Showing papers by "Tommaso Treu published in 2001"

The properties of field elliptical galaxies at intermediate redshift — II. Photometry and spectroscopy of an HST-selected sample

Abstract: A sample of field early-type galaxies (E/S0) at intermediate redshift (z∼0.1–0.6) is selected, based on morphology and colours from HST-WFPC2 parallel images. Photometric structural parameters (effective radius Re and effective surface brightness SBe) are derived through the F606W and F814W filters, using luminosity profile fitting and two-dimensional fitting techniques. The combined parameter that enters the Fundamental Plane (log Re−βSBe, with β≈0.32) is shown to suffer from significantly smaller uncertainties (rms 0.03) than the individual structural parameters (e.g. ∼15 per cent rms on the effective radius). High signal-to-noise ratio, intermediate-resolution spectra, taken at the ESO 3.6-m telescope, yield redshifts for 35 galaxies and central velocity dispersions for 22 galaxies. Central velocity dispersions are derived using a library of stellar templates covering a wide range of spectral types, in order to study the effects of template mismatches. The average random error on the central velocity dispersion is found to be 8 per cent, and the average systematic error caused by template mismatch is found to be 5 per cent. The errors in the velocity dispersion measurement and the effects of template mismatches are studied by means of extensive Monte Carlo simulations. In addition, we investigate whether the determination of the velocity dispersion is sensitive to the spectral range used, finding that the value of velocity dispersion is unchanged when the spectral regions that include the absorption features Ca H and K and NaD are masked out during the fit.

The properties of field elliptical galaxies at intermediate redshift – III. The Fundamental Plane and the evolution of stellar populations from z≈0.4 to z=0

Abstract: We report on the study of a sample of 25 field early-type galaxies, in the redshift range z≈0.1–0.5, selected on the basis of colours and morphology from the Medium Deep Survey (MDS) of the Hubble Space Telescope (HST). Surface photometry in two colours (F606W and F814W) and redshifts have been derived for all the galaxies in the sample, while velocity dispersions have been measured for 19 of the sample galaxies, as described in a companion paper. Our study of the evolution of the Fundamental Plane (FP) with redshift yields the following results. Field early-type galaxies define a tight FP out to z≈0.4, with scatter unchanged with respect to local samples, within the observational errors. The intermediate redshift FP is offset with respect to the local FP of the Coma cluster, in the sense that, for a given effective radius and velocity dispersion, galaxies are brighter than expected from the local relation. The offset of the FP is found to increase with redshift. The range of parameters covered by our sample is not sufficiently extended to measure the slopes of the FP at intermediate redshift. Similar results are found for the SBe–Re relation, out to z≈0.5. The evolution of the FP (and of the SBe–Re relation) is studied quantitatively with a Bayesian–Monte Carlo technique. By applying this technique, we find that the offset of the intercept of the FP (Δγ) with respect to the local FP increases as Δγ=τz with the following 68 per cent limits: 0.33<τ<0.44 (for Ω=1,ΩΛ=0) or 0.44<τ<0.56 (for Ω=0.3,ΩΛ=0.7). In addition, we interpret the results in terms of the evolution of the stellar populations, under the assumption of passive evolution. In a single-burst scenario, the observed properties are consistent with those of a stellar population formed at z≳2 (for Ω=1,ΩΛ=0, H0=50 km s−1 Mpc−1) or 0.8

Lyα Emitters with Red Colors at z ≃ 2.4

Abstract: We have carried out a search for Ly? emission from galaxies at z 2.4 over a field of 1200 arcmin2 using the CFH12K camera at the Canada-France-Hawaii Telescope and a custom medium-band filter. The search has uncovered 58 candidates, corresponding to a completeness-corrected source density of ~0.07 arcmin-2. Our sources have red colors ( 1.8) that imply either that a large fraction of the light is highly reddened and that we are detecting Ly? emission through special lines of sight or that these objects contain an underlying older stellar population. While for each individual object we cannot discriminate between these alternatives, we conclude that most of the objects actually contain an older component because the star formation rates inferred from the picture based on reddening, applied to all candidates, would imply an exceedingly high star formation rate, i.e., more than 2 orders of magnitude above the peak cosmic star formation rate.

Lyman alpha emitters with red colors at z~=2.4

Abstract: We have carried out a search for Lyman alpha emission from galaxies at z~=24 over a field of 1200 sq arcmin using the CFH12K camera at the CFHT and a custom medium band filter The search has uncovered 58 candidates, corresponding to a completeness-corrected source density of ~007 sq arcmin^-2 Our sources have red colors (B-I~=18) which imply either that a large fraction of the light is highly reddened and we are detecting Lyman alpha through special lines of sight, or that these objects contain an underlying older stellar population While for each individual object we cannot discriminate between these alternatives, we conclude that most of the objects actually contain an older component because the star formation rates inferred from the picture based on reddening, applied to all candidates, would imply an exceedingly high star formation rate, ie more than two orders of magnitude above the peak cosmic star formation rate (eg Lilly et al 1996)

Near-infrared spectroscopy and Hubble Space Telescope imaging of a dusty starburst extremely red object

Abstract: We present near-infrared spectroscopy and Hubble Space Telescope (HST) imaging of ERO J164023+4644, an Extremely Red Object (ERO) with (R-K) = 5.9 at z = 1.05 that has been detected by Infrared Space Observatory (ISO) at 15 μm. ERO J164023 resembles a disk galaxy, with an optical/infrared spectral energy distribution that is strongly reddened by dust (LFIR/LB 200; AV ~ 5). The combination of the narrow width of the emission lines in our spectra (~300 km s-1) and the relatively high [N II]/Hα line ratio indicate that this is a "composite" starburst-Seyfert galaxy. Assuming that star formation dominates the energy output, we constrain the star formation rate to lie in the broad range ~10-700 M☉ yr-1 from a variety of star formation indicators. We compare ERO J164023 with the only other spectroscopically identified dusty EROs: HR10 (z = 1.44) and ISO J1324-2016 (z = 1.50). ERO J164023 and HR10 have similar disklike morphologies in the rest-frame UV, and both exhibit a variation in the apparent dust obscuration depending upon the diagnostic used, which suggests that there is a complex spatial mix of stellar populations and dust in these galaxies. In contrast, the compact morphology and spectral properties of ISO J1324-2016 indicate that it is a dusty quasar. Overall, our results demonstrate that the population of dusty galaxies identified using photometric ERO criteria includes systems ranging from pure starbursts through transition systems, such as ERO J164023, to dusty quasars. We suggest that the classification of EROs into these subclasses, necessary for the detailed modeling of the population, cannot be reliably achieved from optical/near-infrared photometry and instead requires mid/far-infrared or submillimeter photometry and near-infrared spectroscopy. The advent of efficient multiobject spectrographs working in the near-infrared, as well as the imminent launch of SIRTF, therefore promises the opportunity of rapid progress in our understanding of the elusive ERO population.

Near-infrared Spectroscopy and HST Imaging of a Dusty Starburst ERO

Abstract: We present near-IR spectroscopy and HST imaging of EROJ164023, an Extremely Red Object (ERO) with R-K=5.9 at z=1.05. EROJ164023 is a disk galaxy, with an optical/IR spectral energy distribution which is strongly reddened by dust (L_FIR/L_B<~200; A_V~5). The narrow emission lines (~300km/s) and the high [NII]/Halpha line ratio indicate that this is a ``composite'' starburst-Seyfert galaxy. Assuming that star formation dominates, we constrain the SFR to be 10-700Mo/yr from a variety of indicators. We compare EROJ164023 with the only other spectroscopically identified dusty EROs: HR10 (z=1.44) and ISOJ1324-2016 (z=1.50). EROJ164023 and HR10 have similar disk-like morphologies, and both exhibit a variation in the apparent dust obscuration depending upon the diagnostic used, suggesting that there is a complex spatial mix of stellar populations and dust in these galaxies. In contrast, the compact morphology and spectral properties of ISOJ1324-2016 indicate that it is a dusty quasar. Our results demonstrate that dusty galaxies identified using photometric ERO criteria include pure starbursts, composite systems such as EROJ164023 and dusty quasars. We suggest that the classification of EROs into these sub-classes cannot be reliably achieved from optical/near-IR photometry and instead requires mid/far-IR or sub-mm photometry and near-IR spectroscopy. The advent of efficient multi-object spectrographs working in the near-IR as well as the imminent launch of SIRTF therefore promise the opportunity of rapid progress in our understanding of the elusive ERO population. [Abridged]

The evolution of field early-type galaxies to z~0.7

Abstract: We have measured the Fundamental Plane (FP) parameters for a sample of 30 field early-type galaxies (E/S0) in the redshift range 0.1 5\AA in 22% of an enlarged sample of 42 massive E/S0 in the range 0.1

The Morphology of Extremely Red Objects

Abstract: We briefly discuss our survey of Extremely Red Objects (EROs) with HST/NICMOS and deep ground-based images and summarize its results. We use the high-resolution NICMOS images to sub-divide EROs into morphological classes, tentatively arguing that about 60 % of EROs (18 out of 30) are early-type galaxies. About 20 % (6 out of 30 objects) appear to be flattened disk systems. Another 10 % (3 out of 30) have an irregular morphology. Thus, combining disk and irregular systems, our data are compatible with 30 % of EROs being dusty starbursts. A small fraction (3 out of 30) appear point-like even on the HST images and could therefore be either compact high redshift galaxies or stars in the Milky Way halo.