1,398 research outputs found
On the robustness of the H Lick index as a cosmic clock in passive early-type galaxies
We examine the H Lick index in a sample of massive () and passive early-type galaxies extracted from SDSS at
z<0.3, in order to assess the reliability of this index to constrain the epoch
of formation and age evolution of these systems. We further investigate the
possibility of exploiting this index as "cosmic chronometer", i.e. to derive
the Hubble parameter from its differential evolution with redshift, hence
constraining cosmological models independently of other probes. We find that
the H strength increases with redshift as expected in passive evolution
models, and shows at each redshift weaker values in more massive galaxies.
However, a detailed comparison of the observed index with the predictions of
stellar population synthesis models highlights a significant tension, with the
observed index being systematically lower than expected. By analyzing the
stacked spectra, we find a weak [NII] emission line (not
detectable in the single spectra) which anti-correlates with the mass, that can
be interpreted as a hint of the presence of ionized gas. We estimated the
correction of the H index by the residual emission component exploiting
different approaches, but find it very uncertain and model-dependent. We
conclude that, while the qualitative trends of the observed H-z
relations are consistent with the expected passive and downsizing scenario, the
possible presence of ionized gas even in the most massive and passive galaxies
prevents to use this index for a quantitative estimate of the age evolution and
for cosmological applications.Comment: 20 pages, 11 figures, 1 table. Accepted for publication in MNRAS Main
Journa
An improved measurement of baryon acoustic oscillations from the correlation function of galaxy clusters at
We detect the peak of baryon acoustic oscillations (BAO) in the two-point
correlation function of a spectroscopic sample of clusters selected
from the Sloan Digital Sky Survey. Galaxy clusters, as tracers of massive dark
matter haloes, are highly biased structures. The linear bias of the sample
considered in this work, that we estimate from the projected correlation
function, is . Thanks to the high signal in the
cluster correlation function and to the accurate spectroscopic redshift
measurements, we can clearly detect the BAO peak and determine its position,
, with high accuracy, despite the relative paucity of the sample. Our
measurement, , is in good agreement
with previous estimates from large galaxy surveys, and has a similar
uncertainty. The BAO measurement presented in this work thus provides a new
strong confirmation of the concordance cosmological model and demonstrates the
power and promise of galaxy clusters as key probes for cosmological
applications based on large scale structures.Comment: 10 pages, 7 figure, accepted for publication in MNRA
The surface density of Extremely Red Objects in high-z quasar fields
We report on a study of the surface density of Extremely Red Objects (EROs)
in the fields of 13 radio-loud quasars at 1.8 < z < 3.0 covering a total area
of 61.7 sqr arcmin. There is a large variation in the ERO surface density from
field to field, and as many as 30--40 % of the fields have roughly 4--5 times
more EROs than what is expected from a random distribution. The average surface
density exceeds the value found in large random-field surveys by a factor of
2--3, a result which is significant at the >3 sigma level. Hence, it appears
that the quasar lines of sight are biassed towards regions of high ERO density.
This might be caused by clusters or groups of galaxies physically associated
with the quasars. However, an equally likely possibility is that the observed
ERO excess is part of overdensities in the ERO population along the line of
sight to the quasars. In this case, the non-randomness of quasar fields with
respect to EROs may be explained in terms of gravitational lensing.Comment: 6 pages, 2 figures, to appear in "Radio galaxies: past, present and
future", eds. M. Jarvis et al., Leiden, November 200
Listening to galaxies tuning at z ~ 2.5 - 3.0: The first strikes of the Hubble fork
We investigate the morphological properties of 494 galaxies selected from the
GMASS survey at z>1, primarily in their optical rest frame, using HST images,
from the CANDELS survey. We propose that the Hubble sequence of galaxy
morphologies takes shape at redshift 2.5<z<3. The fractions of both ellipticals
and disks decrease with increasing lookback time at z>1, such that at redshifts
z=2.5-2.7 and above, the Hubble types cannot be identified, and most galaxies
are classified as irregular. The quantitative morphological analysis shows
that, at 1<z<3, morphological parameters are not as effective in distinguishing
the different morphological Hubble types as they are at low redshift. No
significant morphological k-correction was found to be required for the Hubble
type classification, with some exceptions. In general, different morphological
types occupy the two peaks of the rest-frame (U-B) colour bimodality of
galaxies: most irregulars occupy the blue peak, while ellipticals are mainly
found in the red peak, though with some level of contamination. Disks are more
evenly distributed than either irregulars and ellipticals. We find that the
position of a galaxy in a UVJ diagram is related to its morphological type: the
"quiescent" region of the plot is mainly occupied by ellipticals and, to a
lesser extent, by disks. We find that only ~33% of all morphological
ellipticals in our sample are red and passively evolving galaxies. Blue
galaxies morphologically classified as ellipticals show a remarkable structural
similarity to red ones. Almost all irregulars have a star-forming galaxy
spectrum. In addition, the majority of disks show some sign of star-formation
activity in their spectra, though in some cases their red continuum is
indicative of old stellar populations. Finally, an elliptical morphology may be
associated with either passively evolving or strongly star-forming galaxies.Comment: 27 pages, 16 figures, 5 tables. "Morphological atlas" in the
appendix. Revised version accepted for publication in A&
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 datasets. Requiring
BzK=(z-K)-(B-z)>-0.2 (AB) allows to select actively star-forming galaxies at
z>1.4, independently on their dust reddening. Instead, objects with BzK<-0.2
and (z-K)>2.5 (AB) colors include passively evolving galaxies at z>1.4, often
with spheroidal morphologies. Simple recipes to estimate the reddening, SFRs
and masses of BzK-selected galaxies are derived, and calibrated on K<20
galaxies. 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 Msun yr^-1, and median reddening E(B-V)~0.4. 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) star-formation rate 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. 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 the RJL photometry is proposed to
complement the BzK selection and to identify massive galaxies at 2.5<z<4.0.
These color criteria should help in completing the census of the stellar mass
and of the star-formation rate density at high redshift (abridged).Comment: 19 pages, 17 figures, to appear on ApJ (20 December 2004 issue
UV Spectropolarimetry of Narrow-line Radio Galaxies
We present the results of UV spectropolarimetry (2000 - 3000A) and far-UV
spectroscopy (1500 - 2000A) of two low-redshift narrow-line radio galaxies
(NLRGs) taken with the Faint Object Spectrograph onboard the Hubble Space
Telescope (HST). Spectropolarimetry of several NLRGs has shown that, by the
presence of broad permitted lines in polarized flux spectrum, they have hidden
quasars seen through scattered light. Imaging polarimetry has shown that NLRGs
including our targets often have large scattering regions of a few kpc to >~10
kpc scale. This has posed a problem about the nature of the scatterers in these
radio galaxies. Their polarized continuum has the spectral index similar to or
no bluer than that of quasars, which favors electrons as the dominant
scattering particles. The large scattering region size, however, favors dust
scattering, because of its higher scattering efficiency compared to electrons.
In this paper, we investigate the polarized flux spectrum over a wide
wavelength range, combining our UV data with previous optical/infrared
polarimetry data. We infer that the scattering would be often caused by opaque
dust clouds in the NLRGs and this would be a part of the reason for the
apparently grey scattering. In the high-redshift radio galaxies, these opaque
clouds could be the proto-galactic subunits inferred to be seen in the HST
images. However, we still cannot rule out the possibility of electron
scattering, which could imply the existence of a large gas mass surrounding
these radio galaxies.Comment: 25 pages, 21 figures. To appear in Ap
The formation and evolution of early-type galaxies : solid results and open questions
The most recent results and some of the open key questions on the evolution
of early-type galaxies are reviewed in the general cosmological context of
massive galaxy formation.Comment: 8 pages, invited review at the workshop "Probing Stellar Populations
out to the Distant Universe", Cefalu` (Italy), September 7 - 19, 200
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 0.88 < z < 1.3 plus two at z=0.67, 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 z~1, and compare them with the
local ones. The FP at z~1 keeps 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 show 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.Comment: 14 pages, 15 figures, accepted by A&
CO excitation in the Seyfert galaxy NGC 34: stars, shock or AGN driven?
We present a detailed analysis of the X-ray and molecular gas emission in the
nearby galaxy NGC 34, to constrain the properties of molecular gas, and assess
whether, and to what extent, the radiation produced by the accretion onto the
central black hole affects the CO line emission. We analyse the CO Spectral
Line Energy Distribution (SLED) as resulting mainly from Herschel and ALMA
data, along with X-ray data from NuSTAR and XMM-Newton. The X-ray data analysis
suggests the presence of a heavily obscured AGN with an intrinsic luminosity of
L erg s. ALMA high
resolution data () allows us to scan the nuclear region
down to a spatial scale of pc for the CO(6-5) transition. We
model the observed SLED using Photo-Dissociation Region (PDR), X-ray-Dominated
Region (XDR), and shock models, finding that a combination of a PDR and an XDR
provides the best fit to the observations. The PDR component, characterized by
gas density and temperature K,
reproduces the low-J CO line luminosities. The XDR is instead characterised by
a denser and warmer gas (, K), and is
necessary to fit the high-J transitions. The addition of a third component to
account for the presence of shocks has been also tested but does not improve
the fit of the CO SLED. We conclude that the AGN contribution is significant in
heating the molecular gas in NGC 34.Comment: Accepted for publication in MNRAS. 10 pages, 6 figure
The star formation rate cookbook at 1 < z < 3: Extinction-corrected relations for UV & [OII]{\lambda}3727 luminosities
We use a spectroscopic sample of 286 star-forming galaxies (SFGs) at 1<z<3
from the GMASS survey to study different star formation rate (SFR) estimators.
Infrared (IR) data are used to derive empirical calibrations to correct
ultraviolet (UV) and [OII]{\lambda}3727 luminosities for dust extinction and
dust-corrected estimates of SFR. In the selection procedure we fully exploit
the available spectroscopic information. On the basis of three continuum
indices, we are able to identify and exclude from the sample galaxies in which
old stellar populations might bring a non-negligible contribution to IR
luminosity (LIR) and continuum reddening. Using Spitzer-MIPS and Herschel-PACS
data we derive LIR for two-thirds of our sample. The LIR/LUV ratio is used as a
probe of effective attenuation (AIRX) to search for correlations with continuum
and spectroscopic features. The relation between AIRX and UV continuum slope
({\beta}) was tested for our sample and found to be broadly consistent with the
literature results at the same redshift, though with a larger dispersion with
respect to UV-selected samples. We find a correlation between the rest-frame
equivalent width (EW) of the [OII]{\lambda}3727 line and {\beta}, which is the
main result of this work. We therefore propose the [OII]{\lambda}3727 line EW
as a dust attenuation probe and calibrate it through AIRX, though the
assumption of a reddening curve is still needed to derive the actual
attenuation towards the [OII]{\lambda}3727 line. We tested the issue of
differential attenuation towards stellar continuum and nebular emission: our
results are in line with the traditional prescription of extra attenuation
towards nebular lines. A set of relations is provided that allows the recovery
of the total unattenuated SFR from UV and [OII]{\lambda}3727 luminosities.
(Abridged)Comment: Accepted for publication in A&A; 20 pages, 19 figures, 5 table
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