135 research outputs found
Knots in Four Dimensions and the Fundamental Group
This paper is an introduction to knotted spheres in four dimensions (analogous to knotted circles in three dimensions). We define what a knotted sphere is and describe a to visually represent them, via movies. The fundamental group of the complement of a knot is a powerful invariant and we describe this invariant in detail giving a convenient algorithm for computing it. Lots of examples are given, including the simplest non-trivial locally flat knot
Evidence for non-stellar rest-frame near-IR emission associated with increased star formation in galaxies at
We explore the presence of non-stellar rest-frame near-IR () emission in galaxies at . Previous studies identified
this excess in relatively small samples and suggested that such non-stellar
emission, which could be linked to the polycyclic
aromatic hydrocarbons feature or hot dust emission, is associated with an
increased star formation rate (SFR). In this Letter, we confirm and quantify
the presence of an IR excess in a significant fraction of galaxies in the
3D-HST GOODS catalogs. By constructing a matched sample of galaxies with and
without strong non-stellar near-IR emission, we find that galaxies with such
emission are predominantly star-forming galaxies. Moreover, star-forming
galaxies with an excess show increased mid- and far-IR and H emission
compared to other star-forming galaxies without. While galaxies with a near-IR
excess show a larger fraction of individually detected X-ray active galactic
nuclei (AGNs), an X-ray stacking analysis, together with the IR-colors and
H profiles, shows that AGNs are unlikely to be the dominant source of
the excess in the majority of galaxies. Our results suggest that non-stellar
near-IR emission is linked to increased SFRs and is ubiquitous among
star-forming galaxies. As such, the near-IR emission might be a powerful tool
to measure SFRs in the era of the James Webb Space Telescope.Comment: 6 pages, 5 figures, accepted for publication in ApJ
Ages of massive galaxies at from 3D-HST rest-frame optical spectroscopy
We present low-resolution near-infrared stacked spectra from the 3D-HST
survey up to and fit them with commonly used stellar population
synthesis models: BC03 (Bruzual & Charlot, 2003), FSPS10 (Flexible Stellar
Population Synthesis, Conroy & Gunn 2010), and FSPS-C3K (Conroy, Kurucz,
Cargile, Castelli, in prep). The accuracy of the grism redshifts allows the
unambiguous detection of many emission and absorption features, and thus a
first systematic exploration of the rest-frame optical spectra of galaxies up
to . We select massive galaxies (), we
divide them into quiescent and star-forming via a rest-frame color-color
technique, and we median-stack the samples in 3 redshift bins between
and . We find that stellar population models fit the observations well
at wavelengths below rest-frame, but show systematic residuals
at redder wavelengths. The FSPS-C3K model generally provides the best fits
(evaluated with a statistics) for quiescent galaxies, while BC03
performs the best for star-forming galaxies. The stellar ages of quiescent
galaxies implied by the models, assuming solar metallicity, vary from 4 Gyr at
to 1.5 Gyr at , with an uncertainty of a factor of 2
caused by the unknown metallicity. On average the stellar ages are half the age
of the Universe at these redshifts. We show that the inferred evolution of ages
of quiescent galaxies is in agreement with fundamental plane measurements,
assuming an 8 Gyr age for local galaxies. For star-forming galaxies the
inferred ages depend strongly on the stellar population model and the shape of
the assumed star-formation history.Comment: 13 pages, 15 figures, accepted for publication in Ap
Predicting Quiescence: The Dependence of Specific Star Formation Rate on Galaxy Size and Central Density at 0.5<z<2.5
In this paper, we investigate the relationship between star formation and
structure, using a mass-complete sample of 27,893 galaxies at
selected from 3D-HST. We confirm that star-forming galaxies are larger than
quiescent galaxies at fixed stellar mass (M). However, in contrast
with some simulations, there is only a weak relation between star formation
rate (SFR) and size within the star-forming population: when dividing into
quartiles based on residual offsets in SFR, we find that the sizes of
star-forming galaxies in the lowest quartile are 0.270.06 dex smaller than
the highest quartile. We show that 50% of star formation in galaxies at fixed
M takes place within a narrow range of sizes (0.26 dex). Taken
together, these results suggest that there is an abrupt cessation of star
formation after galaxies attain particular structural properties. Confirming
earlier results, we find that central stellar density within a 1 kpc fixed
physical radius is the key parameter connecting galaxy morphology and star
formation histories: galaxies with high central densities are red and have
increasingly lower SFR/M, whereas galaxies with low central densities
are blue and have a roughly constant (higher) SFR/M at a given
redshift. We find remarkably little scatter in the average trends and a strong
evolution of 0.5 dex in the central density threshold correlated with
quiescence from . Neither a compact size nor high- are
sufficient to assess the likelihood of quiescence for the average galaxy;
rather, the combination of these two parameters together with M
results in a unique quenching threshold in central density/velocity.Comment: 20 pages, 15 figures, and 2 tables; Accepted for publication in the
Astrophysical Journa
Quiescent Galaxies in the 3D-HST Survey: Spectroscopic Confirmation of a Large Number of Galaxies with Relatively Old Stellar Populations at z~2
Quiescent galaxies at z~2 have been identified in large numbers based on
rest-frame colors, but only a small number of these galaxies have been
spectroscopically confirmed to show that their rest-frame optical spectra show
either strong Balmer or metal absorption lines. Here, we median stack the
rest-frame optical spectra for 171 photometrically-quiescent galaxies at 1.4 <
z < 2.2 from the 3D-HST grism survey. In addition to Hbeta (4861A), we
unambiguously identify metal absorption lines in the stacked spectrum,
including the G-band (4304A), Mg I (5175A), and Na I (5894A). This finding
demonstrates that galaxies with relatively old stellar populations already
existed when the universe was ~3 Gyr old, and that rest-frame color selection
techniques can efficiently select them. We find an average age of 1.3^0.1_0.3
Gyr when fitting a simple stellar population to the entire stack. We confirm
our previous result from medium-band photometry that the stellar age varies
with the colors of quiescent galaxies: the reddest 80% of galaxies are
dominated by metal lines and have a relatively old mean age of 1.6^0.5_0.4 Gyr,
whereas the bluest (and brightest) galaxies have strong Balmer lines and a
spectroscopic age of 0.9^0.2_0.1 Gyr. Although the spectrum is dominated by an
evolved stellar population, we also find [OIII] and Hbeta emission.
Interestingly, this emission is more centrally concentrated than the continuum
with L_[OIII] = 1.7 +/- 0.3 x 10^40 erg s^-1, indicating residual central star
formation or nuclear activity.Comment: 6 pages, 4 figures, accepted for publication in the Astrophysical
Journal Letter
Direct measurements of dust attenuation in z~1.5 star-forming galaxies from 3D-HST: Implications for dust geometry and star formation rates
The nature of dust in distant galaxies is not well understood, and until
recently few direct dust measurements have been possible. We investigate dust
in distant star-forming galaxies using near-infrared grism spectra of the
3D-HST survey combined with archival multi-wavelength photometry. These data
allow us to make a direct comparison between dust around star-forming regions
() and the integrated dust content ().
We select a sample of 163 galaxies between with H
signal-to-noise ratio and measure Balmer decrements from stacked spectra
to calculate . First, we stack spectra in bins of
, and find that
, with a significance of
. Our result is consistent with the two-component dust model, in
which galaxies contain both diffuse and stellar birth cloud dust. Next, we
stack spectra in bins of specific star formation rate (),
star formation rate (), and stellar mass (). We
find that on average increases with SFR and mass, but
decreases with increasing SSFR. Interestingly, the data hint that the amount of
extra attenuation decreases with increasing SSFR. This trend is expected from
the two-component model, as the extra attenuation will increase once older
stars outside the star-forming regions become more dominant in the galaxy
spectrum. Finally, using Balmer decrements we derive dust-corrected H
SFRs, and find that stellar population modeling produces incorrect SFRs if
rapidly declining star formation histories are included in the explored
parameter space.Comment: Accepted for publication in the Astrophysical Journal (13 pages, 9
figures
A Massive Galaxy in its Core Formation Phase Three Billion Years After the Big Bang
Most massive galaxies are thought to have formed their dense stellar cores at
early cosmic epochs. However, cores in their formation phase have not yet been
observed. Previous studies have found galaxies with high gas velocity
dispersions or small apparent sizes but so far no objects have been identified
with both the stellar structure and the gas dynamics of a forming core. Here we
present a candidate core in formation 11 billion years ago, at z=2.3.
GOODS-N-774 has a stellar mass of 1.0x10^11 Msun, a half-light radius of 1.0
kpc, and a star formation rate of 90[+45-20]Msun/yr. The star forming gas has a
velocity dispersion 317+-30 km/s, amongst the highest ever measured. It is
similar to the stellar velocity dispersions of the putative descendants of
GOODS-N-774, compact quiescent galaxies at z~2 and giant elliptical galaxies in
the nearby Universe. Galaxies such as GOODS-N-774 appear to be rare; however,
from the star formation rate and size of the galaxy we infer that many star
forming cores may be heavily obscured, and could be missed in optical and
near-infrared surveys.Comment: To appear in Natur
Exploring the chemical link between local ellipticals and their high-redshift progenitors
We present Keck/MOSFIRE K-band spectroscopy of the first mass-selected sample
of galaxies at . Targets are selected from the 3D-HST Treasury
survey. The six detected galaxies have a mean [NII]6584/H
ratio of , with a small standard deviation of 0.05. This mean
value is similar to that of UV-selected galaxies of the same mass. The mean
gas-phase oxygen abundance inferred from the [NII]/H ratios depends on
the calibration method, and ranges from 12+log(O/H) for the
{Pettini} & {Pagel} (2004) calibration to 12+log(O/H) for the
{Maiolino} {et~al.} (2008) calibration. Measurements of the stellar oxygen
abundance in nearby quiescent galaxies with the same number density indicate
12+log(O/H), similar to the gas-phase abundances of the
galaxies if the {Maiolino} {et~al.} (2008) calibration is used. This
suggests that these high-redshift star forming galaxies may be progenitors of
today's massive early-type galaxies. The main uncertainties are the absolute
calibration of the gas-phase oxygen abundance and the incompleteness of the
sample: the galaxies with detected H tend to be larger and
have higher star formation rates than the galaxies without detected H,
and we may still be missing the most dust-obscured progenitors.Comment: 7 pages, 5 figures. Accepted for publication in ApJ
First results from the VIRIAL survey: the stellar content of -selected quiescent galaxies at from KMOS
We investigate the stellar populations of 25 massive, galaxies
() at using data obtained with
the K-band Multi-Object Spectrograph (KMOS) on the ESO VLT. Targets were
selected to be quiescent based on their broadband colors and redshifts using
data from the 3D-HST grism survey. The mean redshift of our sample is , where KMOS YJ-band data probe age- and metallicity-sensitive
absorption features in the rest-frame optical, including the band, Fe I,
and high-order Balmer lines. Fitting simple stellar population models to a
stack of our KMOS spectra, we derive a mean age of Gyr.
We confirm previous results suggesting a correlation between color and age for
quiescent galaxies, finding mean ages of Gyr and
Gyr for the reddest and bluest galaxies in our sample.
Combining our KMOS measurements with those obtained from previous studies at
we find evidence for a Gyr spread in the formation epoch of
massive galaxies. At the measured stellar ages are consistent with
passive evolution, while at they appear to saturate at
1 Gyr, which likely reflects changing demographics of the (mean)
progenitor population. By comparing to star-formation histories inferred for
"normal" star-forming galaxies, we show that the timescales required to form
massive galaxies at are consistent with the enhanced
-element abundances found in massive local early-type galaxies.Comment: 6 pages, 5 figures, accepted for publication in ApJ
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