1,053 research outputs found
Charting the evolution of the ages and metallicities of massive galaxies since z=0.7
The stellar populations of intermediate-redshift galaxies can shed light onto
the growth of massive galaxies in the last 8 billion years. We perform deep,
multi-object rest-frame optical spectroscopy with IMACS/Magellan of ~70
galaxies in the E-CDFS with redshift 0.6522.7 and
stellar mass >10^{10}Msun. Following the Bayesian approach adopted for previous
low-redshift studies, we constrain the stellar mass, mean stellar age and
stellar metallicity of individual galaxies from stellar absorption features. We
characterize for the first time the dependence of stellar metallicity and age
on stellar mass at z~0.7 for all galaxies and for quiescent and star-forming
galaxies separately. These relations for the whole sample have a similar shape
as the z=0.1 SDSS analog, but are shifted by -0.28 dex in age and by -0.13 dex
in metallicity, at odds with simple passive evolution. We find that no
additional star formation and chemical enrichment are required for z=0.7
quiescent galaxies to evolve into the present-day quiescent population.
However, this must be accompanied by the quenching of a fraction of z=0.7
Mstar>10^{11}Msun star-forming galaxies with metallicities comparable to those
of quiescent galaxies, thus increasing the scatter in age without affecting the
metallicity distribution. However rapid quenching of the entire population of
massive star-forming galaxies at z=0.7 would be inconsistent with the
age/metallicity--mass relation for the population as a whole and with the
metallicity distribution of star-forming galaxies only, which are on average
0.12 dex less metal-rich than their local counterparts. This indicates chemical
enrichment until the present in at least a fraction of the z=0.7 massive
star-forming galaxies.[abridged]Comment: accepted for publication on ApJ, 26 pages, 13 figure
Galaxies with Wolf-Rayet signatures in the low-redshift Universe - A survey using the Sloan Digital Sky Survey
We have carried out a search for Wolf-Rayet galaxies in all galaxies with
EW(Hb)>2AA in the SDSS DR6. We identify Wolf-Rayet features using a mixture of
automatic and visual classification and find a total of 570 galaxies with
significant Wolf-Rayet (WR) features and a further 1115 potential candidates,
several times more than even the largest heterogeneously assembled catalogues.
We discuss in detail the properties of galaxies showing Wolf-Rayet features
with a focus on their empirical properties. We are able to accurately quantify
the incidence of Wolf-Rayet galaxies with redshift and show that the likelihood
of otherwise similar galaxies showing Wolf-Rayet features increases with
increasing metallicity, but that WR features are found in galaxies of a wide
range in morphology. The large sample allows us to show explicitly that there
are systematic differences in the metal abundances of WR and non-WR galaxies.
The most striking result is that, below EW(Hb)=100AA, Wolf-Rayet galaxies show
an elevated N/O relative to non-WR galaxies. We interpret this as a rapid
enrichment of the ISM from WR winds. We also show that the model predictions
for WR features strongly disagree with the observations at low metallicity;
while they do agree quite well with the data at solar abundances. We discuss
possible reasons for this and show that models incorporating binary evolution
reproduce the low-metallicity results reasonably well. Finally we combine the
WR sample with a sample of galaxies with nebular He II 4686 to show that, at 12
+ log O/H< 8, the main sources of He II ionising photons appears to be O stars,
arguing for a less dense stellar wind at these metallicities, while at higher
abundances WN stars might increasingly dominate the ionisation budget.Comment: 22 pages text with 40 pages tables at end. See accompanying web site
at http://www.strw.leidenuniv.nl/~jarle/WRinSDSS
Unveiling the Milky Way: A New Technique for Determining the Optical Color and Luminosity of Our Galaxy
Article / Letter to editorSterrewach
The extended HeII4686-emitting region in IZw18 unveiled: clues for peculiar ionizing sources
New integral field spectroscopy has been obtained for IZw18, the nearby
lowest-metallicity galaxy considered our best local analog of systems forming
at high-z. Here we report the spatially resolved spectral map of the nebular
HeII4686 emission in IZw18, from which we derived for the first time its total
HeII-ionizing flux. Nebular HeII emission implies the existence of a hard
radiation field. HeII-emitters are observed to be more frequent among high-z
galaxies than for local objects. So investigating the HeII-ionizing source(s)
in IZw18 may reveal the ionization processes at high-z. HeII emission in
star-forming galaxies, has been suggested to be mainly associated with
Wolf-Rayet stars (WRs), but WRs cannot satisfactorily explain the
HeII-ionization at all times, in particular at lowest metallicities. Shocks
from supernova remnants, or X-ray binaries, have been proposed as additional
potential sources of HeII-ionizing photons. Our data indicate that conventional
HeII-ionizing sources (WRs, shocks, X-ray binaries) are not sufficient to
explain the observed nebular HeII4686 emission in IZw18. We find that the
HeII-ionizing radiation expected from models for either low-metallicity
super-massive O stars or rotating metal-free stars could account for the
HeII-ionization budget measured, while only the latter models could explain the
highest values of HeII4686/Hbeta observed. The presence of such peculiar stars
in IZw18 is suggestive and further investigation in this regard is needed. This
letter highlights that some of the clues of the early Universe can be found
here in our cosmic backyard.Comment: 6 pages, 3 figures. Accepted for publication in ApJ Letter
The triggering probability of radio-loud AGN: A comparison of high and low excitation radio galaxies in hosts of different colors
Low luminosity radio-loud active galactic nuclei (AGN) are generally found in
massive red elliptical galaxies, where they are thought to be powered through
gas accretion from their surrounding hot halos in a radiatively inefficient
manner. These AGN are often referred to as "low-excitation" radio galaxies
(LERGs). When radio-loud AGN are found in galaxies with a young stellar
population and active star formation, they are usually high-power
radiatively-efficient radio AGN ("high-excitation", HERG). Using a sample of
low-redshift radio galaxies identified within the Sloan Digital Sky Survey
(SDSS), we determine the fraction of galaxies that host a radio-loud AGN,
, as a function of host galaxy stellar mass, , star formation
rate, color (defined by the 4000 \angstrom break strength), radio luminosity
and excitation state (HERG/LERG).
We find the following: 1. LERGs are predominantly found in red galaxies. 2.
The radio-loud AGN fraction of LERGs hosted by galaxies of any color follows a
power law. 3. The fraction of red galaxies
hosting a LERG decreases strongly for increasing radio luminosity. For massive
blue galaxies this is not the case. 4. The fraction of green galaxies hosting a
LERG is lower than that of either red or blue galaxies, at all radio
luminosities. 5. The radio-loud AGN fraction of HERGs hosted by galaxies of any
color follows a power law. 6. HERGs have a
strong preference to be hosted by green or blue galaxies. 7. The fraction of
galaxies hosting a HERG shows only a weak dependence on radio luminosity cut.
8. For both HERGs and LERGs, the hosting probability of blue galaxies shows a
strong dependence on star formation rate. This is not observed in galaxies of a
different color.[abridged]Comment: 7 pages, 6 figure
Spatially resolved integral field spectroscopy of the ionized gas in IZw18
We present a detailed 2D study of the ionized ISM of IZw18 using new PMAS-IFU
optical observations. IZw18 is a high-ionization galaxy which is among the most
metal-poor starbursts in the local Universe. This makes IZw18 a local benchmark
for understanding the properties most closely resembling those prevailing at
distant starbursts. Our IFU-aperture (~ 1.4 kpc x 1.4 kpc) samples the entire
IZw18 main body and an extended region of its ionized gas. Maps of relevant
emission lines and emission line ratios show that higher-excitation gas is
preferentially located close to the NW knot and thereabouts. We detect a
Wolf-Rayet feature near the NW knot. We derive spatially resolved and
integrated physical-chemical properties for the ionized gas in IZw18. We find
no dependence between the metallicity-indicator R23 and the ionization
parameter (as traced by [OIII]/[OII]) across IZw18. Over ~ 0.30 kpc^2, using
the [OIII]4363 line, we compute Te[OIII] values (~ 15000 - 25000 K), and oxygen
abundances are derived from the direct determinations of Te[OIII]. More than
70% of the higher-Te[OIII] (> 22000 K) spaxels are HeII4686-emitting spaxels
too. From a statistical analysis, we study the presence of variations in the
ISM physical-chemical properties. A galaxy-wide homogeneity, across hundreds of
parsecs, is seen in O/H. Based on spaxel-by-spaxel measurements, the
error-weighted mean of 12 + log(O/H) = 7.11 +/- 0.01 is taken as the
representative O/H for IZw18. Aperture effects on the derivation of O/H are
discussed. Using our IFU data we obtain, for the first time, the IZw18
integrated spectrum.Comment: Accepted for publication in MNRAS, 13 pages, 10 figures, 4 table
Absorption-line probes of the prevalence and properties of outflows in present-day star-forming galaxies
We analyze star forming galaxies drawn from SDSS DR7 to show how the
interstellar medium (ISM) Na I 5890, 5896 (Na D) absorption lines depend on
galaxy physical properties, and to look for evidence of galactic winds. We
combine the spectra of galaxies with similar geometry/physical parameters to
create composite spectra with signal-to-noise ~300. The stellar continuum is
modeled using stellar population synthesis models, and the continuum-normalized
spectrum is fit with two Na I absorption components. We find that: (1) ISM Na D
absorption lines with equivalent widths EW > 0.8A are only prevalent in disk
galaxies with specific properties -- large extinction (Av), high star formation
rates (SFR), high star formation rate per unit area (), or
high stellar mass (M*). (2) the ISM Na D absorption lines can be separated into
two components: a quiescent disk-like component at the galaxy systemic velocity
and an outflow component; (3) the disk-like component is much stronger in the
edge-on systems, and the outflow component covers a wide angle but is stronger
within 60deg of the disk rotation axis; (4) the EW and covering factor of the
disk component correlate strongly with dust attenuation, highlighting the
importance that dust shielding may play the survival of Na I. (5) The EW of the
outflow component depends primarily on and secondarily on
Av; (6) the outflow velocity varies from ~120 to 160km/s but shows little hint
of a correlation with galaxy physical properties over the modest dynamic range
that our sample probes (1.2 dex in log and 1 dex in log M*).Comment: 18 pages, 18 figures, accepted by A
The host galaxies of radio-loud AGN: mass dependencies, gas cooling and AGN feedback
The properties of the host galaxies of a well-defined sample of 2215
radio-loud AGN with redshifts 0.03 < z < 0.3, defined from the SDSS, are
investigated. These are predominantly low radio luminosity sources, with 1.4GHz
luminosities of 10^23 to 10^25 W/Hz. The fraction of galaxies that host
radio-loud AGN with L(1.4GHz) > 10^23 W/Hz is a strong function of stellar
mass, rising from nearly zero below a stellar mass of 10^10 Msun to more than
30% at 5x10^11 Msun. The integral radio luminosity function is derived in six
ranges of stellar and black hole mass. Its shape is very similar in all of
these ranges and can be well fitted by a broken power-law. Its normalisation
varies strongly with mass, as M_*^2.5 or M_BH^1.6; this scaling only begins to
break down when the predicted radio-loud fraction exceeds 20-30%. There is no
correlation between radio and emission line luminosities for the radio-loud AGN
in the sample and the probability that a galaxy of given mass is radio-loud is
independent of whether it is optically classified as an AGN. The host galaxies
of the radio-loud AGN have properties similar to those of ordinary galaxies of
the same mass.
All of these findings support the conclusion that the optical AGN and low
radio luminosity AGN phenomena are independent and are triggered by different
physical mechanisms. Intriguingly, the dependence on black hole mass of the
radio-loud AGN fraction mirrors that of the rate at which gas cools from the
hot atmospheres of elliptical galaxies. It is speculated that gas cooling
provides a natural explanation for the origin of the radio-loud AGN activity,
and it is argued that AGN heating could plausibly balance the cooling of the
gas over time. [Abridged]Comment: Accepted for publication in MNRAS. LaTeX, 16 pages. Figure 10 is in
colou
Astrophysical Probes of Fundamental Physics
I review the theoretical motivation for varying fundamental couplings and
discuss how these measurements can be used to constrain a number of fundamental
physics scenarios that would otherwise be inacessible to experiment. As a case
study I will focus on the relation between varying couplings and dark energy,
and explain how varying coupling measurements can be used to probe the nature
of dark energy, with important advantages over the standard methods. Assuming
that the current observational evidence for varying and is
correct, a several-sigma detection of dynamical dark energy is feasible within
a few years, using currently operational ground-based facilities. With
forthcoming instruments like CODEX, a high-accuracy reconstruction of the
equation of state may be possible all the way up to redshift .Comment: Invited Review talk at the ESO Precision Spectroscopy in Astrophysics
conference, to appear in the proceeding
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