524 research outputs found
AGN feedback at z~2 and the mutual evolution of active and inactive galaxies
The relationships between galaxies of intermediate stellar mass and moderate
luminosity active galactic nuclei (AGNs) at 1<z<3 are investigated with the
Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS) sample
complemented with public data in the GOODS-South field. Using X-ray data,
hidden AGNs are identified in unsuspected star-forming galaxies with no
apparent signs of non-stellar activity. In the color-mass plane, two parallel
trends emerge during the ~2 Gyr between the average redshifts z~2.2 and z~1.3:
while the red sequence becomes significantly more populated by ellipticals, the
majority of AGNs with L(2-10 keV)>10^42.3 erg s^-1 disappear from the blue
cloud/green valley where they were hosted predominantly by star-forming systems
with disk and irregular morphologies. These results are even clearer when the
rest-frame colors are corrected for dust reddening. At z~2.2, the ultraviolet
spectra of active galaxies (including two Type 1 AGNs) show possible gas
outflows with velocities up to about -500 km s^-1 that are not observed neither
in inactive systems at the same redshift, nor at lower redshifts. Such outflows
indicate the presence of gas that can move faster than the escape velocities of
active galaxies. These results suggest that feedback from moderately luminous
AGNs (logL_X~2 by contributing to
outflows capable of ejecting part of the interstellar medium and leading to a
rapid decrease in the star formation in host galaxies with stellar masses
10<logM<11 M_Sun.Comment: Astrophysical Journal Letters, in press (6 pages, 4 figures
A Coherent Study of Emission Lines from Broad-Band Photometry: Specific Star-Formation Rates and [OIII]/H{\beta} Ratio at 3 < z < 6
We measure the H{\alpha} and [OIII] emission line properties as well as
specific star-formation rates (sSFR) of spectroscopically confirmed 3<z<6
galaxies in COSMOS from their observed colors vs. redshift evolution. Our model
describes consistently the ensemble of galaxies including intrinsic properties
(age, metallicity, star-formation history), dust-attenuation, and optical
emission lines. We forward-model the measured H{\alpha} equivalent-widths (EW)
to obtain the sSFR out to z~6 without stellar mass fitting. We find a strongly
increasing rest-frame H{\alpha} EW that is flattening off above z~2.5 with
average EWs of 300-600A at z~6. The sSFR is increasing proportional to
(1+z)^2.4 at z<2.2 and (1+z)^1.5 at higher redshifts, indicative of a fast mass
build-up in high-z galaxies within e-folding times of 100-200Myr at z~6. The
redshift evolution at z>3 cannot be fully explained in a picture of cold
accretion driven growth. We find a progressively increasing
[OIII]{\lambda}5007/H{\beta} ratio out to z~6, consistent with the ratios in
local galaxies selected by increasing H{\alpha} EW (i.e., sSFR). This
demonstrates the potential of using "local high-z analogs" to investigate the
spectroscopic properties and relations of galaxies in the re-ionization epoch.Comment: 18 pages, 11 figures, 3 table
Constraining The Assembly Of Normal And Compact Passively Evolving Galaxies From Redshift z=3 To The Present With CANDELS
We study the evolution of the number density, as a function of the size, of
passive early-type galaxies with a wide range of stellar masses
10^10<M*/Msun<10^11.5) from z~3 to z~1, exploiting the unique dataset available
in the GOODS-South field, including the recently obtained WFC3 images as a part
of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey
(CANDELS). In particular, we select a sample of 107 massive (M*>10^10 M_sun),
passive (SSFR<10^-2 Gyr^-1) and morphologically spheroidal galaxies at 1.2<z<3,
taking advantage of the panchromatic dataset available for GOODS, including
VLT, CFHT, Spitzer, Chandra and HST ACS+WFC3 data. We find that at 1<z<3 the
passively evolving early-type galaxies are the reddest and most massive objects
in the Universe, and we prove that a correlation between mass, morphology,
color and star-formation activity is already in place at that epoch. We measure
a significant evolution in the mass-size relation of passive early-type
galaxies (ETGs) from z~3 to z~1, with galaxies growing on average by a factor
of 2 in size in a 3 Gyr timescale only. We witness also an increase in the
number density of passive ETGs of 50 times over the same time interval. We find
that the first ETGs to form at z>2 are all compact or ultra-compact, while
normal sized ETGs (meaning ETGs with sizes comparable to those of local
counterparts of the same mass) are the most common ETGs only at z<1. The
increase of the average size of ETGs at 0<z<1 is primarily driven by the
appearance of new large ETGs rather than by the size increase of individual
galaxies.Comment: 9 pages, 5 figures, submitted to Ap
The Progenitors of the Compact Early-Type Galaxies at High-Redshift
We use GOODS and CANDELS images to identify progenitors of massive (log M >
10 Msun) compact "early-type" galaxies (ETGs) at z~1.6. Since merging and
accretion increase the size of the stellar component of galaxies, if the
progenitors are among known star-forming galaxies, these must be compact
themselves. We select candidate progenitors among compact Lyman-break galaxies
at z~3 based on their mass, SFR and central stellar density and find that these
account for a large fraction of, and possibly all, compact ETGs at z~1.6. We
find that the average far-UV SED of the candidates is redder than that of the
non-candidates, but the optical and mid-IR SED are the same, implying that the
redder UV of the candidates is inconsistent with larger dust obscuration, and
consistent with more evolved (aging) star-formation. This is in line with other
evidence that compactness is a sensitive predictor of passivity among
high-redshift massive galaxies. We also find that the light distribution of
both the compact ETGs and their candidate progenitors does not show any
extended "halos" surrounding the compact "core", both in individual images and
in stacks. We argue that this is generally inconsistent with the morphology of
merger remnants, even if gas-rich, as predicted by N-body simulations. This
suggests that the compact ETGs formed via highly dissipative, mostly gaseous
accretion of units whose stellar components are very small and undetected in
the HST images, with their stellar mass assembling in-situ, and that they have
not experienced any major merging until the epoch of observations at z~1.6.Comment: 25 pages, 20 figures; Accepted for publication in Ap
HeII emitters in the VIMOS VLT Deep Survey: PopIII star formation or peculiar stellar populations in galaxies at 2<z<4.6?
The aim of this work is to identify HeII emitters at 2<z<4.6 and to constrain
the source of the hard ionizing continuum that powers the HeII emission. We
have assembled a sample of 277 galaxies with a high quality spectroscopic
redshift at 2<z<4.6 from the VVDS survey, and we have identified 39 HeII1640A
emitters. We study their spectral properties, measuring the fluxes, equivalent
widths (EW) and FWHM for most relevant lines. About 10% of galaxies at z~3 show
HeII in emission, with rest frame equivalent widths EW0~1-7A, equally
distributed between galaxies with Lya in emission or in absorption. We find 11
high-quality HeII emitters with unresolved HeII line (FWHM_0<1200km/s), 13
high-quality emitters with broad He II emission (FWHM_0>1200km/s), 3 AGN, and
an additional 12 possible HeII emitters. The properties of the individual broad
emitters are in agreement with expectations from a W-R model. On the contrary,
the properties of the narrow emitters are not compatible with such model,
neither with predictions of gravitational cooling radiation produced by gas
accretion. Rather, we find that the EW of the narrow HeII line emitters are in
agreement with expectations for a PopIII star formation, if the episode of star
formation is continuous, and we calculate that a PopIII SFR of 0.1-10 Mo yr-1
only is enough to sustain the observed HeII flux. We conclude that narrow HeII
emitters are either powered by the ionizing flux from a stellar population rare
at z~0 but much more common at z~3, or by PopIII star formation. As proposed by
Tornatore et al. (2007), incomplete ISM mixing may leave some small pockets of
pristine gas at the periphery of galaxies from which PopIII may form, even down
to z~2 or lower. If this interpretation is correct, we measure at z~3 a SFRD in
PopIII stars of 10^6Mo yr^-1 Mpc^-3 qualitatively comparable to the value
predicted by Tornatore et al. (2007).Comment: accepted for publication in A&
Morphological studies of the Spitzer Wide-Area Infrared Extragalactic survey galaxy population in the UGC 10214 Hubble space telescope/advanced camera for surveys field
We present the results of a morphological analysis of a small subset of the Spitzer Wide-Area Infrared Extragalactic survey (SWIRE) galaxy population. The analysis is based on public Advanced Camera for Surveys (ACS) data taken inside the SWIRE N1 field, which are the deepest optical high-resolution imaging available within the SWIRE fields as of today. Our reference sample includes 156 galaxies detected by both ACS and SWIRE. Among the various galaxy morphologies, we disentangle two main classes, spheroids (or bulge-dominated galaxies) and disc-dominated ones, for which we compute the number counts as a function of flux. We then limit our sample to objects with Infrared Array Camera (IRAC) fluxes brighter than 10 μJy, estimated ~90 per cent completeness limit of the SWIRE catalogues, and compare the observed counts to model predictions. We find that the observed counts of the spheroidal population agree with the expectations of a hierarchical model while a monolithic scenario predicts steeper counts. Both scenarios, however, underpredict the number of late-type galaxies. These observations show that the large majority (close to 80 per cent) of the 3.6- and 4.5-μm galaxy population, even at these moderately faint fluxes, is dominated by spiral and irregular galaxies or mergers
Rest-Frame UV-Optical Selected Galaxies at 2.3 ≾ z ≾ 3.5: Searching for Dusty Star-forming and Passively Evolving Galaxies
A new set of color selection criteria (VJL) analogous with the BzK method is designed to select both star-forming galaxies (SFGs) and passively evolving galaxies (PEGs) at 2.3 ≾ z ≾ 3.5 by using rest-frame UV-optical (V – J versus J – L) colors. The criteria are thoroughly tested with theoretical stellar population synthesis models and real galaxies with spectroscopic redshifts to evaluate their efficiency and contamination. We apply the well-tested VJL criteria to the HST/WFC3 Early Release Science field and study the physical properties of selected galaxies. The redshift distribution of selected SFGs peaks at z ~ 2.7, slightly lower than that of Lyman break galaxies at z ~ 3. Comparing the observed mid-infrared fluxes of selected galaxies with the prediction of pure stellar emission, we find that our VJL method is effective at selecting massive dusty SFGs that are missed by the Lyman break technique. About half of the star formation in massive (M_(star) > 10^(10) M_☉) galaxies at 2.3 ≾ z ≾ 3.5 is contributed by dusty (extinction E(B – V) > 0.4) SFGs, which, however, only account for ~20% of the number density of massive SFGs. We also use the mid-infrared fluxes to clean our PEG sample and find that galaxy size can be used as a secondary criterion to effectively eliminate the contamination of dusty SFGs. The redshift distribution of the cleaned PEG sample peaks at z ~ 2.5. We find six PEG candidates at z > 3 and discuss possible methods to distinguish them from dusty contamination. We conclude that at least part of our candidates are real PEGs at z ~ 3, implying that these types of galaxies began to form their stars at z ≳ 5. We measure the integrated stellar mass density (ISMD) of PEGs at z ~ 2.5 and set constraints on it at z > 3. We find that the ISMD grows by at least about a factor of 10 in 1 Gyr at 3 < z <5 and by another factor of 10 in the next 3.5 Gyr (1 < z < 3)
Multi-Wavelength View of Kiloparsec-Scale Clumps in Star-Forming Galaxies at z~2
This paper studies the properties of kiloparsec-scale clumps in star-forming
galaxies at z~2 through multi-wavelength broad band photometry. A sample of 40
clumps is identified through auto-detection and visual inspection from 10
galaxies with 1.5<z<2.5 in the Hubble Ultra Deep Field, where deep and
high-resolution HST/WFC3 and ACS images enable us to resolve structures of z~2
galaxies down to kpc scale in the rest-frame UV and optical bands as well as to
detect clumps toward the faint end. The physical properties of clumps are
measured through fitting spatially resolved seven-band (BVizYJH) spectral
energy distribution to models. On average, the clumps are blue and have similar
median rest-frame UV--optical color as the diffuse components of their host
galaxies, but the clumps have large scatter in their colors. Although the star
formation rate (SFR)--stellar mass relation of galaxies is dominated by the
diffuse components, clumps emerge as regions with enhanced specific SFRs,
contributing individually ~10% and together ~50% of the SFR of the host
galaxies. However, the contributions of clumps to the rest-frame UV/optical
luminosity and stellar mass are smaller, typically a few percent individually
and ~20% together. On average, clumps are younger by 0.2 dex and denser by a
factor of 8 than diffuse components. Clump properties have obvious radial
variations in the sense that central clumps are redder, older, more extincted,
denser, and less active on forming stars than outskirts clumps. Our results are
broadly consistent with a widely held view that clumps are formed through
gravitational instability in gas-rich turbulent disks and would eventually
migrate toward galactic centers and coalesce into bulges. Roughly 40% of the
galaxies in our sample contain a massive clump that could be identified as a
proto-bulge, which seems qualitatively consistent with such a bulge-formation
scenario.Comment: Accepted by ApJ. This updated version matches the in-press one. 50
pages (single column), 10 figures, 3 table
Discovery Of Cold, Pristine Gas Possibly Accreting Onto An Overdensity Of Star-Forming Galaxies At Redshift z ~ 1.6
We report the discovery of large amounts of cold (T ~ 10^4 K), chemically
young gas in an overdensity of galaxies at redshift z ~ 1.6 in the Great
Observatories Origins Deep Survey southern field (GOODS-S). The gas is
identified thanks to the ultra-strong Mg II absorption features it imprints in
the rest-frame UV spectra of galaxies in the background of the overdensity.
There is no evidence that the optically-thick gas is part of any massive galaxy
(i.e. M_star > 4x10^9 M_sun), but rather is associated with the overdensity;
less massive and fainter galaxies (25.5 < z_850 < 27.5 mag) have too large an
impact parameter to be causing ultra-strong absorption systems, based on our
knowledge of such systems. The lack of corresponding Fe II absorption features,
not detected even in co-added spectra, suggests that the gas is chemically more
pristine than the ISM and outflows of star-forming galaxies at similar
redshift, including those in the overdensity itself, and comparable to the most
metal-poor stars in the Milky Way halo. A crude estimate of the projected
covering factor of the high-column density gas (N_H >~ 10^20 cm-2) based on the
observed fraction of galaxies with ultra-strong absorbers is C_F ~ 0.04. A
broad, continuum absorption profile extending to the red of the interstellar Mg
II absorption line by <~ 2000 km/s is possibly detected in two independent
co-added spectra of galaxies of the overdensity, consistent with a large-scale
infall motion of the gas onto the overdensity and its galaxies. Overall, these
findings provides the first tentative evidence of accretion of cold, chemically
young gas onto galaxies at high redshift, possibly feeding their star formation
activity. The fact that the galaxies are members of a large structure, as
opposed to field galaxies, might play a significant role in our ability to
detect the accreting gas.Comment: 57 pages, 17 figures, 1 table; accepted for publication by ApJ (Aug
9, 2011); minor modifications to match the accepted versio
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