arXiv subjects

A. Humphrey

Publications and source records attributed to A. Humphrey.

63 records · Page 4Linked to original sources

arXiv2007

Deep spectroscopy of the FUV-optical emission lines from a sample of radio galaxies at z~2.5: metallicity and ionization

We present long-slit NIR spectra, obtained using the ISAAC instrument at the Very Large Telescope, for nine radio galaxies at z~2.5. One-dimensional spectra have been extracted and cross calibrated with optical spectra from the literature to produce line spectra spanning a rest wavelength of ~1200-7000 A. We have also produced a composite of the rest-frame UV-optical line fluxes of powerful, z~2.5 radio galaxies. We have investigated the relative strengths of Ly-alpha, H-beta, H-alpha, HeII 1640 and HeII 4687, and we find that Av can vary significantly from object to object. In addition, we identify new line ratios to calculate electron temperature: [NeV] 1575/[NeV] 3426, [NeIV] 1602/[NeIV] 2423, OIII] 1663/[OIII] 5008 and [OII] 2471/[OII]3728. We model the emission line spectra and conclude they are best explained by AGN-photoionization with the ionization parameter U varying between objects. Single slab photoionization models are unable to reproduce the high- and the low-ionization lines simultaneously: this may be alleviated either by combining two or more single slab photoionization models with different U, or by using mixed-medium models such as those of Binette, Wilson & Storchi-Bergmann (1996). On the basis of NV/NIV] and NIV]/CIV we argue that shocks make a fractional contribution to the ionization of the EELR. We find that in the EELR of z~2 radio galaxies the N/H abundance ratio is close to its solar value. We conclude that N/H and metallicity do not vary by more than a factor of two in our sample. This is consistent with the idea that massive ellipticals are assembled very early in the history of the universe, and then evolve relatively passively up to the present day.

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arXiv2007

UV and optical emission lines from the z=2.6 radio galaxy 0828+193: spatially resolved measurements

We present an investigation into the spatial variation of the rest-frame UV and optical line and continuum emission along the radio axis of the z=2.6 radio galaxy 0828+193, using long-slit spectra from the Keck II and Subaru telescopes. Line brightnesses, line ratios and electron temperatures are examined, and their relationship with the arm-length asymmetry of the radio source is also investigated. We find that on the side of the nucleus with the shortest radio lobe, the gas covering factor is higher, and the ionization parameter is lower. The contrasts in environmental density required to explain the asymmetries in the line brightness and the radio arm-length asymmetries are in fair agreement with each other. These results add further weight to the conclusion of McCarthy, van Breugel & Kapahi (1991) - lobe distance asymmetries in powerful radio sources are the result of an asymmetry in the environmental density. We also note that the brightness of both the UV and optical continuum emission shows a similar spatial asymmetry to that shown by the line emission. While the UV continuum asymmetry can be wholly explained by the expected asymmetry in the nebular continuum, the optical continuum asymmetry cannot. We argue that, at least at optical wavelengths, the starlight and/or the scattered light must also be strongly spatially asymmetric.

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arXiv2007

VIMOS-VLT spectroscopy of the giant Ly-alpha nebulae associated with three z~2.5 radio galaxies

The morphological and spectroscopic properties of the giant (>60 kpc) Ly-alpha nebulae associated with three radio galaxies at z~2.5 (MRC 1558-003, MRC 2025-218 and MRC 0140-257) have been investigated using integral field spectroscopic data obtained with VIMOS on VLT. The morphologies are varied. The nebula of one source has a centrally peaked, rounded appearance. In the other two objects, it consists of two spatial components. The three nebulae are aligned with the radio axis within <30 deg. The total Ly-alpha luminosities are in the range (0.3-3.4) x 1e44 erg s-1. The Ly-alpha spectral profile shows strong variation through the nebulae, with FWHM values in the range ~400-1500 km s-1 and velocity shifts V~120-600 km s-1. We present an infall model which can explain successfully most Ly-alpha morphological and spectroscopic properties of the nebula associated with MRC 1558-003. This adds further support to our previous conclusion that the _quiescent_ giant nebulae associated with this and other high redshift powerful radio galaxies are in infall. A problem for this model is the difficulty to reproduce the large Ly-alpha FWHM values. We have discovered a giant (~85 kpc) Ly-alpha nebula associated with the radio galaxy MRC 0140-257 at z=2.64. It shows strikingly relaxed kinematics (FWHM< 300 km s-1 and V<120 km s-1), unique among high z (>2) radio galaxies.

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arXiv2006

Ly-alpha excess in high redshift radio galaxies: a signature of star formation

About 54% of radio galaxies at z>3 and 8% of radio galaxies at 2 2) radio galaxies. These Ly-alpha excess objects (LAEs) show Ly-alpha/HeII values consistent with or above standard photoionization model predictions. We show that the most successful explanation is the presence of a young stellar population which provides the extra supply of ionizing photons required to explain the Ly-alpha excess in at least the most extreme LAEs (probably in all of them). The measurement of unusually high Ly-alpha ratios in the extended gas of some high redshift radio galaxies suggests that star formation activity occurs in spatial scales of tens of kpc. We argue that, although the fraction of LAEs may be incompletely determined, both at 2 3, the much larger fraction of LAEs found at z>3 is a genuine redshift evolution and not due to selection effects. Therefore, our results suggest that the radio galaxy phenomenon is more often associated with a massive starburst at z>3 than at z<3.

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arXiv2006

Giant Lyman-alpha nebulae around z>2 radio galaxies: evidence for infall

We present an investigation into the possible relationship between side-to-side asymmetries of powerful radio galaxies at high redshift, with the goal of understanding the geometry, orientation and gas dynamics of these sources. Our sample consists of 11 radio galaxies at 2.3<z<3.6 previously known to have giant, kinematically quiescent nebulae. We identify several correlated asymmetries: on the side of the brightest radio jet and hotspot (i) the redshift of the kinematically quiescent nebula is highest, (ii) Ly-alpha is brighter relative to the other lines and continuum, (iii) the radio spectrum is flattest and (iv) the radio structure has its highest polarization. These asymmetries are not found to be correlated with either the radio arm length asymmetry or the brightness asymmetry of the UV-optical emitting material. The correlation between the radio brightness asymmetry and the radial velocity of the quiescent gas also appears to be present in powerful radio galaxies with 0<z<1. Collectively, these asymmetries are most naturally explained as an effect of orientation, with the quiescent nebulae in infall: this is the first study to distinguish between the rotation, infall, outflow and chaotic motion scenarios for the kinematically quiescent emission line nebulae around powerful active galactic nuclei.

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arXiv2006

Jet-gas interactions in z~2.5 radio galaxies: evolution of the ultraviolet line and continuum emission with radio morphology

We present an investigation into the nature of the jet-gas interactions in a sample of 10 radio galaxies at 2.3<z<2.9 using deep spectroscopy of the UV line and continuum emission obtained at Keck II and the Very Large Telescope. Kinematically perturbed gas, which we have shown to be within the radio structure in previous publications, is always blueshifted with respect to the kinematically quiescent gas, is usually spatially extended, and is usually detected on both sides of the nucleus. In the three objects from this sample for which we are able to measure line ratios for both the perturbed and quiescent gases, we suggest that the former has a lower ionization state than the latter. We propose that the perturbed gas is part of a jet-induced outflow, with dust obscuring the outflowing gas that lies on the far side of the object. The spatial extent of the blueshifted perturbed gas, typically ~35 kpc, implies that the dust is spatially extended at least on similar spatial scales. We also find interesting interrelationships between UV line, UV continuum and radio continuum properties of this sample.

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arXiv2005

VIMOS-VLT and Spitzer observations of a radio galaxy at z=2.5

We present: 1) a kinematic and morphological study of the giant Lya nebula associated with the radio galaxy MRC2104-242 (z = 2.49) based on integral field spectroscopic VIMOS data from VLT; 2) a photometric study of the host (proto?) galaxy based on Spitzer Space Telescope data. The galaxy appears to be embedded in a giant (>120 kpc) gas reservoir that surrounds it completely. The kinematic properties of the nebula suggest that it is a rotating structure, which would imply a lower limit to the dynamical mass of ~3 x 1e11 Msun. An alternate scenario is that the gas is infalling. Such a process would be able to initiate and sustain significant central starburst activity, although it is likely to contribute with less than 10% of the total stellar mass of the galaxy. The near- to mid-IR spectral energy distribution of the radio galaxy suggests the existence of a reddened, E(B-V)=0.4(+/-)0 .1$, evolved stellar population of age >~ 1.8 Gyr and mass 5(+/-)2 x 1e11 Msun. The implied formation redshift is z_f >~ 6. This stellar mass is similar to the stellar masses found for massive early-type galaxies at z~2 in deep, near-infrared surveys.

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arXiv2003

Kinematically quiet halos around z~2.5 radio galaxies. Keck spectroscopy

We present the results of the kinematic study of the extended gas in a sample of 10 high redshift radio galaxies (z~2.5) based on high signal to noise Keck II and VLT long slit spectroscopy. In addition to the typical high surface brightness kinematically perturbed regions (FWHM and velocity shifts >1000 km/s), we find in all objects giant low surface brightness halos which show quieter kinematics with typical emission line FWHM and velocity shifts of several hundred km/s. The giant halos often extend for more than 100 kpc and sometimes beyond the radio structures. Their emission line specta are typical of high redshift active galaxies. Continuum is also often detected. The halos are enriched with heavy elements at tens of kpc from the active nucleus. The quasar continuum is the dominant source of ionization of the quiescent halos along the radio axis. The detection of giant quiescent halos in all objects suggests that they could be a common ingredient of high redshift radio galaxies. The radio galaxies seem to be embedded within the halos. The nature, origin of the halos and the cosmological implications are also discussed.

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arXiv2002

Giant low surface brightness halos in distant radio galaxies: USS0828+193

We present results on the spectroscopic study of the ionized gas in the high redshift radio galaxy USS0828+193 at z=2.57. Thanks to the high S/N of the emission lines in the Keck spectrum, we have been able to perform a detailed kinematic study by means of the spectral decomposition of the emission line profiles. This study reveals the existence of two types of material in this object: a) a low surface brightness component with apparent quiescent kinematics consistent with gravitational motions and b) a perturbed component with rather extreme kinematics. The quiescent halo extends across the entire object for ~80 kpc. It is enriched with heavy elements and apparently ionized by the continuum from the active nucleus. The properties of the quiescent halo and its origin are discussed in this paper. We propose that it could be part of a structure that surrounds the entire object, although its nature is not clear (a rotating disc? low surface brightness satellites? a cooling flow nebula? material ejected in galactic winds? other?).

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