arXiv subjects

A. von der Linden

Publications and source records attributed to A. von der Linden.

51 records · Page 3Linked to original sources

arXiv2014

The origin of cold gas in giant elliptical galaxies and its role in fueling radio-mode AGN feedback

We present a multi-wavelength study of the interstellar medium in eight nearby, X-ray and optically bright, giant elliptical galaxies. Using Herschel PACS, we map the cold gas in the lines of [CII], [OI], and [OIb]. Additionally, we present Ha+[NII] imaging of warm ionized gas with the SOAR telescope, and a study of the hot X-ray emitting plasma with Chandra. All systems with extended Ha emission in our sample (6/8 galaxies) display significant [CII] line emission indicating the presence of cold gas. This emission is co-spatial with the Ha+[NII] emitting nebulae and the lowest entropy X-ray emitting plasma. The entropy profiles of the hot galactic atmospheres show a clear dichotomy, with the systems displaying extended emission line nebulae having lower entropies beyond r~1 kpc than the cold-gas-poor systems. We show that while the hot atmospheres of the cold-gas-poor galaxies are thermally stable outside of their innermost cores, the atmospheres of the cold-gas-rich systems are prone to cooling instabilities. This result indicates that the cold gas is produced chiefly by thermally unstable cooling from the hot phase. We show that cooling instabilities may develop more easily in rotating systems and discuss an alternative condition for thermal instability for this case. The hot atmospheres of cold-gas-rich galaxies display disturbed morphologies indicating that the accretion of clumpy multiphase gas in these systems may result in variable power output of the AGN jets, potentially triggering sporadic, larger outbursts. In the two cold-gas-poor, X-ray morphologically relaxed galaxies of our sample, NGC 1399 and NGC 4472, powerful AGN outbursts may have destroyed or removed most of the cold gas from the cores, allowing the jets to propagate and deposit most of their energy further out, increasing the entropy of the hot galactic atmospheres and leaving their cores relatively undisturbed.

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arXiv2013

X-ray Bright Active Galactic Nuclei in Massive Galaxy Clusters II: The Fraction of Galaxies Hosting Active Nuclei

We present a measurement of the fraction of cluster galaxies hosting X-ray bright Active Galactic Nuclei (AGN) as a function of clustercentric distance scaled in units of $r_{500}$. Our analysis employs high quality Chandra X-ray and Subaru optical imaging for 42 massive X-ray selected galaxy cluster fields spanning the redshift range of $0.2 < z < 0.7$. In total, our study involves 176 AGN with bright ($R <23$) optical counterparts above a $0.5-8.0$ keV flux limit of $10^{-14} \rm{erg} \ \rm{cm}^{-2} \ \rm{s}^{-1}$. When excluding central dominant galaxies from the calculation, we measure a cluster-galaxy AGN fraction in the central regions of the clusters that is $\sim 3 $ times lower that the field value. This fraction increases with clustercentric distance before becoming consistent with the field at $\sim 2.5 r_{500}$. Our data exhibit similar radial trends to those observed for star formation and optically selected AGN in cluster member galaxies, both of which are also suppressed near cluster centers to a comparable extent. These results strongly support the idea that X-ray AGN activity and strong star formation are linked through their common dependence on available reservoirs of cold gas.

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arXiv2013

Spectroscopic Needs for Imaging Dark Energy Experiments: Photometric Redshift Training and Calibration

Large sets of objects with spectroscopic redshift measurements will be needed for imaging dark energy experiments to achieve their full potential, serving two goals:_training_, i.e., the use of objects with known redshift to develop and optimize photometric redshift algorithms; and_calibration_, i.e., the characterization of moments of redshift (or photo-z error) distributions. Better training makes cosmological constraints from a given experiment stronger, while highly-accurate calibration is needed for photo-z systematics not to dominate errors. In this white paper, we investigate the required scope of spectroscopic datasets which can serve both these purposes for ongoing and next-generation dark energy experiments, as well as the time required to obtain such data with instruments available in the next decade. Large time allocations on kilo-object spectrographs will be necessary, ideally augmented by infrared spectroscopy from space. Alternatively, precision calibrations could be obtained by measuring cross-correlation statistics using samples of bright objects from a large baryon acoustic oscillation experiment such as DESI. We also summarize the additional work on photometric redshift methods needed to prepare for ongoing and future dark energy experiments.

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arXiv2013

Probing the extreme realm of AGN feedback in the massive galaxy cluster, RX J1532.9+3021

We present a detailed Chandra, XMM-Newton, VLA and HST analysis of one of the strongest cool core clusters known, RX J1532.9+3021 (z=0.3613). Using new, deep 90 ks Chandra observations, we confirm the presence of a western X-ray cavity or bubble, and report on a newly discovered eastern X-ray cavity. The total mechanical power associated with these AGN-driven outflows is (22+/-9)*10^44 erg/s, and is sufficient to offset the cooling, indicating that AGN feedback still provides a viable solution to the cooling flow problem even in the strongest cool core clusters. Based on the distribution of the optical filaments, as well as a jet-like structure seen in the 325 MHz VLA radio map, we suggest that the cluster harbours older outflows along the north to south direction. The jet of the central AGN is therefore either precessing or sloshing-induced motions have caused the outflows to change directions. There are also hints of an X-ray depression to the north aligned with the 325 MHz jet-like structure, which might represent the highest redshift ghost cavity discovered to date. We further find evidence of a cold front (r=65kpc) that coincides with the outermost edge of the western X-ray cavity and the edge of the radio mini-halo. The common location of the cold front with the edge of the radio mini-halo supports the idea that the latter originates from electrons being reaccelerated due to sloshing induced turbulence. Alternatively, its coexistence with the edge of the X-ray cavity may be due to cool gas being dragged out by the outburst. We confirm that the central AGN is highly sub-Eddington and conclude that a >10^10M_Sun or a rapidly spinning black hole is favoured to explain both the radiative-inefficiency of the AGN and the powerful X-ray cavities.

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arXiv2013

Measuring cosmic distances with galaxy clusters

In addition to cosmological tests based on the mass function and clustering of galaxy clusters, which probe the growth of cosmic structure, nature offers two independent ways of using clusters to measure cosmic distances. The first uses measurements of the X-ray emitting gas mass fraction, which is an approximately standard quantity, independent of mass and redshift, for the most massive clusters. The second uses combined millimeter (mm) and X-ray measurements of cluster pressure profiles. We review these methods, their current status and the prospects for improvements over the next decade. For the first technique, which currently provides comparable dark energy constraints to type Ia supernova studies, improvements of a factor of 6 or more should be readily achievable, together with tight constraints on the mean matter density that are largely independent of the cosmological model assumed. Realizing this potential will require a coordinated, multiwavelength approach, utilizing new cluster surveys, X-ray, optical and mm facilities, and a continued emphasis on improved hydrodynamical simulations.

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arXiv2013

The Hot and Energetic Universe: The evolution of galaxy groups and clusters

Major astrophysical questions related to the formation and evolution of structures, and more specifically of galaxy groups and clusters, will still be open in the coming decade and beyond: what is the interplay of galaxy, supermassive black hole, and intergalactic gas evolution in the most massive objects in the Universe - galaxy groups and clusters? What are the processes driving the evolution of chemical enrichment of the hot diffuse gas in large-scale structures? How and when did the first galaxy groups in the Universe, massive enough to bind more than 10^7 K gas, form? Focussing on the period when groups and clusters assembled (0.5<z<2.5), we show that, due to the continuum and line emission of this hot intergalactic gas at X-ray wavelengths, Athena+, combining high sensitivity with excellent spectral and spatial resolution, will deliver breakthrough observations in view of the aforementioned issues. Indeed, the physical and chemical properties of the hot intra-cluster gas, and their evolution across time, are a key to understand the co-evolution of galaxy and supermassive black hole within their environments.

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arXiv2013

The nature of filamentary cold gas in the core of the Virgo Cluster

We present a multi-wavelength study of the emission-line nebulae located southeast of the nucleus of M87, the central dominant galaxy of the Virgo Cluster. We report the detection of far-infrared (FIR) [CII] line emission from the nebulae using observations made with Herschel PACS. The infrared line emission is extended and cospatial with optical Hα+[NII], far-ultraviolet CIV lines, and soft X-ray emission. The filamentary nebulae evidently contain multi-phase material spanning a temperature range of at least 5 orders of magnitude, from ~100 K to ~10^7 K. This material has most likely been uplifted by the AGN from the center of M87. The thermal pressure of the 10^4 K phase appears to be significantly lower than that of the surrounding hot intra-cluster medium (ICM) indicating the presence of additional turbulent and magnetic pressure in the filaments. If the turbulence in the filaments is subsonic then the magnetic field strength required to balance the pressure of the surrounding ICM is B~30-70 μG. The spectral properties of the soft X-ray emission from the filaments indicate that it is due to thermal plasma with kT~0.5-1 keV, which is cooling by mixing with the cold gas and/or radiatively. Charge exchange can be ruled out as a significant source of soft X-rays. Both cooling and mixing scenarios predict gas with a range of temperatures. This is at first glance inconsistent with the apparent lack of X-ray emitting gas with kT<0.5 keV. However, we show that the missing very soft X-ray emission could be absorbed by the cold gas in the filaments with an integrated hydrogen column density of ~1.6x10^21 cm^-2, providing a natural explanation for the apparent temperature floor to the X-ray emission at kT~0.5 keV. The FIR through ultra-violet line emission is most likely primarily powered by the ICM particles penetrating the cold gas following a shearing induced mixing process.

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arXiv2012

X-ray Bright Active Galactic Nuclei in Massive Galaxy Clusters I: Number Counts and Spatial Distribution

We present an analysis of the X-ray bright point source population in 43 massive clusters of galaxies observed with the Chandra X-ray Observatory. We have constructed a catalog of 4210 rigorously selected X-ray point sources in these fields, which span a survey area of 4.2 square degrees. This catalog reveals a clear excess of sources when compared to deep blank-field surveys, which amounts to roughly 1 additional source per cluster, likely Active Galactic Nuclei (AGN) associated with the clusters. The excess sources are concentrated within the virial radii of the clusters, with the largest excess observed near the cluster centers. The average radial profile of the excess X-ray sources of the cluster are well described by a power law (N(r) ~ r^β) with an index of β~ -0.5. An initial analysis using literature results on the mean profile of member galaxies in massive X-ray selected clusters indicates that the fraction of galaxies hosting X-ray AGN rises with increasing clustercentric radius, being approximately 5 to 10 times higher near the virial radius than in the central regions. This trend is qualitatively similar to that observed for star formation in cluster member galaxies.

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arXiv2011

Spatial kinematics of Brightest Cluster Galaxies and their close companions from Integral Field Unit spectroscopy

We present Integral Field Unit (IFU) spectroscopy of four brightest cluster galaxies (BCGs) at z~0.1. Three of the BCGs have close companions within a projected radius of 20 kpc and one has no companion within that radius. We calculate the dynamical masses of the BCGs and their companions to be 1.4x10^11<M_dyn (M_solar)<1.5x10^12. We estimate the probability that the companions of the BCGs are bound using the observed masses and velocity offsets. We show that the lowest mass companion (1:4) is not bound while the two nearly equal mass (1:1.45 and 1:1.25) companions are likely to merge with their host BCGs in 0.35 Gyr in major, dry mergers. We conclude that some BCGs continue to grow from major merging even at z~0. We analyse the stellar kinematics of these systems using the λ_R parameter developed by the SAURON team. This offers a new and unique means to measure the stellar angular momentum of BCGs and make a direct comparison to other early-type galaxies. The BCGs and their companions have similar ellipticities to those of other early-type galaxies but are more massive. We find that not all these massive galaxies have low λ_R_e as one might expect. One of the four BCGs and the two massive companions are found to be fast-rotating galaxies with high angular momentum, thereby providing a new test for models of galaxy evolution and the formation of Intra-Cluster Light.

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arXiv2010

Feedback under the microscope II: heating, gas uplift, and mixing in the nearest cluster core

Using a combination of deep 574ks Chandra data, XMM-Newton high-resolution spectra, and optical Halpha+NII images, we study the nature and spatial distribution of the multiphase plasma in M87. Our results provide direct observational evidence of `radio mode' AGN feedback in action, stripping the central galaxy of its lowest entropy gas and preventing star-formation. This low entropy gas was entrained with and uplifted by the buoyantly rising relativistic plasma, forming long "arms". These arms are likely oriented within 15-30 degrees of our line-of-sight. The mass of the uplifted gas in the arms is comparable to the gas mass in the approximately spherically symmetric 3.8 kpc core, demonstrating that the AGN has a profound effect on its immediate surroundings. The coolest X-ray emitting gas in M87 has a temperature of ~0.5 keV and is spatially coincident with Halpha+NII nebulae, forming a multiphase medium where the cooler gas phases are arranged in magnetized filaments. We place strong upper limits of 0.06 Msun/yr on the amount of plasma cooling radiatively from 0.5 keV and show that a uniform, volume-averaged heating mechanism could not be preventing the cool gas from further cooling. All of the bright Halpha filaments appear in the downstream region of the <3 Myr old shock front, at smaller radii than ~0.6'. We suggest that shocks induce shearing around the filaments, thereby promoting mixing of the cold gas with the ambient hot ICM via instabilities. By bringing hot thermal particles into contact with the cool, line-emitting gas, mixing can supply the power and ionizing particles needed to explain the observed optical spectra. Mixing of the coolest X-ray emitting plasma with the cold optical line emitting filamentary gas promotes efficient conduction between the two phases, allowing non-radiative cooling which could explain the lack of X-ray gas with temperatures under 0.5 keV.

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arXiv2009

Photometric redshifts and cluster tomography in the ESO Distant Cluster Survey

This paper reports the results obtained on the photometric redshifts measurement and accuracy, and cluster tomography in the ESO Distant Cluster Survey (EDisCS) fields. Photometric redshifts were computed using two independent codes (Hyperz and G. Rudnick's code). The accuracy of photometric redshifts was assessed by comparing our estimates with the spectroscopic redshifts of ~1400 galaxies in the 0.3<z<1.0 domain. The accuracy for galaxies fainter than the spectroscopic control sample was estimated using a degraded version of the photometric catalog for the spectroscopic sample. The accuracy of photometric redshifts is typically sigma(Delta z/(1+z)) ~ 0.05+/-0.01, depending on the field, the filter set, and the spectral type of the galaxies. The quality of the photometric redshifts degrades by a factor of two in sigma(Delta z/(1+z)) between the brightest (I~22) and the faintest (I~24-24.5) galaxies in the EDisCS sample. The photometric determination of cluster redshifts in the EDisCS fields using a simple algorithm based on photoz is in excellent agreement with the spectroscopic values (delta(z) ~0.03-0.04 in the high-z sample and ~0.05 in the low-z sample). We also developed a method that uses both photz codes jointly to reject interlopers at magnitudes fainter than the spectroscopic limit. When applied to the spectroscopic sample, this method rejects ~50-90% of all spectroscopically confirmed non-members, while retaining at least 90% of all confirmed members. Photometric redshifts are found to be particularly useful for the identification and study of clusters of galaxies in large surveys. They enable efficient and complete pre-selection of cluster members for spectroscopy, allow accurate determinations of the cluster redshifts, and provide a means of determining cluster membership. (Abridged)

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arXiv2008

The evolution of the brightest cluster galaxies since z~1 from the ESO Distant Cluster Survey (EDisCS)

[Abridged] We present K-band data for the brightest cluster galaxies (BCGs) from the ESO Distant Cluster Survey. These data are combined with photometry from Aragon-Salamanca et al. (1998) and a low-redshift comparison sample from von der Linden et al. (2007). The K-band Hubble diagram for BCGs exhibits very low scatter (~0.35mag) since z=1. The colour and $K$-band luminosity evolution of the BCGs are in good agreement with passively-evolving stellar populations formed at z>2. We do not detect any significant change in the stellar mass of the BCG since z~1. These results do not seem to depend on the velocity dispersion of the parent cluster. There is a correlation between the 1D velocity dispersion of the clusters and the K-band luminosity of the BCGs (after correcting for passive evolution). The clusters with large velocity dispersions tend to have brighter BCGs, i.e., BCGs with larger stellar masses. This dependency, although significant, is relatively weak: the stellar mass of the BCGs changes only by ~70% over a two-order-of-magnitude range in cluster mass. This dependency doesn't change significantly with redshift. The models of De Lucia & Blaizot (2007) predict colours which are in reasonable agreement with the observations because the growth in stellar mass is dominated by the accretion of old stars. However, the stellar mass in the model BCGs grows by a factor of 3-4 since z=1, a growth rate which seems to be ruled out by the observations. The models predict a dependency between the BCG's stellar mass and the velocity dispersion of the parent cluster in the same sense as the data, but the dependency is significantly stronger than observed. However, one major difficulty in this comparison is that we have measured fixed metric aperture magnitudes while the models compute total luminosities.

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arXiv2007

On the prevalence of radio-loud AGN in brightest cluster galaxies: implications for AGN heating of cooling flows

(Abridged) The prevalence of radio-loud AGN activity in present-day massive halos is determined using a sample of 625 nearby groups and clusters from the SDSS. Brightest group and cluster galaxies (BCGs) are more likely to host a radio-loud AGN than other galaxies of the same stellar mass (by less than a factor of two at a stellar mass of 5e11 M_sun, but over an order of magnitude below 1e11 M_sun). The distribution of radio luminosities for BCGs does not depend on stellar mass, however, and is similar to that of field galaxies of the same mass. Neither the radio-loud fraction nor the radio luminosity distribution of BCGs depends strongly on the cluster velocity dispersion. The radio-AGN fraction is also studied as a function of clustercentric distance: only within 0.2 r_200 do cluster galaxies exhibit an enhanced likelihood of radio-loud AGN activity, approaching that of the BCGs. The radio-loud AGN properties of both BCGs and non-BCGs can naturally be explained if this activity is fuelled by cooling from hot gas surrounding the galaxy. Using observational estimates of the mechanical output of the radio jets, the time-averaged energy output associated with recurrent radio source activity is estimated for all group/cluster galaxies. Within the cooling radius of the cluster, the radio-mode heating from the BCG dominates over that of all other galaxies combined. The scaling between total radio-AGN energy output and cluster velocity dispersion is considerably shallower than the sigma^4 scaling of the radiative cooling rate. Thus, unless either the mechanical-to-radio luminosity ratio or the efficiency of converting AGN mechanical energy into heating increases by 2-3 orders of magnitude between groups and rich clusters, radio-mode heating will not balance cooling in systems of all masses.

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arXiv2005

The evolution of the star formation activity in galaxies and its dependence on environment

We study how the proportion of star-forming galaxies evolves between z=0.8 and z=0 as a function of galaxy environment, using the [OII] line in emission as a signature of ongoing star formation. Our high-z dataset comprises 16 clusters, 10 groups and another 250 galaxies in poorer groups and the field at z=0.4-0.8 from the ESO Distant Cluster Survey, plus another 9 massive clusters at similar redshifts. As a local comparison, we use samples of galaxy systems selected from the Sloan Digital Sky Survey at 0.04< z < 0.08. At high-z most systems follow a broad anticorrelation between the fraction of star-forming galaxies and the system velocity dispersion. At face value, this suggests that at z=0.4-0.8 the mass of the system largely determines the proportion of galaxies with ongoing star formation. At these redshifts the strength of star formation (as measured by the [OII] equivalent width) in star-forming galaxies is also found to vary systematically with environment. Sloan clusters have much lower fractions of star-forming galaxies than clusters at z=0.4-0.8 and, in contrast with the distant clusters, show a plateau for velocity dispersions $ \ge 550 km s^-1$, where the fraction of galaxies with [OII] emission does not vary systematically with velocity dispersion. We quantify the evolution of the proportion of star-forming galaxies as a function of the system velocity dispersion and find it is strongest in intermediate-mass systems (sigma ~ 500-600 km s^-1 at z=0). To understand the origin of the observed trends, we use the Press-Schechter formalism and the Millennium Simulation and show that galaxy star formation histories may be closely related to the growth history of clusters and groups. We propose a scheme that is able to account for the observed relations between the star-forming fraction and σ[abridged].

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arXiv2005

The dark clump near Abell 1942: dark matter halo or statistical fluke?

Weak lensing surveys provide the possibility of identifying dark matter halos based on their total matter content rather than just the luminous matter content. On the basis of two sets of observations carried out with the CFHT, Erben et al. (2000) presented the first candidate dark clump, i.e. a dark matter concentration identified by its significant weak lensing signal without a corresponding galaxy overdensity or X-ray emission. We present a set of HST mosaic observations which confirms the presence of an alignment signal at the dark clump position. The signal strength, however, is weaker than in the ground-based data. It is therefore still unclear whether the signal is caused by a lensing mass or is just a chance alignment. We also present Chandra observations of the dark clump, which fail to reveal any significant extended emission. A comparison of the ellipticity measurements from the space-based HST data and the ground-based CFHT data shows a remarkable agreement on average, demonstrating that weak lensing studies from high-quality ground-based observations yield reliable results.

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