag. Two of the three brightest UCD candidates (M-V similar or equal to -17 mag) are slightly resolved. At the distance of Abell 1689, these two objects would have King profile core radii of similar or equal to35 pc and r(eff) similar or equal to 300 pc, implying luminosities and sizes 2-3 times those of M32's bulge. Additional photometric redshifts obtained with late-type stellar and elliptical galaxy templates support the assignment of these two resolved sources to Abell 1689 but also allow for up to four foreground stars among the six brightest UCD candidates. Our findings imply that in Abell 1689 there are greater than or equal to10 UCDs with M-V < -12.7 mag, probably created by stripping "normal" dwarf or spiral galaxies. Compared with the UCDs in the Fornax Cluster - the location of their original discovery - they are brighter, larger, and have colors closer to normal dwarf galaxies. This suggests that they may be in an intermediate stage of the stripping process. Checking the photometric redshifts of the brightest UCD candidates with spectroscopy would be the next step to definitely confirm the existence of UCDs in Abell 1689