arXiv subjects

C. Leipski

Publications and source records attributed to C. Leipski.

13 recordsLinked to original sources

arXiv2015

The unification of powerful quasars and radio galaxies and their relation to other massive galaxies

The unification model for powerful radio galaxies and radio-loud quasars postulates that these objects are intrinsically the same but viewed along different angles. Herschel Space Observatory data permit the assessment of that model in the far-infrared spectral window. We analyze photometry from Spitzer and Herschel for the distant 3CR hosts, and find that radio galaxies and quasars have different mid-infrared, but indistinguishable far-infrared colors. Both these properties, the former being orientation dependent and the latter orientation invariant, are in line with expectations from the unification model. Adding powerful radio-quiet active galaxies and typical massive star-forming galaxies to the analysis, we demonstrate that infrared colors not only provide an orientation indicator, but can also distinguish active from star-forming galaxies.

astro-ph.GA↗

arXiv2015

Star formation in z>1 3CR host galaxies as seen by Herschel

We present Herschel (PACS and SPIRE) far-infrared (FIR) photometry of a complete sample of z>1 3CR sources, from the Herschel GT project The Herschel Legacy of distant radio-loud AGN (PI: Barthel). Combining these with existing Spitzer photometric data, we perform an infrared (IR) spectral energy distribution (SED) analysis of these landmark objects in extragalactic research to study the star formation in the hosts of some of the brightest active galactic nuclei (AGN) known at any epoch. Accounting for the contribution from an AGN-powered warm dust component to the IR SED, about 40% of our objects undergo episodes of prodigious, ULIRG-strength star formation, with rates of hundreds of solar masses per year, coeval with the growth of the central supermassive black hole. Median SEDs imply that the quasar and radio galaxy hosts have similar FIR properties, in agreement with the orientation-based unification for radio-loud AGN. The star-forming properties of the AGN hosts are similar to those of the general population of equally massive non-AGN galaxies at comparable redshifts, thus there is no strong evidence of universal quenching of star formation (negative feedback) within this sample. Massive galaxies at high redshift may be forming stars prodigiously, regardless of whether their supermassive black holes are accreting or not.

astro-ph.GA↗

arXiv2014

Spectral Energy Distributions of QSOs at z>5: common AGN-heated dust and occasionally strong star-formation

We present spectral energy distributions (SEDs) of 69 QSOs at z>5, covering a rest frame wavelength range of 0.1mu to ~80mu, and centered on new Spitzer and Herschel observations. The detection rate of the QSOs with Spitzer is very high (97% at lambda_rest ~< 4mu), but drops towards the Herschel bands with 30% detected in PACS (rest frame mid-infrared) and 15% additionally in the SPIRE (rest frame far-infrared; FIR). We perform multi-component SED fits for Herschel-detected objects and confirm that to match the observed SEDs, a clumpy torus model needs to be complemented by a hot (~1300K) component and, in cases with prominent FIR emission, also by a cold (~50K) component. In the FIR detected cases the luminosity of the cold component is on the order of 10^13 L_sun which is likely heated by star formation. From the SED fits we also determine that the AGN dust-to-accretion disk luminosity ratio declines with UV/optical luminosity. Emission from hot (~1300K) dust is common in our sample, showing that nuclear dust is ubiquitous in luminous QSOs out to redshift 6. However, about 15% of the objects appear under-luminous in the near infrared compared to their optical emission and seem to be deficient in (but not devoid of) hot dust. Within our full sample, the QSOs detected with Herschel are found at the high luminosity end in L_UV/opt and L_NIR and show low equivalent widths (EWs) in H_alpha and in Ly_alpha. In the distribution of H_alpha EWs, as determined from the Spitzer photometry, the high-redshift QSOs show little difference to low redshift AGN.

astro-ph.GA↗

arXiv2013

Complete infrared spectral energy distributions of mm detected quasars at z>5

We present Herschel far-infrared (FIR) photometry of eleven quasars at redshift z>5 that have previously been detected at 1.2mm. We perform full spectral energy distribution (SED) fits over the wavelength range lambda_rest ~0.1-400mu for those objects with good Herschel detections. These fits reveal the need for an additional far-infrared (FIR) component besides the emission from a dusty AGN-powered torus. This additional FIR component has temperatures of T_FIR ~ 40-60K with luminosities of L_(8-1000mu) ~ 10^13 L_sun (accounting for 25-60% of the bolometric FIR luminosity). If the FIR dust emission is due to star formation it would suggest star formation rates in excess of 1000 solar masses per year. We show that at long wavelengths (lambda_rest > 50mu) the contribution of the AGN-powered torus emission is negligible. This explains how previous FIR studies of high-redshift quasars that relied on single component fits to (ground-based) observations at lambda_obs > 350mu reached T_FIR and L_FIR values similar to our complete SED fits. Stacking the Herschel data of four individually undetected sources reveals a significant average signal in the PACS bands but not in SPIRE. The average SED of sources with individual Herschel detections shows a striking surplus in near- and mid-infrared emission when compared to common AGN templates. The comparison between two average SEDs (sources with and without individual Herschel detections) matched in the UV/optical indicates that for these objects the strength of the MIR emission may correlate with the strength of the FIR emission.

astro-ph.CO↗

arXiv2012

The dust emission of high-redshift quasars

The detection of powerful near-infrared emission in high redshift (z>5) quasars demonstrates that very hot dust is present close to the active nucleus also in the very early universe. A number of high-redshift objects even show significant excess emission in the rest frame NIR over more local AGN spectral energy distribution (SED) templates. In order to test if this is a result of the very high luminosities and redshifts, we construct mean SEDs from the latest SDSS quasar catalogue in combination with MIR data from the WISE preliminary data release for several redshift and luminosity bins. Comparing these mean SEDs with a large sample of z>5 quasars we could not identify any significant trends of the NIR spectral slope with luminosity or redshift in the regime 2.5 < z < 6 and 10^45 < nuL_nu(1350AA) < 10^47 erg/s. In addition to the NIR regime, our combined Herschel and Spitzer photometry provides full infrared SED coverage of the same sample of z>5 quasars. These observations reveal strong FIR emission (L_FIR > 10^13 L_sun) in seven objects, possibly indicating star-formation rates of several thousand solar masses per year. The FIR excess emission has unusally high temperatures (T ~ 65 K) which is in contrast to the temperature typically expected from studies at lower redshift (T ~ 45 K). These objects are currently being investigated in more detail.

astro-ph.CO↗

arXiv2011

Quantifying the anisotropy in the infrared emission of powerful AGN

We use restframe near- and mid-IR data of an isotropically selected sample of quasars and radio galaxies at 1.0 \leq z \leq 1.4, which have been published previously, to study the wavelength-dependent anisotropy of the IR emission. For that we build average SEDs of the quasar subsample (= type 1 AGN) and radio galaxies (= type 2 AGN) from ~1-17 μm and plot the ratio of both average samples. From 2 to 8 μm restframe wavelength the ratio gradually decreases from 20 to 2 with values around 3 in the 10μm silicate feature. Longward of 12μm the ratio decreases further and shows some high degree of isotropy at 15 μm (ratio ~1.4). The results are consistent with upper limits derived from the X-ray/mid-IR correlation of local Seyfert galaxies. We find that the anisotropy in our high-luminosity radio-loud sample is smaller than in radio-quiet lower-luminosity AGN which may be interpreted in the framework of a receding torus model with luminosity-dependent obscuration properties. It is also shown that the relatively small degree of anisotropy is consistent with clumpy torus models.

astro-ph.CO↗

arXiv2010

Mid-infrared spectroscopy of high-redshift 3CRR sources

Using the Spitzer Space Telescope, we have obtained rest frame 9-16mu spectra of 11 quasars and 9 radio galaxies from the 3CRR catalog at redshifts 1.0<z<1.4. This complete flux-limited 178MHz-selected sample is unbiased with respect to orientation and therefore suited to study orientation-dependent effects in the most powerful active galactic nuclei (AGN). The mean radio galaxy spectrum shows a clear silicate absorption feature (tau_9.7mu = 1.1) whereas the mean quasar spectrum shows silicates in emission. The mean radio galaxy spectrum matches a dust-absorbed mean quasar spectrum in both shape and overall flux level. The data for individual objects conform to these results. The trend of the silicate depth to increase with decreasing core fraction of the radio source further supports that for this sample, orientation is the main driver for the difference between radio galaxies and quasars, as predicted by AGN unification. However, comparing our high-z sample with lower redshift 3CRR objects reveals that the absorption of the high-z radio galaxy MIR continuum is lower than expected from a scaled up version of lower luminosity sources, and we discuss some effects that may explain these trends.

astro-ph.CO↗

arXiv2010

Herschel/PACS far-infrared photometry of two z>4 quasars

We present Herschel far-infrared (FIR) observations of two sub-mm bright quasars at high redshift: SDSS J1148+5251 (z=6.42) and BR 1202-0725 (z=4.69) obtained with the PACS instrument. Both objects are detected in the PACS photometric bands. The Herschel measurements provide additional data points that constrain the FIR spectral energy distributions (SEDs) of both sources, and they emphasise a broad range of dust temperatures in these objects. For lambda_rest ~< 20mu, the two SEDs are very similar to the average SEDs of quasars at low redshift. In the FIR, however, both quasars show excess emission compared to low-z QSO templates, most likely from cold dust powered by vigorous star formation in the QSO host galaxies. For SDSS J1148+5251 we detect another object at 160mu with a distance of ~10 arcseconds from the QSO. Although no physical connection between the quasar and this object can be shown with the available data, it could potentially confuse low-resolution measurements, thus resulting in an overestimate of the FIR luminosity of the z=6.42 quasar.

astro-ph.CO↗

arXiv2009

The Spitzer view of FR-I radio galaxies: on the origin of the nuclear mid-infrared continuum

We present Spitzer MIR spectra of 25 FR-I radio galaxies and investigate the nature of their MIR continuum emission. MIR spectra of star-forming galaxies and quiescent elliptical galaxies are used to identify host galaxy contributions while radio/optical core data are used to isolate the nuclear non-thermal emission. Out of the 15 sources with detected optical compact cores, four sources are dominated by emission related to the host galaxy. Another four sources show signs of warm, nuclear dust emission: 3C15, 3C84, 3C270, and NGC 6251. It is likley that these warm dust sources result from hidden AGN of optical spectral type 1. The MIR spectra of seven sources are dominated by synchrotron emission, with no significant component of nuclear dust emission. In parabolic SED fits of the non-thermal cores FR-Is tend to have lower peak frequencies and stronger curvature than blazars. This is roughly consistent with the common picture in which the core emission in FR-Is is less strongly beamed than in blazars.

astro-ph.CO↗

arXiv2008

Nuclear activity in nearby galaxies: Mid-infrared imaging with the VLT

Dust enshrouded activity can ideally be studied by mid-infrared (MIR) observations. In order to explore the AGN versus star forming origin of the nuclear MIR emission of galaxies, observations of high spatial resolution are required. Here we report on 11.3mic. observations with VISIR at the VLT, reaching 0.35'' spatial resolution (FWHM). During the scientific verification of VISIR we have observed a sample of 36 nearby galaxies having a variety of optically classified nuclear activity: 17 black hole driven active galactic nuclei (AGN), 10 starbursts (SBs) and 9 quiet spirals. 16/17 AGN are detected and unresolved, 5/10 SBs are detected and resolved with structured emission up to a few arcsec, while for 5/10 SB and all 9 quiet nuclei low upper limits are provided. The morphology of the resolved SB nuclei follows that seen at radio frequencies. The compactness of AGN and the extent of the SB nuclei is consistent with predictions from radiative transfer models and with MIR spectra of lower spatial resolution. We explore the nuclear MIR surface brightness as a quantitative measure. While AGN and SB cannot be distinguished with MIR data from 4m class telescopes, our data provide evidence that, up to a distance of 100 Mpc, AGN and SB can well be separated by means of MIR surface brightness when using 8m class telescopes.

astro-ph↗

arXiv2007

The reddest ISO-2MASS quasar

In the course of the NIR/MIR AGN search combining the 6.7 mu ISOCAM Parallel Survey and 2MASS we have discovered 24 type-1 quasars about a third of which are too red to be discriminated by optical/UV search techniques. Here we report on a detailed case study of the reddest type-1 quasar of our sample (J2341) at redshift z=0.236 with M_K=-25.8 and J-K=1.95. We performed spectroscopy in the optical with VLT/FORS1 and in the MIR with Spitzer as well as NIR imaging with ISPI at CTIO. The optical and NIR observations reveal a star forming emission-line galaxy at the same redshift as the quasar with a projected linear separation of 1.8 arcsec (6.7 kpc). The quasar and its companion are embedded in diffuse extended continuum emission. Compared with its companion the quasar exhibits redder optical-NIR colours, which we attribute to hot nuclear dust. The MIR spectrum shows only few emission lines superimposed on a power-law spectral energy distribution. However, the lack of strong FIR emission suggests that our potentially interacting object contains much less gas and dust and is in a stage different from dust reddened ULIRG-AGN like Mrk 231. The optical spectrum shows signatures for reddening in the emission-lines and no post-starburst stellar population is detected in the host galaxy of the quasar. The optical continuum emission of the active nucleus appears absorbed and diluted. Even the combination of absorption and host dilution is not able to match J2341 with standard quasar templates. While the BLR shows only a rather moderate absorption of E_(B-V)=0.3, the continuum shorter than 4500 AA requires strong obscuration with E_(B-V)=0.7, exceeding the constraints from the low upper limit on the 9.7 mu silicate absorption. This leads us to conclude that the continuum of J2341 is intrinsically redder than that of typical quasars.

astro-ph↗

arXiv2007

Narrow-line AGN in the ISO-2MASS Survey

Context: A long-standing challenge of observational AGN research is to find type 2 quasars, the luminous analogues of Seyfert-2 galaxies. Aims: We search for luminous narrow-line type 2 AGN, characterise their properties, and compare them with broad-line type 1 AGN. Methods: Combining the ISOCAM parallel survey at 6.7 mu with 2MASS, we have selected AGN via near-mid-infrared colours caused by the hot nuclear dust emission. We performed spectroscopy in the optical and, for a subset of the sample, also in the mid-infrared with Spitzer. Results: We find nine type 2 AGN at redshift 0.1 0.5 we did not find type 2 AGN, probably because the hottest dust emission, still covered by the NIR filters, is obscured. The optical spectra of the type 2 host galaxies show young and old stellar populations. Only one object is an ultraluminous infrared galaxy with starburst. The 5-38 mu spectra of the two type 2 sources observed show a strong continuum with PAH emission in one case and silicate absorption in the other case. Conclusions: The near-mid-infrared selection is a successful strategy to find luminous type 2 AGN at low z. The objects exhibit a large range of properties so that it is difficult to infer details by means of popular SED fitting with simple average templates.

astro-ph↗

arXiv2005

The ISO-2MASS AGN survey: On the type-1 sources

We combined the ISOCAM Parallel Mode Survey at 6.7 micron (LW2 filter) with the Two Micron All Sky Survey in order to obtain a powerful tool to search for AGN independent of dust extinction. Using moderate colour criteria H-K>0.5 and K-LW2>2.7 we have selected a sample of 77 AGN candidates in an effective area of about 10 square degrees. By means of optical spectroscopy we find 24 (30%) type-1 QSOs at redshifts 0.1 0.8. About one third of the ISO-2MASS QSOs show so red optical colours, that they are missed in optical and UV AGN surveys like SDSS, 2DF, or HES. With a surface density of about 2 deg^(-2) down to R<18 mag the ISO-2MASS QSOs outnumber the 1.35 deg^(-2) of the SDSS quasar survey by 50%; we find a combined optical-IR QSO surface density of 2.7 deg^(-2). Since only two of the ISO-2MASS QSOs have also J-K>2, the inclusion of the ISO mid-infrared photometry significantly extends the capabilities of the pure 2MASS red AGN survey. We suggest that the newly found red AGN resemble young members of the quasar population, and that quasars spend much of their lifetime in a dust enshrouded phase.

astro-ph↗