We studied the chronology of galactic bulge and disc formation by analysing the relative contributions of these components to the B-band rest-frame luminosity density at different epochs. We present ...the first estimate of the evolution of the fraction of rest-frame B-band light in galactic bulges and discs since redshift z similar to 0.8. We performed a bulge-to-disc decomposition of HST/ACS images of 3266 galaxies in the zCOSMOS-bright survey with spectroscopic redshifts in the range 0.7 \textless= z \textless= 0.9. We find that the fraction of B-band light in bulges and discs is (26 4)% and (74 4)%, respectively. When compared with rest-frame B-band measurements of galaxies in the local Universe in the same mass range (10(9)M circle dot M10(11.5).M circle dot),we find that the B-band light in discs decreases by -30% from z similar to 0.7-0.9 to z 0, while the light from the bulge increases by -30% over the same period of time. We interpret this evolution as the consequence of star formation and mass assembly processes, as well as morphological transformation, which gradually shift stars formed at half the age of the Universe from star-forming late-type/irregular galaxies to earlier types and ultimately into spheroids.
Context. The mass assembly of galaxies can proceed through different physical processes. Here we report on the spectroscopic identification of close physical pairs of galaxies at redshifts 2 less ...than or similar to z \textless 4 and discuss the impact of major mergers in building galaxies at these early cosmological times. Aims. We aim to identify and characterize close physical pairs of galaxies destined to merge and use their properties to infer the contribution of merging processes to the early mass assembly of galaxies. Methods. We searched for galaxy pairs with a transverse separation r(p) \textless= 25 h(-1) kpc and a velocity difference Delta(v) \textless= 500 km s(-1) using early data from the VIMOS Ultra Deep Survey (VUDS) that comprise a sample of 1111 galaxies with spectroscopic redshifts measurements at redshifts 1.8 \textless= z \textless= 4 in the COSMOS. ECDFS, and VVDS-02h fields, combined with VVDS data. We analysed their spectra and associated visible and near-infrared photometry to assess the main properties of merging galaxies that have an average stellar mass M-* = 2.3 x 10(10) M-circle dot at these redshifts. Results. Using the 12 physical pairs found in our sample we obtain a first robust measurement of the major merger fraction at these redshifts, f(MM)= 19.4(-6)(+9)%. These pairs are expected to merge within 1 Gyr on average each producing a more massive galaxy by the time the cosmic star formation peaks at z similar to 1-2. Using the pairs' merging time scales, we derive a merging rate of R-MM = 0.17(-0.05)(+0.08) Gyr(-1). From the average mass ratio between galaxies in the pairs, the stellar mass of the resulting galaxy after merging will be similar to 60% higher than the most massive galaxy in the pair before merging. We conclude that major merging of galaxy pairs is on-going at 2 less than or similar to z \textless 4 and is significantly contributing to the major mass assembly phase of galaxies at this early epoch.
The longest pointed observation (152 ks) with the ROSAT PSPC, in the direction of the absolutely lowest neutral hydrogen column density (5.7 x 10 exp 19/sq cm) is discussed. In addition, 26 shallower ...fields from the ROSAT medium sensitivity survey are analyzed. 1176 X-ray sources have been detected in at least one ROSAT energy band in these fields covering a total solid angle of 9.3 sq deg; 661 of these sources constitute a statistically complete sample detected in the hard band with 0.5-2 keV fluxes greater than 2.5 x 10 exp 15 erg/sq cm s. At the faintest limiting flux of our survey the surface density of X-ray sources is about 413/sq deg. The differential source counts, corrected for systematic effects on the basis of extensive Monte Carlo simulations, are well fitted with two power laws with slopes of 2.7 and 1.9 at bright and faint fluxes, respectively.
Context. The study of large and representative samples of low-metallicity star-forming galaxies at different cosmic epochs is of great interest to the detailed understanding of the assembly history ...and evolution of low-mass galaxies. Aims. We present a thorough characterization of a large sample of 183 extreme emission-line galaxies (EELGs) at redshift 0.11 ≤ z ≤ 0.93 selected from the 20k zCOSMOS bright survey because of their unusually large emission line equivalent widths. Methods. We use multiwavelength COSMOS photometry, HST-ACS I-band imaging, and optical zCOSMOS spectroscopy to derive the main global properties of star-forming EELGs, such as sizes, stellar masses, star formation rates (SFR), and reliable oxygen abundances using both “direct” and “strong-line” methods. Results. The EELGs are extremely compact (r50 ~ 1.3 kpc), low-mass (M∗ ~ 107−1010 M⊙) galaxies forming stars at unusually high specific star formation rates (sSFR ≡ SFR/M⋆ up to 10-7 yr-1) compared to main sequence star-forming galaxies of the same stellar mass and redshift. At rest-frame UV wavelengths, the EELGs are luminous and show high surface brightness and include strong Lyα emitters, as revealed by GALEX spectroscopy. We show that zCOSMOS EELGs are high-ionization, low-metallicity systems, with median 12+log (O/H) = 8.16 ± 0.21 (0.2 Z⊙) including a handful of extremely metal-deficient (<0.1 Z⊙) EELGs. While ~80% of the EELGs show non-axisymmetric morphologies, including clumpy and cometary or tadpole galaxies, we find that ~29% of them show additional low-surface-brightness features, which strongly suggests recent or ongoing interactions. As star-forming dwarfs in the local Universe, EELGs are most often found in relative isolation. While only very few EELGs belong to compact groups, almost one third of them are found in spectroscopically confirmed loose pairs or triplets. Conclusions. The zCOSMOS EELGs are galaxies caught in a transient and probably early period of their evolution, where they are efficiently building up a significant fraction of their present-day stellar mass in an ongoing, galaxy-wide starburst. Therefore, the EELGs constitute an ideal benchmark for comparison studies between low- and high-redshift low-mass star-forming galaxies.
The Vimos VLT Deep Survey Scodeggio, M.; Vergani, D.; Cucciati, O. ...
Astronomy and astrophysics (Berlin),
07/2009, Letnik:
501, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Context. Hierarchical models of galaxy formation predict that the properties of a dark matter halo depend on the large-scale environment surrounding the halo. As a result of this correlation, we ...expect massive haloes to be present in larger number in overdense regions than in underdense ones. Given that a correlation exists between a galaxy stellar mass and the hosting dark matter halo mass, the segregation in dark matter halo mass should then result in a segregation in the distribution of stellar mass in the galaxy population. Aims. In this work we study the distribution of galaxy stellar mass and rest-frame optical color as a function of the large-scale galaxy distribution using the VLT VIMOS Deep Survey sample, in order to verify the presence of segregation in the properties of the galaxy population. Methods. We use VVDS redshift measurements and multi-band photometric data to derive estimates of the stellar mass, rest-frame optical color, and of the large-scale galaxy density, on a scale of approximately 8 Mpc, for a sample of 5619 galaxies in the redshift range $0.2<z<1.4$. Results. We observe a significant mass and optical color segregation over the whole redshift interval covered by our sample, such that the median value of the mass distribution is larger and the rest-frame optical color is redder in regions of high galaxy density. The amplitude of the mass segregation changes little with redshift, at least in the high stellar mass regime that we can uniformly sample over the $0.2 < z < 1.4$ redshift interval. The color segregation, instead, decreases significantly for $z > 0.7$. However, when we consider only galaxies in narrow bins of stellar mass, in order to exclude the effects of stellar mass segregation on galaxy properties, we no longer observe any significant color segregation.
Context. Ultra-deep imaging of small parts of the sky has revealed many populations of distant galaxies, providing insight into the early stages of galaxy evolution. Spectroscopic follow-up has ...mostly targeted galaxies with strong emission lines at z > 2 or concentrated on galaxies at z < 1. Aims. The populations of both quiescent and actively star-forming galaxies at 1 < z < 2 are still under-represented in our general census of galaxies throughout the history of the Universe. In the light of galaxy formation models, however, the evolution of galaxies at these redshifts is of pivotal importance and merits further investigation. In addition, photometry provides only limited clues about the nature and evolutionary status of these galaxies. We therefore designed a spectroscopic observing campaign of a sample of both massive, quiescent and star-forming galaxies at z > 1.4. Methods. To determine redshifts and physical properties, such as metallicity, dust content, dynamical masses, and star formation history, we performed ultra-deep spectroscopy with the red-sensitive optical spectrograph FORS2 at the Very Large Telescope. We first constructed a sample of objects, within the CDFS/GOODS area, detected at 4.5 μm, to be sensitive to stellar mass rather than star formation intensity. The spectroscopic targets were selected with a photometric redshift constraint (z > 1.4) and magnitude constraints (BAB < 26, IAB < 26.5), which should ensure that these are faint, distant, and fairly massive galaxies. Results. We present the sample selection, survey design, observations, data reduction, and spectroscopic redshifts. Up to 30 h of spectroscopy of 174 spectroscopic targets and 70 additional objects enabled us to determine 210 redshifts, of which 145 are at z > 1.4. The redshift distribution is clearly inhomogeneous with several pronounced redshift peaks. From the redshifts and photometry, we deduce that the BzK selection criteria are efficient (82%) and suffer low contamination (11%). Several papers based on the GMASS survey show its value for studies of galaxy formation and evolution. We publicly release the redshifts and reduced spectra. In combination with existing and on-going additional observations in CDFS/GOODS, this data set provides a legacy for future studies of distant galaxies.
Aims. Our aim is to perform the same colour − density analysis on galaxy mock samples as was carried out on a 5 h-1 Mpc scale using the VIMOS-VLT Deep Survey (VVDS), and to compare the results from ...these mock samples with observed data. This allows us to test galaxy evolution in the model and to understand the relation between the studied environment and the underlying dark matter distribution. Methods. We used galaxy mock catalogues with the same flux limits as the VVDS-Deep (IAB ≤ 24) survey (Cmocks), constructed using a semi-analytic model for galaxy evolution applied to the Millennium Simulation. From each Cmock, we extracted a sub-sample of galaxies mimicking the VVDS observational strategy (Omocks). We then computed the B-band luminosity function LF and the colour − density relation in the mock samples using the same methods as employed for the VVDS data. Results. We find that the B-band LF in mock samples roughly agrees with the observed LF, but at 0.2 < z < 0.8 the faint-end slope of the model LF is steeper than the observed one. Computing the LF for early- and late-type galaxies separately, we show that mock samples have an excess of faint early-type galaxies and of bright late-type galaxies compared with the data. We find that the colour − density relation in Omocks agrees excellently with that in Cmocks. This suggests that the VVDS observational strategy does not introduce any severe bias to the observed colour − density relation. At z ~ 0.7, the colour − density relation in mock samples agrees qualitatively with observations, with red galaxies residing preferentially in high densities. However, the strength of the colour − density relation in mock samples does not vary within 0.2 < z < 1.5, while the observed relation flattens with increasing redshift and possibly inverts at z ~ 1.3. We argue that the lack of evolution in the colour − density relation in the model cannot be due only to inaccurate prescriptions for the evolution of satellite galaxies, but indicates that the treatment of the central galaxies has also to be revised. Conclusions. The reversal of the colour − density relation can be explained by wet mergers between young galaxies, producing a starburst event. This should be seen on group scales, where mergers are frequent, with possibly some residual trend on larger scales. This residual is found in observations at z = 1.5 on a scale of ~5 h-1 Mpc, but not in the model, suggesting that the treatment of physical processes influencing both satellites and central galaxies in models should be revised. A detailed analysis would be desirable on small scales as well, which requires flux limits fainter than those of the VVDS data.
This paper aims to perform the same colour-density analysis on galaxy mock samples as was carried out on a 5 ... Mpc scale using the VIMOS-VLT Deep Survey (VVDS), and to compare the results from ...these mock samples with observed data. The authors find that the B-band LF in mock samples roughly agrees with the observed LF, but at 0.2 less than z less than 0.8 the faint-end slope of the model LF is steeper than the observed one. The reversal of the colour-density relation can be explained by wet mergers between young galaxies, producing a starburst event. This should be seen on group scales, where mergers are frequent, with possibly some residual trend on larger scales. This residual is found in observations at ... on a scale of ... Mpc suggesting that the treatment of physical processes influencing both satellites and central galaxies in models should be revised.(ProQuest: ... denotes formulae/symbols omitted.)
PDF
