24 research outputs found
Intrinsic AGN SED & black hole growth in the Palomar--Green quasars
We present a new analysis of the PG quasar sample based on Spitzer and
Herschel observations. (I) Assuming PAH-based star formation luminosities
(L_SF) similar to Symeonidis et al. (2016, S16), we find mean and median
intrinsic AGN spectral energy distributions (SEDs). These, in the FIR, appear
hotter and significantly less luminous than the S16 mean intrinsic AGN SED. The
differences are mostly due to our normalization of the individual SEDs, that
properly accounts for a small number of very FIR-luminous quasars. Our median,
PAH-based SED represents ~ 6% increase on the 1-243 micron luminosity of the
extended Mor & Netzer (2012, EM12) torus SED, while S16 find a significantly
larger difference. It requires large-scale dust with T ~ 20 -- 30 K which, if
optically thin and heated by the AGN, would be outside the host galaxy. (II) We
also explore the black hole and stellar mass growths, using L_SF estimates from
fitting Herschel/PACS observations after subtracting the EM12 torus
contribution. We use rough estimates of stellar mass, based on scaling
relations, to divide our sample into groups: on, below and above the star
formation main sequence (SFMS). Objects on the SFMS show a strong correlation
between star formation luminosity and AGN bolometric luminosity, with a
logarithmic slope of ~ 0.7. Finally we derive the relative duty cycles of this
and another sample of very luminous AGN at z = 2 -- 3.5. Large differences in
this quantity indicate different evolutionary pathways for these two
populations characterised by significantly different black hole masses.Comment: Accepted for publication in MNRA
Exploring the progenitors of brightest cluster galaxies at z ∼ 2
We present a new method for tracing the evolution of brightest cluster galaxies (BCGs) from z ∼ 2 to z ∼ 0. We conclude on the basis of semi-analytical models that the best method to select BCG progenitors at z ∼ 2 is a hybrid environmental density and stellar mass ranking approach. Ultimately, we are able to retrieve 45 per cent of BCG progenitors. We apply this method on the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Ultra Deep Survey data to construct a progenitor sample at high redshift. We furthermore populate the comparisons in local Universe by using Sloan Digital Sky Survey data with statistically likely contamination to ensure a fair comparison between high and low redshifts. Using these samples we demonstrate that the BCG sizes have grown by a factor of ∼3.2 since z ∼ 2, and BCG progenitors are mainly late-type galaxies, exhibiting less concentrated profiles than their early type local counterparts. We find that BCG progenitors have more disturbed morphologies. In contrast, local BCGs have much smoother profiles. Moreover, we find that the stellar masses of BCGs have grown by a factor of ∼2.5 since z ∼ 2, and the star formation rate of BCG progenitors has a median value of 13.5 Mʘ yr‾¹, much higher than their quiescent local descendants. We demonstrate that over z = 1–2 star formation and merging contribute equally to BCG mass growth. However, merging plays a dominant role in BCG assembly at z ≲ 1. We also find that BCG progenitors at high z are not significantly different from other galaxies of similar mass at the same epoch. This suggests that the processes which differentiate BCGs from normal massive elliptical galaxies must occur at z ≲ 2
Evidence for a correlation between the sizes of quiescent galaxies and local environment to z ~ 2
We present evidence for a strong relationship between galaxy size and
environment for the quiescent population in the redshift range 1 < z < 2.
Environments were measured using projected galaxy overdensities on a scale of
400 kpc, as determined from ~ 96,000 K-band selected galaxies from the UKIDSS
Ultra Deep Survey (UDS). Sizes were determined from ground-based K-band
imaging, calibrated using space-based CANDELS HST observations in the centre of
the UDS field, with photometric redshifts and stellar masses derived from
11-band photometric fitting. From the resulting size-mass relation, we confirm
that quiescent galaxies at a given stellar mass were typically ~ 50 % smaller
at z ~ 1.4 compared to the present day. At a given epoch, however, we find that
passive galaxies in denser environments are on average significantly larger at
a given stellar mass. The most massive quiescent galaxies (M_stellar > 2 x
10^11 M_sun) at z > 1 are typically 50 % larger in the highest density
environments compared to those in the lowest density environments. Using Monte
Carlo simulations, we reject the null hypothesis that the size-mass relation is
independent of environment at a significance > 4.8 sigma for the redshift range
1 < z < 2. In contrast, the evidence for a relationship between size and
environment is much weaker for star-forming galaxies.Comment: Accepted for publication in MNRAS. 16 pages, 11 figures, 6 table
Star-forming Blue ETGs in Two Newly Discovered Galaxy Overdensities in the HUDF at z=1.84 and 1.9: Unveiling the Progenitors of Passive ETGs in Cluster Cores
We present the discovery of two galaxy overdensities in the Hubble Space Telescope UDF: a proto-cluster, HUDFJ0332.4-2746.6 at z=1.84 ± 0.01, and a group, HUDFJ0332.5-2747.3 at z=1.90 ± 0.01. Assuming viralization, the velocity dispersion of HUDFJ0332.4-2746.6 implies a mass of M_(200) = (2.2 ± 1.8) x 10^(14) M_☉, consistent with the lack of extended X-ray emission. Neither overdensity shows evidence of a red sequence. About of their members show interactions and/or disturbed morphologies, which are signatures of merger remnants or disk instability. Most of their ETGs have blue colors and show recent star formation. These observations reveal for the first time large fractions of spectroscopically confirmed star-forming blue ETGs in proto-clusters at ≈ z. These star-forming ETGs are most likely among the progenitors of the quiescent population in clusters at more recent epochs. Their mass–size relation is consistent with that of passive ETGs in clusters at z ~ 0.7-1.5. If these galaxies are the progenitors of cluster ETGs at these lower redshifts, their size would evolve according to a similar mass-size relation. It is noteworthy that quiescent ETGs in clusters at z = 1.8-2 also do not show any significant size evolution over this redshift range, contrary to field ETGs. The ETG fraction is ≾50%, compared to the typical quiescent ETG fraction of ≈80% in cluster cores at z < 1. The fraction, masses, and colors of the newly discovered ETGs imply that other cluster ETGs will be formed/accreted at a later time
Deconstructing the galaxy stellar mass function with UKIDSS and CANDELS: the impact of colour, structure and environment
We combine photometry from the Ultra Deep Survey (UDS), Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) UDS and CANDELS the Great Observatories Origins Deep Survey-South (GOODS-S) surveys to construct the galaxy stellar mass function probing both the low- and high-mass end accurately in the redshift range 0.326.0), affording us robust measures of structural parameters. We construct stellar mass functions for the entire sample as parametrized by the Schechter function, and find that there is a decline in the values of ϕ and of α with higher redshifts, and a nearly constant M* up to z∼3. We divide the galaxy stellar mass function by colour, structure, and environment and explore the links between environmental overdensity, morphology, and the quenching of star formation. We find that a double Schechter function describes galaxies with high Sérsic index (n>2.5), similar to galaxies which are red or passive. The low-mass end of the n>2.5 stellar mass function is dominated by blue galaxies, whereas the high-mass end is dominated by red galaxies. This shows that there is a possible link between morphological evolution and star formation quenching in high mass galaxies, which is not seen in lower mass systems. This in turn suggests that there are strong mass-dependent quenching mechanisms. In addition, we find that the number density of high-mass systems is elevated in dense environments, suggesting that an environmental process is building up massive galaxies quicker in over densities than in lower densitie
Deconstructing the Galaxy Stellar Mass Function with UKIDSS and CANDELS: the Impact of Colour, Structure and Environment
We combine photometry from the UDS, and CANDELS UDS and CANDELS GOODS-S
surveys to construct the galaxy stellar mass function probing both the low and
high mass end accurately in the redshift range 0.3<z<3. The advantages of using
a homogeneous concatenation of these datasets include meaningful measures of
environment in the UDS, due to its large area (0.88 deg^2), and the high
resolution deep imaging in CANDELS (H_160 > 26.0), affording us robust measures
of structural parameters. We construct stellar mass functions for the entire
sample as parameterised by the Schechter function, and find that there is a
decline in the values of phi and of alpha with higher redshifts, and a nearly
constant M* up to z~3. We divide the galaxy stellar mass function by colour,
structure, and environment and explore the links between environmental
over-density, morphology, and the quenching of star formation. We find that a
double Schechter function describes galaxies with high Sersic index (n>2.5),
similar to galaxies which are red or passive. The low-mass end of the n>2.5
stellar mass function is dominated by blue galaxies, whereas the high-mass end
is dominated by red galaxies. This hints that possible links between
morphological evolution and star formation quenching are only present in
high-mass galaxies. This is turn suggests that there are strong mass dependent
quenching mechanisms. In addition, we find that the number density of high mass
systems is elevated in dense environments, suggesting that an environmental
process is building up massive galaxies quicker in over densities than in lower
densities.Comment: 26 pages, 14 figures, Accepted for publication in MNRA
The redshift and mass dependence on the formation of the Hubble sequence at z > 1 from CANDELS/UDS
In this paper we present a detailed study of the structures and morphologies of a sample of 1188 massive galaxies with M-* >= 10(10) M-circle dot between redshifts z = 1 and 3 within the Ultra Deep Survey (UDS) region of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) field. Using this sample we determine how galaxy structure and morphology evolve with time, and investigate the nature of galaxy structure at high redshift. We visually classify our sample into discs, ellipticals and peculiar systems and correct for redshift effects on these classifications through simulations. We find significant evolution in the fractions of galaxies at a given visual classification as a function of redshift. The peculiar population is dominant at z > 2 with a substantial spheroid population, and a negligible disc population. We compute the transition redshift, z(trans), where the combined fraction of spheroidal and disc galaxies is equal to that of the peculiar population, as z(trans) = 1.86 +/- 0.62 for galaxies in our stellar mass range. We find that this transition changes as a function of stellar mass, with Hubble-type galaxies becoming dominant at higher redshifts for higher mass galaxies (z(trans) = 2.22 +/- 0.82), than for the lower mass galaxies (z(trans) = 1.73 +/- 0.57). Higher mass galaxies become morphologically settled before their lower mass counterparts, a form of morphological downsizing. We furthermore compare our visual classifications with the Sersic index, the concentration, asymmetry and clumpiness (CAS) parameters, star formation rate and rest-frame U - B colour. We find links between the colour of a galaxy, its star formation rate and how extended or peculiar it appears. Finally, we discuss the negligible z > 2 disc fraction based on visual morphologies and speculate that this is an effect of forming disc appearing peculiar through processes such as violent disc instabilities or mergers. We conclude that to properly define and measure high-redshift morphology and structure a new and more exact classification scheme is needed
CANDELS Observations of the Environmental Dependence of the Color-Mass-Morphology Relation at z = 1.6
We study the environmental dependence of color, stellar mass, and morphology
by comparing galaxies in a forming cluster to those in the field at z = 1:6
with Hubble Space Telescope near-infrared imaging in the CANDELS/UDS field. We
quantify the morphology of the galaxies using the effective radius, reff, and
S\'ersic index, n. In both the cluster and field, approximately half of the
bulge-dominated galaxies (n > 2) reside on the red sequence of the
color-magnitude diagram, and most disk-dominated galaxies (n < 2) have colors
expected for star-forming galaxies. There is weak evidence that cluster
galaxies have redder rest-frame U - B colors and higher stellar masses compared
to the field. Star-forming galaxies in both the cluster and field show no
significant differences in their morphologies. In contrast, there is evidence
that quiescent galaxies in the cluster have larger median effective radii and
smaller S\'ersic indices compared to the field with a significance of 2?. These
differences are most pronounced for galaxies at clustercentric distances 1 Mpc
< Rproj < 1.5 Mpc, which have low S\'ersic indices and possibly larger
effective radii, more consistent with star-forming galaxies at this epoch and
in contrast to other quiescent galaxies. We argue that star-forming galaxies
are processed under the influence of the cluster environment at distances
greater than the cluster-halo virial radius. Our results are consistent with
models where gas accretion onto these galaxies is suppressed from processes
associated with the cluster environment.Comment: ApJ accepted, 19 pages, 10 figure
The Redshift and Mass Dependence on the Formation of the Hubble Sequence at \u3cem\u3ez\u3c/em\u3e \u3e 1 from CANDELS/UDS
In this paper we present a detailed study of the structures and morphologies of a sample of 1188 massive galaxies with M* ≥ 1010 M⊙between redshifts z = 1 and 3 within the Ultra Deep Survey (UDS) region of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) field. Using this sample we determine how galaxy structure and morphology evolve with time, and investigate the nature of galaxy structure at high redshift. We visually classify our sample into discs, ellipticals and peculiar systems and correct for redshift effects on these classifications through simulations. We find significant evolution in the fractions of galaxies at a given visual classification as a function of redshift. The peculiar population is dominant at z \u3e 2 with a substantial spheroid population, and a negligible disc population. We compute the transition redshift, ztrans, where the combined fraction of spheroidal and disc galaxies is equal to that of the peculiar population, as ztrans = 1.86 ± 0.62 for galaxies in our stellar mass range. We find that this transition changes as a function of stellar mass, with Hubble-type galaxies becoming dominant at higher redshifts for higher mass galaxies (ztrans = 2.22 ± 0.82), than for the lower mass galaxies (ztrans = 1.73 ± 0.57). Higher mass galaxies become morphologically settled before their lower mass counterparts, a form of morphological downsizing. We furthermore compare our visual classifications with the Sérsic index, the concentration, asymmetry and clumpiness (CAS) parameters, star formation rate and rest-frame U − B colour. We find links between the colour of a galaxy, its star formation rate and how extended or peculiar it appears. Finally, we discuss the negligible z \u3e 2 disc fraction based on visual morphologies and speculate that this is an effect of forming disc appearing peculiar through processes such as violent disc instabilities or mergers. We conclude that to properly define and measure high-redshift morphology and structure a new and more exact classification scheme is needed