arXiv subjects

K. Jahnke

Publications and source records attributed to K. Jahnke.

At least 343 records · Page 19Linked to original sources

arXiv2006

A wide angle tail radio galaxy in the COSMOS field: evidence for cluster formation

We have identified a complex galaxy cluster system in the COSMOS field via a wide angle tail (WAT) radio galaxy consistent with the idea that WAT galaxies can be used as tracers of clusters. The WAT galaxy, CWAT-01, is coincident with an elliptical galaxy resolved in the HST-ACS image. Using the COSMOS multiwavelength data set, we derive the radio properties of CWAT-01 and use the optical and X-ray data to investigate its host environment. The cluster hosting CWAT-01 is part of a larger assembly consisting of a minimum of four X-ray luminous clusters within ~2 Mpc distance. We apply hydrodynamical models that combine ram pressure and buoyancy forces on CWAT-01. These models explain the shape of the radio jets only if the galaxy's velocity relative to the intra-cluster medium (ICM) is in the range of about 300-550 km/s which is higher than expected for brightest cluster galaxies (BCGs) in relaxed systems. This indicates that the CWAT-01 host cluster is not relaxed, but is possibly dynamically young. We argue that such a velocity could have been induced through subcluster merger within the CWAT-01 parent cluster and/or cluster-cluster interactions. Our results strongly indicate that we are witnessing the formation of a large cluster from an assembly of multiple clusters, consistent with the hierarchical scenario of structure formation. We estimate the total mass of the final cluster to be approximately 20% of the mass of the Coma cluster.

astro-ph↗

arXiv2006

Extended Lyman alpha emission around bright quasars

Quasars trace the most massive structures at high redshifts and their presence may influence the evolution of the massive host galaxies. We study the extended Lyman alpha emission line regions (EELRs) around seven bright, mostly radio-quiet quasars (QSOs) at 2.7<z<4.5, and compare luminosities with EELRs around radio-loud QSOs reported in the literature. Using integral field spectroscopy, we analyse the morphology and kinematics of the quiescent Lya EELRs around the QSOs. We find evidence for the presence of EELRs around four radio-quiet and one radio-loud QSO. All EELRs appear asymmetric and the optically brightest QSOs also have the brightest Lya nebulae. For the two brightest nebulae we find velocities between ~600 km s^-1 at the QSO position to ~200 km s^-1 at a distance of 3-4 arcsec from the QSO and surface flux densities up to 2-3*10^{-16} erg cm^-2 s^-1 arcsec^-2. The five EELRs have total Lya luminosities which correspond to ~0.5% of the luminosities from the QSOs broad Lya emission lines. This fraction is an order of magnitude smaller than found for EELRs around radio-loud, steep spectrum QSOs reported in the literature. While the nebulae luminosities are correlated with the QSO Lya luminosities, we find that nebulae luminosities are not correlated with the central QSO ionising fluxes. The presence of gas in the EELRs can be interpreted based on two competing scenarios: either from quasar feedback mechanisms, or from infalling matter. Apart from these two effects, the Lya flux around radio-loud objects can be enhanced due to interactions with the radio jets. The relatively fainter nebulae around radio-quiet QSOs compared to lobe-dominated radio-loud QSOs can be ascribed to this effect, or to significant differences in the environments between the two classes.

astro-ph↗

arXiv2006

Seeing the sky through Hubble's eye: The COSMOS SkyWalker

Large, high-resolution space-based imaging surveys produce a volume of data that is difficult to present to the public in a comprehensible way. While megapixel-sized images can still be printed out or downloaded via the World Wide Web, this is no longer feasible for images with 10^9 pixels (e.g., the Hubble Space Telescope Advanced Camera for Surveys [ACS] images of the Galaxy Evolution from Morphology and SEDs [GEMS] project) or even 10^10 pixels (for the ACS Cosmic Evolution Survey [COSMOS]). We present a Web-based utility called the COSMOS SkyWalker that allows viewing of the huge ACS image data set, even through slow Internet connections. Using standard HTML and JavaScript, the application successively loads only those portions of the image at a time that are currently being viewed on the screen. The user can move within the image by using the mouse or interacting with an overview image. Using an astrometrically registered image for the COSMOS SkyWalker allows the display of calibrated world coordinates for use in science. The SkyWalker "technique" can be applied to other data sets. This requires some customization, notably the slicing up of a data set into small (e.g., 256^2 pixel) subimages. An advantage of the SkyWalker is the use of standard Web browser components; thus, it requires no installation of any software and can therefore be viewed by anyone across many operating systems.

astro-ph↗

arXiv2006

A jet-cloud interaction in the 3C 196 environment

Powerful radio galaxies and radio-loud quasars at high redshifts are frequently associated with extended emission-line regions (EELRs). Here we investigate the [O II] EELR around the quasar 3C 196 at z=0.871 using integral field spectroscopy. We also detect extended [Ne II] emission at a distance of about 30 kpc from the core. The emission is aligned with the radio hot spots and shows a redshifted and a blueshifted component with a velocity difference of ~800 km s^-1. The alignment effect and large velocities support the hypothesis that the EELR is caused by a jet-cloud interaction, which is furthermore indicated by the presence of a pronounced bend in the radio emission at the location of the radio hot spots. We also report observations of two other systems which do not show as clear indications of interactions. We find a weaker alignment of an [O II] EELR from the z=0.927 quasar 3C 336, while no EELR is found around the core-dominated quasar OI 363 at z=0.63.

astro-ph↗

arXiv2005

GEMS Survey Data and Catalog

We describe the data reduction and object cataloging for the GEMS survey, a large-area (800 arcmin^2) two-band (F606W and F850LP) imaging survey with the Advanced Camera for Surveys on HST, centered on the Chandra Deep Field South.

astro-ph↗

arXiv2005

High-redshift quasar host galaxies with adaptive optics

We present K band adaptive optics observations of three high-redshift (z ~ 2.2) high-luminosity quasars, all of which were studied for the first time. We also bserved several point spread function (PSF) calibrators, non-simultaneously because of the small field of view. The significant temporal PSF variations on timescales of minutes inhibited a straightforward scaled PSF removal from the quasar images. Characterising the degree of PSF concentration by the radii encircling 20% and 80% of the total flux, respectively, we found that even under very different observing conditions the r_20 vs. r_80 relation varied coherently between individual short exposure images, delineating a well-defined relation for point sources. Placing the quasar images on this relation, we see indications that all three objects were resolved. We designed a procedure to estimate the significance of this result, and to estimate host galaxy parameters, by reproducing the statistical distribution of the individual short exposure images. We find in all three cases evidence for a luminous host galaxy, with a mean absolute magnitude of M_R = -27.0 and scale lengths around ~ 4-12 kpc. Together with a rough estimate of the central black hole masses obtained from C_iv line widths, the location of the objects on the bulge luminosity vs. black hole mass relation is not significantly dfferent from the low-redshift regime, assuming only passive evolution of the host galaxy. Corresponding Eddington luminosities are L_nuc/L_Edd ~ 0.1-0.6.

astro-ph↗

arXiv2005

GEMS: which galaxies dominate the z~0.7 ultraviolet luminosity density?

We combine high-resolution images from GEMS with redshifts and spectral energy distributions from COMBO-17 to explore the morphological types of galaxies that dominate the z~0.7 UV luminosity density. We analysed rest-frame 280 nm and V-band luminosities of 1483 galaxies with 0.65<z< 0.75, combining these with visual morphological classifications from F850LP images (approximately rest-frame V-band) taken with HST/ACS on the Extended Chandra Deep Field South. We derive UV luminosity functions and j_280 luminosity densities for spheroid-dominated galaxies, spiral galaxies, Magellanic irregulars, and clearly-interacting galaxies with morphologies suggestive of ongoing major mergers. We check the reliability of GEMS morphologies against the deeper GOODS images and quantify an incompleteness of the GEMS merger identification at the faint end. We derive the fractions of the global UV luminosity density j_280 originating from the galaxy types, and find that spiral galaxies and Magellanic irregulars dominate with about 40% each. Interacting and merging galaxies account for roughly 20% of j_280, while the contribution of early types is negligible. These results imply that the strong decline in the UV luminosity density of the Universe observed from z~1 until today is dominated by the decreasing UV luminosities of normal spiral galaxies, accompanied by the migration of UV-luminous star formation in irregular galaxies to systems of progressively lower mass and luminosity. (abridged)

astro-ph↗

arXiv2004

The merging/AGN connection II. Ionization of the circumnuclear regions

We report the first results of a study of a sample of 20 galaxy mergers/interacting systems, using the VIMOS and PMAS integral field spectrographs. For each object, we extracted the integrated spectrum of the central regions and analyzed the ionization state using classical diagnostic diagrams (Veilleux & Osterbrock 1987). There is evidence of AGN ionization in 4 of the objects, i.e. 20% of the sample, a considerably higher fraction than found in previous studies ~4%

astro-ph↗

arXiv2004

Bar Evolution Over the Last Eight Billion Years: A Constant Fraction of Strong Bars in GEMS

One third of present-day spirals host optically visible strong bars that drive their dynamical evolution. However, the fundamental question of how bars evolve over cosmological times has yet to be addressed, and even the frequency of bars at intermediate redshifts remains controversial. We investigate the frequency of bars out to z~1.0 drawing on a sample of 1590 galaxies from the GEMS survey, which provides morphologies from HST ACS two-color images, and highly accurate redshifts from the COMBO-17 survey. We identify spiral galaxies using the Sersic index, concentration parameter, and rest-frame color. We characterize bars and disks by fitting ellipses to F606W and F850LP images, taking advantage of the two bands to minimize bandpass shifting. We exclude highly inclined (i>60 deg) galaxies to ensure reliable morphological classifications, and apply completeness cuts of M_v <= -19.3 and -20.6. More than 40% of the bars that we detect have semi major axes a<0.5" and would be easily missed in earlier surveys without the small PSF of ACS. The bars that we can reliably detect are fairly strong (with ellipticities e>=0.4) and have a in the range ~1.2-13 kpc. We find that the optical fraction of such strong bars remains at ~(30% +- 6%) from the present-day out to look-back times of 2-6 Gyr (z~0.2-0.7) and 6-8 Gyr (z~0.7-1.0); it certainly shows no sign of a drastic decline at z>0.7. Our findings of a large and similar bar fraction at these three epochs favor scenarios in which cold gravitationally unstable disks are already in place by z~1, and where on average bars have a long lifetime (well above 2 Gyr). The distributions of structural bar properties in the two slices are, however, not statistically identical and therefore allow for the possibility that the bar strengths and sizes may evolve over time.

astro-ph↗

arXiv2004

Abundances and kinematics of a candidate sub-damped Lyman-alpha galaxy toward PHL 1226

The spectrum of the quasar PHL 1226 is known to have a strong Mg II and sub-damped Lyman-alpha (sub-DLA) absorption line system with N(HI)=(5+/-2)*10^19 cm^-2 at z=0.1602. Using integral field spectra from the Potsdam Multi Aperture Spectrophotometer (PMAS) we investigate a galaxy at an impact parameter of 6".4 which is most probably responsible for the absorption lines. A fainter galaxy at a similar redshift and a slightly larger distance from the QSO is known to exist, but we assume that the absorption is caused by the more nearby galaxy. From optical Balmer lines we estimate an intrinsic reddening consistent with 0, and a moderate star formation rate of 0.5 M_sun yr^-1, is inferred from the Halpha luminosity. Using nebular emission line ratios we find a solar oxygen abundance 12+log(O/H)=8.7+/-0.1 and a solar nitrogen to oxygen abundance ratio log(N/O)=-1.0+/-0.2. This abundance is larger than those of all known sub-DLA systems derived from analyses of metal absorption lines in quasar spectra. On the other hand, the properties are compatible with the most metal rich galaxies responsible for strong Mg II absorption systems. These two categories can be reconciled if we assume an abundance gradient similar to local galaxies. Under that assumption we predict abundances 12+log(O/H)=7.1 and log(N/O)=-1.9 for the sub-DLA cloud, which is similar to high redshift DLA and sub-DLA systems. We find evidence for a rotational velocity of ~200 km s^-1 over a length of ~7 kpc. From the geometry and kinematics of the galaxy we estimate that the absorbing cloud does not belong to a rotating disk, but could originate in a rotating halo.

astro-ph↗

arXiv2004

Integral field observations of DLA galaxies

We report preliminary results from a targeted investigation on quasars containing damped Lyman-alpha absorption (DLA) lines as well strong metal absorption lines, carried out with the Potsdam Multi Aperture Spectrophotometer (PMAS). We search for line-emitting objects at the same redshift as the absorption lines and close to the line of sight of the QSOs. We have observed and detected the already confirmed absorbing galaxies in Q2233+131 (z_abs=3.15) and Q0151+045 (z_abs=0.160), while failing to find spectral signatures for the z=0.091 absorber in Q0738+313. From the Q2233+131 DLA galaxy, we have detected extended Ly-alpha emission from an area of 3"*5".

astro-ph↗

arXiv2004

Evolution and Impact of Bars over the Last Eight Billion Years: Early Results from GEMS

Bars drive the dynamical evolution of disk galaxies by redistributing mass and angular momentum, and they are ubiquitous in present-day spirals. Early studies of the Hubble Deep Field reported a dramatic decline in the rest-frame optical bar fraction f_opt to below 5% at redshifts z>0.7, implying that disks at these epochs are fundamentally different from present-day spirals. The GEMS bar project, based on ~8300 galaxies with HST-based morphologies and accurate redshifts over the range 0.2-1.1, aims at constraining the evolution and impact of bars over the last 8 Gyr. We present early results indicating that f_opt remains nearly constant at ~30% over the range z=0.2-1.1,corresponding to lookback times of ~2.5-8 Gyr. The bars detected at z>0.6 are primarily strong with ellipticities of 0.4-0.8. Remarkably, the bar fraction and range of bar sizes observed at z>0.6 appear to be comparable to the values measured in the local Universe for bars of corresponding strengths. Implications for bar evolution models are discussed.

astro-ph↗

arXiv2004

UV light from young stars in GEMS quasar host galaxies at 1.8<z<2.75

We have performed HST imaging of a sample of 23 high-redshift (1.8<z<2.75) AGN, drawn from the COMBO-17 survey. The sample contains moderately luminous quasars (M_B ~ -23). The data are part of the GEMS imaging survey that provides high resolution optical images obtained with ACS in two bands (F606W and F850LP), sampling the rest-frame UV flux of the targets. To deblend the AGN images into nuclear and resolved (host galaxy) components we use a PSF subtraction technique that is strictly conservative with respect to the flux of the host galaxy. We resolve the host galaxies in both filter bands in 9 of the 23 AGN, whereas the remaining 14 objects are considered non-detections, with upper limits of less than 5% of the nuclear flux. However, when we coadd the unresolved AGN images into a single high signal-to-noise composite image we find again an unambiguously resolved host galaxy. The recovered host galaxies have apparent magnitudes of 23.0<F606W<26.0 and 22.5<F850LP<24.5 with rest-frame UV colours in the range -0.2<(F606W-F850LP)_obs<2.3. The rest-frame absolute magnitudes at 200 nm are -20.0<M_200nm<-22.2. The photometric properties of the composite host are consistent with the individual resolved host galaxies. We find that the UV colors of all host galaxies are substantially bluer than expected from an old population of stars with formation redshift z=5, independent of the assumed metallicities. These UV colours and luminosities range up to the values found for Lyman-break galaxies (LBGs) at z=3. Our results agree with the recent discovery of enhanced blue stellar light in AGN hosts at lower redshifts. [abridged]

astro-ph↗

arXiv2004

Colors of AGN host galaxies at 0.5<z<1.1 from GEMS

We present the results from a study of the host galaxies of 15 optically selected AGNs with 0.5<z<1.1 from GEMS. GEMS is a Hubble Space Telescope imaging survey of a ~28"x28" contiguous field centered on the Chandra Deep Field South in the F606W and F850LP filter bands. It incorporates the SEDs and redshifts of ~10000 objects, obtained by the COMBO-17 project. We have detected the host galaxies of all 15 AGNs in the F850LP-band (and 13/15 in the F606W-band), recovering their fluxes, morphologies and structural parameters. We find that 80% of the host galaxies have early-type (bulge-dominated) morphologies, while the rest have structures characteristic of late-type (disk-dominated) galaxies. We find that 25% of the early types, and 30% of the late types, exhibit disturbances consistent with galaxy interactions. The hosts show a wide range of colors, from those of red-sequence galaxies to blue colors consistent with ongoing star formation. Roughly 70% of the morphologically early-type hosts have rest-frame blue colors, a much larger fraction than those typical of non-active morphologically early-type galaxies in this redshift and luminosity range. Yet, we find that the early-type hosts are structurally similar to red-sequence ellipticals, inasmuch as they follow an absolute magnitude versus half-light size correlation that are consistent with the mean relation for early-type galaxies at similar redshifts.

astro-ph↗

arXiv2004

On-axis spectroscopy of the z=0.144 radio-loud quasar HE 1434-1600: an elliptical host with a highly ionized ISM

VLT on-axis optical spectroscopy of the z=0.144 radio-loud quasar HE 1434-1600 is presented. The spatially resolved spectra of the host galaxy are deconvolved and separated from those of the central quasar in order to study the dynamics of the stars and gas as well as the physical conditions of the ISM. We find that the host of HE 1434-1600 is an elliptical galaxy that resides in a group of at least 5 member galaxies, and that most likely experienced a recent collision with its nearest companion. Compared with other quasar host galaxies, HE 1434-1600 has a highly ionized ISM. The ionization state corresponds to that of typical Seyferts, but the ionized regions are not distributed in a homogeneous way around the QSO, and are located preferentially several kiloparsecs away from it. While the stellar absorption lines do not show any significant velocity field, the gas emission lines do. The observed gas velocity field is hard to reconcile with dynamical models involving rotating disk, modified Hubble laws or power laws, that all require extreme central masses (M > 10^9 M_sun) to provide only poor fit to the data. Power law models, which best fit the data, provide a total mass of M(< 10 kpc)=9.2 10^10 M_sun. We conclude that the recent interaction between HE 1434-1600 and its closest companion has strongly affected the gas velocity and ionization state, from the center of the galaxy to its most external parts.

astro-ph↗

arXiv2004

Evolution of optically faint AGN from COMBO-17 and GEMS

We have mapped the AGN luminosity function and its evolution between z=1 and z=5 down to apparent magnitudes of $R<24$. Within the GEMS project we have analysed HST-ACS images of many AGN in the Extended Chandra Deep Field South, enabling us to assess the evolution of AGN host galaxy properties with cosmic time.

astro-ph↗

arXiv2004

Skywalking GEMS and UDF

The two large colour mosaics of the GEMS and UDF projects, both obtained with the Hubble Space Telescope and ACS, consist of large amounts of data. We present two web application pages (the GEMS and UDF "Skywalker") that allow to pan around in these mosaics with downloading only small parts at a time.

astro-ph↗

arXiv2004

Integral Field Spectroscopy of Extended Lyman-alpha Emission from the DLA Galaxy in Q2233+131

This paper presents observations of an extended Lyman-alpha emission nebula surrounding the galaxy responsible for the Damped Lyman-alpha Absorption (DLA) line in the spectrum of the quasar Q2233+131. With the Potsdam Multi Aperture Spectrophotometer (PMAS) we measure the properties of the extended Ly-alpha emission in an area of 3"* 5" having a total line flux of (2.8+/-0.3)*10^{-16} erg cm-2 s-1, which at redshift z=3.15 corresponds to a luminosity of (2.4 -0.2/+0.3)*10^{43} erg s-1 and a size of 23 times 38 kpc. The location of the emission is spatially coincident with the previously detected DLA galaxy, but extends significantly beyond its limb. We argue that the Ly-alpha emission is likely to be caused by an outflow from the DLA galaxy, presumably powered by star formation. In the case of negligible dust extinction, the Ly-alpha luminosity indicates a star-formation rate of 19+/-10 solar masses per year consistent with that derived from the UV continuum flux from the parent galaxy. The wind velocity indicated by the integral field spectra is of the order of several hundred km s-1. We find no indication of emission originating in a rotating disk.

astro-ph↗