A spiral galaxy partially overlapping a more distant elliptical offers a unique opportunity to measure the dust extinction in the foreground spiral. From the Sloan Digital Sky Survey (SDSS) DR4 ...spectroscopic sample, we selected 83 occulting galaxy pairs and measured disk opacity over the redshift range z = 0.0-0.2 with the goal of determining the recent evolution of disk dust opacity. The enrichment of the ISM changes over the lifetime of a disk, and it is reasonable to expect the dust extinction properties of spiral disks as a whole to change over their lifetime. When they do, the change will affect our measurements of galaxies over the observable universe. From the SDSS pairs we conclude that spiral disks show evidence of extinction to ~2 effective radii. However, no evidence for recent evolution of disk opacity is evident, due to the limited redshift range and our inability to distinguish other factors on disk opacity such as the presence of spiral arms and Hubble type. Such effects also mask any relation between surface brightness and optical depth that has been found in nearby galaxies. Hence, we conclude that the SDSS spectral catalog is an excellent way to find occulting pairs and construct a uniform local sample. However, a higher resolution than that of the SDSS images is needed to disentangle the effects of spiral arms and Hubble type from evolution since z = 0.2.
We report on deep near-infrared F125W (J) and F160W (H) Hubble Space Telescope Wide Field Camera 3 images of the z = 6.42 quasar J1148+5251 to attempt to detect rest-frame near-ultraviolet emission ...from the host galaxy. These observations included contemporaneous observations of a nearby star of similar near-infrared colors to measure temporal variations in the telescope and instrument point-spread function (PSF). We subtract the quasar point source using both this direct PSF and a model PSF. Using direct subtraction, we measure an upper limit for the quasar host galaxy of m sub(J) > 22.8 and m sub(H) > 23.0 AB mag (2sigma). After subtracting our best model PSF, we measure a limiting surface brightness from 0".3 to 0".5 radius of mu sub(J) > 23.5 and mu sub(H) > 23.7 AB mag arcsec super(-2) (2sigma). We test the ability of the model subtraction method to recover the host galaxy flux by simulating host galaxies with varying integrated magnitude, effective radius, and Sersic index, and conducting the same analysis. These models indicate that the surface brightness limit ( mu sub(J) > 23.5 AB mag arcsec super(-2)) corresponds to an integrated upper limit of m sub(J) > 22-23 AB mag, consistent with the direct subtraction method. Combined with existing far-infrared observations, this gives an infrared excess log(IRX) > 1.0 and corresponding ultraviolet spectral slope beta > -1.2 + or - 0.2. These values match those of most local luminous infrared galaxies, but are redder than those of almost all local star-forming galaxies and z Asymptotically = to 6 Lyman break galaxies.
We present multi-wavelength radio observations in the direction of the spiral galaxy IC 2497 and the neighbouring emission nebula known as “Hanny's Voorwerp”. Our WSRT continuum observations at ...$1.4~\rm GHz$ and $4.9~\rm GHz$ reveal the presence of extended emission at the position of the nebulosity, although the bulk of the emission remains unresolved at the centre of the galaxy. e-VLBI $1.65~\rm GHz$ observations show that on the milliarcsecond-scale, a faint central compact source is present in IC 2497 with a brightness temperature in excess of 4 $\times$ $10^5~\rm K$. With the WSRT, we detect a large reservoir of neutral hydrogen in the proximity of IC 2497. One cloud complex with a total mass of 5.6 $\times$ $10^9~{M}_\odot$ to the South of IC 2497 encompasses Hanny's Voorwerp. Another cloud complex is located at the position of a small galaxy group ${\sim}100~\rm kpc$ to the West of IC 2497 with a mass of 2.9 $\times$ $10^9~{M}_\odot$. Our data hint at a physical connection between the complexes. We also detect $\ion{H}{i}$ in absorption against the central continuum source of IC 2497. Our observations strongly support the hypothesis that Hanny's Voorwerp is being ionised by an AGN in the centre of IC 2497. In this scenario, a plasma jet associated with the AGN clears a path through the ISM/IGM in the direction of the nebulosity. The large-scale radio continuum emission possibly originates from the interaction between this jet and the large cloud complex in which Hanny's Voorwerp is embedded. The $\ion{H}{i}$ kinematics do not fit regular rotation, thus the cloud complex around IC 2497 is probably of tidal origin. From the $\ion{H}{i}$ absorption against the central source, we derive a lower limit of 2.8 ± 0.4 $\times$ $10^{21}~{\rm atoms}~{\rm cm}^{-2}$ to the $\ion{H}{i}$ column density. However, assuming non-standard conditions for the detected gas, we cannot exclude the possibility that the AGN in the centre of IC 2497 is Compton-thick.
We use radio, near-IR, optical, and X-ray observations to examine dynamic processes in the central region of the rich galaxy cluster Abell 2125. In addition to the central triple of E and cD ...galaxies, including members of both major dynamical subsystems identified from a redshift survey, this region features a galaxy showing strong evidence for ongoing gas stripping during a high-velocity passage through the gas in the cluster core. The disk galaxy C153 exhibits a plume stretching toward the cluster center seen in soft X-rays by Chandra, parts of which are also seen in O II emission and near-UV continuum light. Hubble Space Telescope imaging shows a distorted disk with star-forming knots asymmetrically distributed and remnant spiral structure possibly defined by dust lanes. The stars and ionized gas in this disk are kinematically decoupled, demonstrating that pressure stripping (possibly turbulent, as well as ram) must be important, and that tidal disruption is not the only mechanism at work. Comparison of the gas properties seen in the X-ray and optical data on the plume highlight significant and poorly known features of the history of stripped gas in the intracluster medium that could be clarified through further observations of this system. The nucleus of C153 also hosts an active galactic nucleus, shown by the weak and distorted extended radio emission and a radio-compact core. The unusual strength of the stripping signatures in this instance is likely related to the very high relative velocity of the galaxy with respect to the intracluster medium during a cluster-cluster merger and its passage very near the core of the cluster. An additional sign of recent dynamical events is the diffuse starlight asymmetrically placed about the central triple in a cD envelope. Transient and extreme dynamical events as seen in A2125 may be important drivers of galaxy evolution in the cores of rich clusters.
We present optical and near-infrared integral field spectroscopy of the SCUBA galaxy SMMJ163650.43+405734.5 (ELAIS N2 850.4) at z = 2.385. We combine Lyα and Hα emission line maps and velocity ...structure with high-resolution HST ACS and NICMOS imaging to probe the complex dynamics of this vigorous starburst galaxy. The imaging data show a complex morphology, consisting of at least three components separated by ∼1 arcsec (8 kpc) in projection. When combined with the Hα velocity field from UKIRT UIST IFU observations we identify two components whose redshifts are coincident with the systemic redshift, measured from previous CO observations, one of which shows signs of AGN activity. A third component is offset by 220 ± 50 km s−1 from the systemic velocity. The total star-formation rate of the whole system (estimated from the narrow-line Hα and uncorrected for reddening) is 340 ± 50 M⊙ yr−1. The Lyα emission mapped by the GMOS IFU covers the complete galaxy and is offset by +270 ± 40 km s−1 from the systemic velocity. This velocity offset is comparable to that seen in rest-frame UV-selected galaxies at similar redshifts and usually interpreted as a starburst-driven wind. The extended structure of the Lyα emission suggests that this wind is not a nuclear phenomenon, but is instead a galactic-scale outflow. Our observations suggest that the vigorous activity in N2 850.4 is arising as a result of an interaction between at least two dynamically-distinct components, resulting in a strong starburst, a starburst-driven wind and actively-fuelled AGN activity. Whilst these observations are based on a single object, our results clearly show the power of combining optical and near-infrared integral field spectroscopy to probe the power sources, masses and metallicities of far-infrared luminous galaxies, as well as understanding the role of AGN- and starburst-driven feedback processes in these high-redshift systems.
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