Galaxy formation and evolution

Abstract:

We measured stellar velocity dispersions σ and derived dynamical masses of nine massive (M ≈ 1011 M ⊙) early-type galaxies (ETGs) from the Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS) sample at redshift 1.4 ≲ z ≲ 2.0. The σ are based on individual spectra for two galaxies at z ≈ 1.4 and on a stacked spectrum for seven galaxies with 1.6 < z < 2.0, with 202 hr of exposure at the ESO Very Large Telescope. We constructed detailed axisymmetric dynamical models for the objects, based on the Jeans equations, taking the observed surface brightness (from deep HST/ACS observations), point-spread function, and slit effects into account. Our dynamical masses M Jeans agree within ≲30% with virial estimates M vir = 5 × Reσ2/ G, although the latter tend to be smaller. Our M Jeans also agrees within a factor ≲2 with the M pop previously derived using stellar population models and 11 bands photometry. This confirms that the galaxies are intrinsically massive. The inferred mass-to-light ratios (M/L) U in the very age-sensitive rest-frame U band are consistent with passive evolution in the past 1 Gyr (formation redshift zf ∼3). A "bottom-light" stellar initial mass function appears to be required to ensure close agreement between M Jeans and M pop at z ∼2, as it does at z ∼0. The GMASS ETGs are on average more dense than their local counterpart. However, a few percent of local ETGs of similar dynamical masses also have comparable σ and mass surface density Σ50 inside R e. © 2009. The American Astronomical Society. All rights reserved.

Abstract:

We report the discovery of an unusual object near the spiral galaxy IC 2497, discovered by visual inspection of the Sloan Digital Sky Survey (SDSS) as part of the Galaxy Zoo project. The object, known as Hanny's Voorwerp, is bright in the SDSS g band due to unusually strong [O iii]4959, 5007 emission lines. We present the results of the first targeted observations of the object in the optical, ultraviolet and X-ray, which show that the object contains highly ionized gas. Although the line ratios are similar to extended emission-line regions near luminous active galactic nucleus (AGN), the source of this ionization is not apparent. The emission-line properties, and lack of X-ray emission from IC 2497, suggest either a highly obscured AGN with a novel geometry arranged to allow photoionization of the object but not the galaxy's own circumnuclear gas, or, as we argue, the first detection of a quasar light echo. In this case, either the luminosity of the central source has decreased dramatically or else the obscuration in the system has increased within 10 5 yr. This object may thus represent the first direct probe of quasar history on these time-scales. © 2009 RAS.

Abstract:

We present mass models of a sample of 14 spiral and 14 S0 galaxies that constrain their stellar and dark matter content. For each galaxy, we derive the stellar mass distribution from near-infrared photometry under the assumptions of axisymmetry and a constant KS-band stellar mass-to-light ratio. To this we add a dark halo assumed to follow a spherically symmetric Navarro, Frenk and White profile and a correlation between concentration and dark mass within the virial radius, MDM. We solve the Jeans equations for the corresponding potential under the assumption of constant anisotropy in the meridional plane, βz. By comparing the predicted second velocity moment to observed long-slit stellar kinematics, we determine the three best-fitting parameters of the model: and βz. These simple axisymmetric Jeans models are able to accurately reproduce the wide range of observed stellar kinematics, which typically extend to ≈2-3Re or, equivalently, ≈0.5-1R25. Although our sample contains barred galaxies, we argue a posteriori that the assumption of axisymmetry does not significantly bias our results. We find a median stellar mass-to-light ratio at KS-band of with an rms scatter of 0.31. We present preliminary comparisons between this large sample of dynamically determined stellar mass-to-light ratios and the predictions of stellar population models. The stellar population models predict slightly lower mass-to-light ratios than we measure. The mass models contain a median of 15 per cent dark matter by mass within an effective radius Re (defined here as the semimajor axis of the ellipse containing half the KS-band light) and 49 per cent within the optical radius R25. Dark and stellar matter contribute equally to the mass within a sphere of radius 4.1Re or 1.0 R25. There is no evidence of any significant difference in the dark matter content of the spirals and S0s in our sample. Models without dark matter are also able to satisfactorily reproduce the observed kinematics in most cases. The improvement when a halo is added is statistically significant, however, and the stellar mass-to-light ratios of mass models with dark haloes match the independent expectations of stellar population models better. © 2009 RAS.

Abstract:

The S3-Tools are a set of Python-based routines and interfaces whose purpose is to provide user-friendly access to the SKA Simulated Skies (S3) set of simulations, an effort led by the University of Oxford in the framework of the European Union's SKADS program (http://www.skads-eu.org). The databases built from the S3 simulations are hosted by the Oxford e-Research Center (OeRC), and can be accessed through a web portal at http://s-cubed.physics.ox.ac.uk. This paper focuses on the practical steps involved to make radio images from the S3-SEX and S3-SAX simulations using the S 3-Map tool and should be taken as a broad overview. For a more complete description, the interested reader should look up the user's guide. The output images can then be used as input to instrument simulators, e.g. to assess technical designs and observational strategies for the SKA and SKA pathfinders.

Abstract:

We present results on low-resolution mid-infrared (MIR) spectra of 70 IR-luminous galaxies obtained with the infrared spectrograph (IRS) onboard Spitzer. We selected sources from the European Large Area Infrared Survey with S15 > 0.8 mJy and photometric or spectroscopic z > 1. About half of the samples are quasi-stellar objects (QSOs) in the optical, while the remaining sources are galaxies, comprising both obscured active galactic nuclei (AGN) and starbursts. Redshifts were obtained from optical spectroscopy, photometric redshifts and the IRS spectra. The later turn out to be reliable for obscured and/or star-forming sources, thus becoming an ideal complement to optical spectroscopy for redshift estimation. We estimate monochromatic luminosities at several rest-frame wavelengths, equivalent widths and luminosities for the polycyclic aromatic hydrocarbon (PAH) features, and strength of the silicate feature in individual spectra. We also estimate integrated 8-1000 μm IR luminosities via spectral energy distribution fitting to MIR and far-IR (FIR) photometry from the Spitzer Wide-Area Infrared Extragalactic survey and the MIR spectrum. Based on these measurements, we classify the spectra using well-known IR diagnostics, as well as a new one that we propose, into three types of source: those dominated by an unobscured AGN, mostly corresponding to optical quasars (QSOs), those dominated by an obscured AGN and starburst-dominated sources. Starbursts concentrate at z ∼ 0.6-1.0 favoured by the shift of the 7.7-μm PAH band into the selection 15-μm band, while AGN spread over the 0.5 < z < 3.1 range. Star formation rates (SFR) are estimated for individual sources from the luminosity of the PAH features. An estimate of the average PAH luminosity in QSOs and obscured AGN is obtained from the composite spectrum of all sources with reliable redshifts. The estimated mean SFR in the QSOs is 50-100 M⊙ yr-1, but the implied FIR luminosity is 3-10 times lower than that obtained from stacking analysis of the FIR photometry, suggesting destruction of the PAH carriers by energetic photons from the AGN. The SFR estimated in obscured AGN is two to three times higher than in QSOs of similar MIR luminosity. This discrepancy might not be due to luminosity effects or selection bias alone, but could instead indicate a connection between obscuration and star formation. However, the observed correlation between silicate absorption and the slope of the NIR to MIR spectrum is compatible with the obscuration of the AGN emission in these sources being produced in a dust torus. © 2009 RAS.