K20 Survey Research Articles

GMASS ultradeep spectroscopy of galaxies at z ~ 2

<i>Aims. <i/>The properties of dust attenuation at rest-frame UV wavelengths are inferred from very high-quality FORS 2 spectra of 78 galaxies from the GMASS survey at redshift . These objects complement a previously investigated sample of 108 UV-luminous, intermediate-mass (i.e., with stellar masses ~) galaxies at similar redshifts, selected from the FDF spectroscopic survey, the K20 survey, and the GDDS. Detection of the broad absorption feature centred on about 2175 Å (“UV bump”) implies that the average UV extinction curve of a galaxy more closely resembles that of the Milky Way (MW) or the Large Magellanic Cloud (LMC), and differs from that of the Small Magellanic Cloud (SMC).<i>Methods. <i/>The shape of the UV extinction curve is constrained by a parametric description of the rest-frame UV continuum with the support of a suite of models combining radiative transfer and stellar population synthesis. The UV bump is further characterised by fitting Lorentzian-like profiles.<i>Results. <i/>Spectra exhibit a significant 2175 Å feature in at least 30% of the cases, especially those suffering from substantial reddening. If attenuation is dominated by dust ejected from the galaxy main body via galactic winds or more localised superwinds, UV extinction curves in-between those of the SMC and LMC characterise UV-luminous, intermediate-mass galaxies at . The fraction of galaxies with extinction curves differing from the SMC one increases, if more dust resides in the galactic plane or dust attenuation depends on stellar age. On average, the width of the manifested UV bumps is about 60% of the values typically measured along sightlines in the LMC and MW. This suggests the presence of dust similar to that found in the “supergiant” shell of ionised filaments LMC 2, close to 30 Dor. The presence of the carriers of the UV bump (probably organic carbon and amorphous silicates) at argues for outflows from asymptotic giant branch (AGB) stars being copious then. Consistent with their higher star-formation rates, the GMASS galaxies with a manifested UV bump are more luminous at rest-frame 8 <i>μ<i/>m, where the emission is dominated by polycyclic aromatic hydrocarbons (also products of AGB stars). In addition, they exhibit larger equivalent widths for prominent UV (metal) absorption features, mostly of interstellar origin, which indicates overall more evolved stellar populations.<i>Conclusions. <i/>We conclude that diversification of the small-size dust component (responsible for the differential extinction at UV wavelengths and the emission at mid-IR wavelengths) has already started in the most evolved star-forming systems at .

Presence of dust with a UV bump in massive, star-forming galaxies at 1 &lt; z &lt; 2.5

Aims. Fundamental properties of the extinction curve, like the slope in the rest-frame ultraviolet (UV) and the presence/absence of a broad absorption excess centred at 2175 A (the UV bump), are investigated for a sample of 108 massive, star-forming galaxies at 1 < z < 2.5, selected from the FDF Spectroscopic Survey, the K20 survey, and the GDDS. Methods. These characteristics are constrained from a parametric description of the UV spectral energy distribution (SED) of a galaxy, as enforced by combined stellar population and radiative transfer models for different geometries, dust/stars configurations, and dust properties. Results. In at least one third of the sample, there is a robust evidence for extinction curves with at least a moderate UV bump. The presence of the carriers of the UV bump is more evident in galaxies with UV SEDs suffering from heavy reddening. We interpret these results as follows. The sample objects possess different mixtures of dust grains and molecules producing extinction curves in between the average ones of the Small and Large Magellanic Cloud, where the UV bump is absent or modest, respectively. Most of the dust embeds the UV-emitting stellar populations or is distributed out of the galaxy mid-plane. Alternatively, even dust with a pronounced UV bump, as for the average Milky-Way extinction curve, can be present and distributed in the galaxy mid-plane. In this case, variations of the continuum scattering albedo with wavelength or an age-dependent extinction are not sufficient to explain the previous trend with reddening. Hence, additional extraplanar dust has to be invoked. The data suggest that the carriers of the UV bump are associated with intermediate-age stellar populations, while they survive in the harshest UV-radiation fields owing to dust self-shielding. Conclusions. The existence of different extinction curves implies that different patterns of evolution and reprocessing of dust exist at high redshift. Ignoring this may produce a non-negligible uncertainty on the star-formation rate estimated from the rest-frame UV.

The evolution of early-type galaxies at z ∼ 1 from the K20 survey

We have performed VLT spectroscopy of an almost complete sample of 18 early-type galaxies with plus two at , selected from the K20 survey, and derived the velocity dispersion for 15+2 of them. By combining these data with HST and VLT images, we study the Fundamental Plane (FP), the Faber–Jackson and the Kormendy relations at . Compared with the local one, the FP at has a remarkably small scatter, and shows both an offset and a rotation, which we interpret in terms of evolution of the mass-to-light ratio, and possibly of the size. We give evidence that the evolution rate depends on galaxy mass, being faster for less massive galaxies. We discuss the possible factors driving the evolution of spheroids and compare our results with the predictions of the hierachical models of galaxy formation.

Massive Galaxies and Extremely Red Objects atz= 1–3 in Cosmological Hydrodynamic Simulations: Near‐Infrared Properties

Recent observations have revealed a population of red massive galaxies at high redshift that are challenging to explain in terms of hierarchical galaxy formation models. We analyze this "massive galaxy problem" with two different types of hydrodynamic simulations—Eulerian total variation diminishing (TVD) and smoothed particle hydrodynamics (SPH)—of a concordance Λ cold dark matter (ΛCDM) universe. We consider two separate but connected aspects of the problem posed by these extremely red objects (EROs): (1) the mass scale of these galaxies and (2) their red colors. We perform spectrophotometric analyses of simulated galaxies in B, z, R, I, Js, Ks, and K filters and compare their near-infrared (near-IR) properties with observations at redshift z = 1-3. We find that the simulated galaxies brighter than the magnitude limit of KVega = 20 mag have stellar masses M⋆ ≳ 1011 h-1 M☉ and a number density of a few times 10-4 h3 Mpc-3 at z ~ 2, in good agreement with the observed number density in the K20 survey. Therefore, our hydrodynamic simulations do not exhibit the mass-scale problem. The answer to the "redness problem" is less clear because of our poor knowledge of the amount of dust extinction in EROs and the uncertain fraction of star-forming EROs. However, our simulations can account for the observed comoving number density of ~1 × 10-4 Mpc-3 at z = 1-2 if we assume a uniform extinction of E(B - V) = 0.4 for the entire population of simulated galaxies. Upcoming observations of the thermal emission of dust in 24 μm by the Spitzer Space Telescope will help to better estimate the dust content of EROs at z = 1-3 and thus to further constrain the star formation history of the universe and theoretical models of galaxy formation.

The K20 survey

The K20 survey is a near infrared-selected, deep () redshift survey targeting galaxies in two independent regions of the sky, the Chandra Deep Field South and the field around the quasar , for a total area of 52 arcmin2. The total Ks-selected sample includes 545 objects. Low-resolution () optical spectra for 525 of them have been obtained with the FORS1/FORS2 spectrographs at the ESO/VLT, providing 501 spectroscopic identifications (including 12 type-1 AGN and 45 stars); consequently, we were able to measure redshifts and identify stars in 96% of the observed objects, whereas the spectroscopic completeness with respect to the total photometrically selected sample is 92% (501/545). The K20 survey is therefore the most complete spectroscopic survey of a near infrared-selected sample to date. The K20 survey contains 444 spectroscopically identified galaxies, covering a redshift range of , with a mean redshift ; excluding the 32 “low-quality” redshifts does not significantly change these values. This paper describes the final K20 spectroscopic catalogue, along with the technique used to determine redshifts, measure the spectral features and characterize the spectra. The classification of the galaxy spectra has been performed according to a simple parametric recipe that uses the equivalent widths of the two main emission lines ([OII]λ3727 and Hα+[NII]) and two continuum indices (the 4000 Å break index, D4000, and a near-UV color index, ). We defined three main spectroscopic classes: red early-type galaxies, blue emission-line galaxies and the intermediate galaxies, which show emission lines but a red continuum. More than 95% of the examined galaxies is included in one of these spectral types and a composite spectrum is built for each of the three galaxy classes. The full spectroscopic catalogue, the reduced individual spectra and the composite spectra are released to the community through the K20 web page (http://www.arcetri.astro.it/~k20/). The blue emission-line and the early-type galaxies have been divided in redshift bins, and the corresponding composite spectra have been built, in order to investigate the evolution of the spectral properties of the K20 galaxies with redshift. The early-type average spectra are remarkable in their similarity, showing only subtle but systematic differences in the D4000 index, which are consistent with the ageing of the stellar population. Conversely, the star-forming galaxies present a significant “blueing” of the optical/near-UV continuum with redshift, although the [OII] equivalent width remains constant (~33 Å) in the same redshift intervals. We reproduce the observed properties with simple, dust-free population synthesis models, suggesting that the high-redshift galaxies are younger and more active than those detected at lower redshift, whilst the equivalent width of the emission lines apparently require a lower metallicity for the low-redshift objects. This may be consistent with the metallicity-luminosity relationship locally observed for star-forming galaxies.

A New Photometric Technique for the Joint Selection of Star‐forming and Passive Galaxies at 1.4 ≲z≲ 2.5

A simple two-color selection based on B-, z-, and K-band photometry is proposed for culling galaxies at 1.4 z 2.5 in K-selected samples and classifying them as star-forming or passive systems. The method is calibrated on the highly complete spectroscopic redshift database of the K20 survey, verified with simulations and tested on other data sets. Requiring BzK = (z - K)AB - (B - z)AB > -0.2 allows us to select actively star-forming galaxies at z 1.4, independently of their dust reddening. On the other hand, objects with BzK 2.5 colors include passively evolving galaxies at z 1.4, often with spheroidal morphologies. Simple recipes to estimate the reddening, star formation rates (SFRs), and masses of BzK-selected galaxies are derived and are calibrated on K < 20 galaxies. These K < 20 galaxies have typical stellar masses of ~1011 M☉ and sky and volume densities of ~1 arcmin-2 and ~10-4 Mpc-3, respectively. Based on their UV (reddening-corrected), X-ray, and radio luminosities, the BzK-selected star-forming galaxies with K < 20 turn out to have average SFR ≈ 200 M☉ yr-1 and median reddening E(B - V) ~ 0.4. This SFR is a factor of 10 higher than that of z ~ 1 dusty extremely red objects, and a factor of 3 higher than found for z ~ 2 UV-selected galaxies, both at similar K limits. Besides missing the passively evolving galaxies, the UV selection appears to miss some relevant fraction of the z ~ 2 star-forming galaxies with K < 20, and hence of the (obscured) SFR density at this redshift. The high SFRs and masses add to other existing evidence that these z = 2 star-forming galaxies may be among the precursors of z = 0 early-type galaxies. A V/Vmax test suggests that such a population may be increasing in number density with increasing redshift. Theoretical models cannot reproduce simultaneously the space density of both passively evolving and highly star-forming galaxies at z = 2. In view of Spitzer Space Telescope observations, an analogous technique based on RJL photometry is proposed to complement the BzK selection and to identify massive galaxies at 2.5 z 4.0. By selecting passively evolving galaxies as well as actively star-forming galaxies (including strongly dust-reddened ones), these color criteria should help in completing the census of the stellar mass and of the SFR density at high redshift.

Metal Enrichment in Near-Infrared Luminous Galaxies at z ~ 2: Signatures of Proto-elliptical Galaxies?

We present the analysis of the co-added rest-frame UV spectrum (1200 A < z < 2000 A) of five K-luminous galaxies at z ~2 from the K20 survey. The composite spectrum is characterized by strong absorption lines over the UV continuum from C, N, O, Al, Si, and Fe in various ionization stages. While some of these lines are interstellar, several among the strongest absorptions are identified with stellar photospheric lines. Most of the photospheric and interstellar features are stronger in the K-luminous composite spectrum than in Lyman break galaxies at z ~3. This suggests higher metallicity and possibly also larger interstellar velocity dispersion caused by macroscopic motions. The absorption lines and the slope of the UV continuum are well matched by the spectrum of the nearby luminous infrared galaxy NGC 6090, which is in the process of merging. A metallicity higher than solar is suggested by comparing the pure photospheric lines (Si III, C III, and Fe V) with starburst models. The evidence of high metallicity, together with the high masses, high star formation rates, and possibly strong clustering, well qualify these galaxies as progenitors of local massive elliptical galaxies.

Near-Infrared Bright Galaxies at z 2. Entering the Spheroid Formation Epoch?

Spectroscopic redshifts have been measured for nine K-band luminous galaxies at 1.7 < z < 2.3, selected with Ks < 20 in the K20 survey region of the Great Observatories Origins Deep Survey (GOODS) area. Star formation rates (SFRs) of ~100-500 M? yr-1 are derived when dust extinction is taken into account. The fitting of their multicolor spectral energy distributions indicates stellar masses of M 1011 M? for most of the galaxies. Their rest-frame UV morphology is highly irregular, suggesting that merging-driven starbursts are going on in these galaxies. Morphologies tend to be more compact in the near-IR, a hint for the possible presence of older stellar populations. Such galaxies are strongly clustered, with seven out of nine belonging to redshift spikes, which indicates a correlation length of r0 ~ 9-17 h-1 Mpc (1 ? range). Current semianalytical models of galaxy formation appear to underpredict by a large factor (30) the number density of such a population of massive and powerful starburst galaxies at z ~ 2. The high masses and SFRs, together with the strong clustering, suggest that at z ~ 2 we may have started to explore the major formation epoch of massive early-type galaxies.

Tracing the Large‐Scale Structure in theChandraDeep Field South

We report the discovery of large-scale structures of X-ray sources in the 1 Ms observation of the Chandra Deep Field South. Two main structures appear as narrow (?z 0.02) spikes in the source redshift distribution at z = 0.67 and 0.73, respectively. Their angular distribution spans a region at least ~17' wide, corresponding to a physical size of 7.3 h Mpc at a redshift of z ~ 0.7 (?m = 0.3, ?? = 0.7). These spikes are populated by 19 sources each, which are mainly identified as active galactic nuclei (AGNs). Two sources in each spike are extended in X-rays, corresponding to galaxy groups/clusters embedded in larger structures. The X-ray source redshift distribution shows other spikes, the most remarkable at z = 1.04, 1.62, and 2.57. This is one of the first evidences for large-scale structure traced by X-ray sources and for spatial clustering of X-ray-selected AGNs. The X-ray data have been complemented with the spectroscopic data from the K20 near-infrared survey (Cimatti et al.), which covers ~1/10 of the X-ray field. In this survey, too, the source redshift distribution shows several spikes. Two narrow structures at z = 0.67 and 0.73 (again with ?z ~ 0.02) are the most significant features, containing 24 and 47 galaxies, respectively. While the K20 structure at z = 0.73 is dominated by a standard galaxy cluster with a significant concentration around a central cD galaxy and morphological segregation, the galaxies at z = 0.67 constitute a loose structure rather uniformly distributed along the K20 field. Moreover, we find a very good correlation (almost one-to-one) between less prominent peaks detected in the redshift distributions of X-ray and K20 sources. In particular, at z < 1.3 we find that five out of the six more significant K20 peaks have a corresponding peak in the X-ray-selected sources and, similarly, all five X-ray peaks below that redshift have a corresponding K20 peak. Since the K20 survey sensitivity drops beyond z ~ 1.3, structures at higher redshift are traced only by the X-ray sources. This correlation suggests that AGNs (from the X-ray data) and (early-type) galaxies (from the K20 survey) are tracing the same underlying structures. We also compared the X-ray and K-band catalogs to search for enhanced X-ray activity in the sources in the two main redshift spikes. While in the structure at z = 0.73 the fraction of X-ray sources is the same as in the field, in the structure at z = 0.67 it is higher by a factor of ~2, suggesting that X-ray activity may be triggered preferentially in the structure at z = 0.67. Given the limited statistics, this result is significant only at the ~2 ? level.

The K20 survey. V. The evolution of the near-IR Luminosity Function

We present the galaxy rest-frame near-IR Luminosity Function (LF) and its cosmic evolution to z 1:5 based on a spectroscopic survey of a magnitude limited sample of galaxies with Ks < 20 (the K20 survey, Cimatti et al. 2002b). The LFs have been derived in the rest-frame J and Ks bands. Their evolution is traced using three dierent redshift bins (zmean ' 0:5; 1; 1:5) and comparing them to the Local near-IR Luminosity Function. The luminosity functions at dierent redshifts are fairly well fitted by Schechter functions at z< 1:3. The faint-end of the LFs (L< L) is consistent with the local estimates, with no evidence for a change either in the slope or normalization up to z< 1:3. At higher redshift this part of the luminosity function is not well sampled by our data. Viceversa, the density of luminous galaxies ( MKs 5l ogh70< 25:5) is higher than locally at all redshifts and relatively constant or mildly increasing with redshift within our sample. The data are consistent with a mild luminosity evolutionboth in the J -a ndKs -band up toz' 1:5, with an amplitude of aboutMJ ' 0:69 0:12 and MK ' 0:54 0:12 at z 1. Pure density evolution is not consistent with the observed LF at z 1. Moreover, we find that red and early-type galaxies dominate the bright-end of the LF , and that their number density shows at most a small decrease (<30%) up to z' 1, thus suggesting that massive elliptical galaxies were already in place at z' 1a ndthey should have formed their stars and assembled their mass at higher redshift.There appears to be a correlation of the optical/near-IR colors with near-IR luminosities, the most luminous/massive galaxies being red/old, the low-luminous galaxies being instead dominated by blue young stellar populations. We also investigate the evolution of the near-IR comoving luminosity density to z' 1:5, finding a slow evolution with redshift ((z)=(z= 0)(1+z) () with(J)' 0:70 and(Ks)' 0:37). Finally, we compare the observed LFs with the predictions of a set of the most updated hierarchical merging models. Such a comparison shows that the current versions of hierarchical models overpredict significantly the density of low luminosity galaxies at z 1 and underpredict the density of luminous galaxies at z 1, whereas passive evolution models are more consistent with the data up to z 1:5. The GIF model (Kaufmann et al. 1999) shows a clear deficiency of red luminous galaxies at z 1 compared to our observations and predicts a decrease of luminous galaxies with redshift not observed in our sample.

The formation and evolution of field massive galaxies

The problem of the formation and evolution of field massive galaxies is briefly reviewed from an observational perspective. The motivations and the characteristics of the K20 survey are outlined. The redshift distribution of Ks<20 galaxies, the evolution of the rest-frame Ks-band luminosity function and luminosity density to z=1.5, the nature and the role of the red galaxy population are presented. Such results are compared with the predictions of models of galaxy evolution.

The K20 survey

The K20 survey is an ESO VLT optical and near-infrared spectroscopic survey aimed at obtaining spectral infor- mation and redshifts of a complete sample of about 550 objects to Ks 20:0 over two independent fields with a total area of 52 arcmin 2 . In this paper we discuss the scientific motivation of such a survey, we describe the photometric and spectroscopic properties of the sample, and we release the Ks-band photometric catalog. Extensive simulations showed that the sample is photometrically highly complete to Ks= 20. The observed galaxy counts and the R Ks color distribution are consistent with literature results. We observed spectroscopically 94% of the sample, reaching a spectroscopic redshift identification complete- ness of 92% to Ks 20:0 for the observed targets, and of 87% for the whole sample (i.e. counting also the unobserved targets). Deep spectroscopy was complemented with multi-band deep imaging in order to derive tested and reliable photometric red- shifts for the galaxies lacking spectroscopic redshifts. The results show a very good agreement between the spectroscopic and the photometric redshifts with = 0:01 and with a dispersion ofz= 0:09. Using both the spectroscopic and the photometric redshifts, we reached an overall redshift completeness of about 98%. The size of the sample, the redshift complete- ness, the availability of high quality photometric redshifts and multicolor spectral energy distributions make the K20 survey database one of the most complete samples available to date for constraining the currently competing scenarios of galaxy formation and for a variety of other galaxy evolution studies.

The K20 survey

We compare the 3D clustering of old passively-evolving and dusty star-forming z~1 EROs from the K20 survey. With detailed simulations of clustering, the comoving correlation length of dusty star-forming EROs is constrained to be less than r_0~2.5 Mpc/h. In contrast, the old EROs are much more positively correlated, with 5.5<~r_0/(Mpc/h)<~16, consistent with previous claims for z~1 field early-type galaxies based on analyses of ERO angular clustering. The low level of clustering of dusty star-forming EROs does not support these to be major mergers building up an elliptical galaxy, or typical counterparts of SCUBA sources, but it is instead consistent with the weak clustering of high redshift blue galaxies and of luminous local IRAS galaxies. Current hierarchical merging models can explain the large r_0 for z~1 field early-type galaxies, but fail in matching their high number density and overall old ages.