50 research outputs found
The rapid formation a large rotating disk galaxy three billion years after the Big Bang
[Abridged] Over the past two decades observations and theoretical simulations
have established a global frame-work of galaxy formation and evolution in the
young Universe. Galaxies formed as baryonic gas cooled at the centres of
collapsing dark matter halos. Mergers of halos led to the build up of galaxy
mass. A major step forward in understanding these issues requires well resolved
physical information on individual galaxies at high redshift. Here we report
adaptive optics, spectroscopic observations of a representative luminous star
forming galaxy when the Universe was only twenty percent of its age. The
superior angular resolution of these data reveals the physical and dynamical
properties of a high redshift galaxy in unprecedented detail. A large and
massive rotating proto-disk is channelling gas towards a growing central
stellar bulge hosting an accreting massive black hole.Comment: Narure, accepted (Released Aug 17th
Alpha element abundances and gradients in the Milky Way bulge from FLAMES-GIRAFFE spectra of 650 K giants
We obtained FLAMES-GIRAFFE spectra (R=22,500) at the ESO Very Large Telescope
for 650 bulge red giant branch (RGB) stars and performed spectral synthesis to
measure Mg, Ca, Ti, and Si abundances. This sample is composed of 474 giant
stars observed in 3 fields along the minor axis of the Galactic bulge and at
latitudes b=-4, b=-6, b=-12. Another 176 stars belong to a field containing the
globular cluster NGC 6553, located at b=-3 and 5 degrees away from the other
three fields along the major axis. Our results confirm, with large number
statistics, the chemical similarity between the Galactic bulge and thick disk,
which are both enhanced in alpha elements when compared to the thin disk. In
the same context, we analyze [alpha/Fe] vs. [Fe/H] trends across different
bulge regions. The most metal rich stars, showing low [alpha/Fe] ratios at b=-4
disappear at higher Galactic latitudes in agreement with the observed
metallicity gradient in the bulge. Metal-poor stars ([Fe/H]<-0.2) show a
remarkable homogeneity at different bulge locations. We have obtained further
constrains for the formation scenario of the Galactic bulge. A metal-poor
component chemically indistinguishable from the thick disk hints for a fast and
early formation for both the bulge and the thick disk. Such a component shows
no variation, neither in abundances nor kinematics, among different bulge
regions. A metal-rich component showing low [alpha/Fe] similar to those of the
thin disk disappears at larger latitudes. This allows us to trace a component
formed through fast early mergers (classical bulge) and a disk/bar component
formed on a more extended timescale.Comment: 13 pages, 17 figures. Accepted for publication in Astronomy and
Astrophysic
Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. II. Ages, metallicities, detailed elemental abundances, and connections to the Galactic thick disc
The Bulge is the least understood major stellar population of the Milky Way.
Most of what we know about the formation and evolution of the Bulge comes from
bright giant stars. The underlying assumption that giants represent all the
stars, and accurately trace the chemical evolution of a stellar population, is
under debate. In particular, recent observations of a few microlensed dwarf
stars give a very different picture of the evolution of the Bulge from that
given by the giant stars. [ABRIDGED] We perform a detailed elemental abundance
analysis of dwarf stars in the Galactic bulge, based on high-resolution spectra
that were obtained while the stars were optically magnified during
gravitational microlensing events. [ABRIDGED] We present detailed elemental
abundances and stellar ages for six new dwarf stars in the Galactic bulge.
Combining these with previous events, here re-analysed with the same methods,
we study a homogeneous sample of 15 stars, which constitute the largest sample
to date of microlensed dwarf stars in the Galactic bulge. We find that the
stars span the full range of metallicities from [Fe/H]=-0.72 to +0.54, and an
average metallicity of =-0.08+/-0.47, close to the average metallicity
based on giant stars in the Bulge. Furthermore, the stars follow well-defined
abundance trends, that for [Fe/H]<0 are very similar to those of the local
Galactic thick disc. This suggests that the Bulge and the thick disc have had,
at least partially, comparable chemical histories. At sub-solar metallicities
we find the Bulge dwarf stars to have consistently old ages, while at
super-solar metallicities we find a wide range of ages. Using the new age and
abundance results from the microlensed dwarf stars we investigate possible
formation scenarios for the Bulge.Comment: New version accepted for publication in Astronomy and Astrophysic
Turbulence and galactic structure
Interstellar turbulence is driven over a wide range of scales by processes
including spiral arm instabilities and supernovae, and it affects the rate and
morphology of star formation, energy dissipation, and angular momentum transfer
in galaxy disks. Star formation is initiated on large scales by gravitational
instabilities which control the overall rate through the long dynamical time
corresponding to the average ISM density. Stars form at much higher densities
than average, however, and at much faster rates locally, so the slow average
rate arises because the fraction of the gas mass that forms stars at any one
time is low, ~10^{-4}. This low fraction is determined by turbulence
compression, and is apparently independent of specific cloud formation
processes which all operate at lower densities. Turbulence compression also
accounts for the formation of most stars in clusters, along with the cluster
mass spectrum, and it gives a hierarchical distribution to the positions of
these clusters and to star-forming regions in general. Turbulent motions appear
to be very fast in irregular galaxies at high redshift, possibly having speeds
equal to several tenths of the rotation speed in view of the morphology of
chain galaxies and their face-on counterparts. The origin of this turbulence is
not evident, but some of it could come from accretion onto the disk. Such high
turbulence could help drive an early epoch of gas inflow through viscous
torques in galaxies where spiral arms and bars are weak. Such evolution may
lead to bulge or bar formation, or to bar re-formation if a previous bar
dissolved. We show evidence that the bar fraction is about constant with
redshift out to z~1, and model the formation and destruction rates of bars
required to achieve this constancy.Comment: in: Penetrating Bars through Masks of Cosmic Dust: The Hubble Tuning
Fork strikes a New Note, Eds., K. Freeman, D. Block, I. Puerari, R. Groess,
Dordrecht: Kluwer, in press (presented at a conference in South Africa, June
7-12, 2004). 19 pgs, 5 figure
Constraints on the assembly and dynamics of galaxies. II. Properties of kiloparsec-scale clumps in rest-frame optical emission of z ~ 2 star-forming galaxies
We study the properties of luminous stellar clumps identified in deep, high
resolution HST/NIC2 F160W imaging at 1.6um of six z~2 star-forming galaxies
with existing near-IR integral field spectroscopy from SINFONI at the VLT.
Individual clumps contribute ~0.5%-15% of the galaxy-integrated rest-frame
~5000A emission, with median of about 2%; the total contribution of clump light
ranges from 10%-25%. The median intrinsic clump size and stellar mass are ~1kpc
and log(Mstar[Msun])~9, in the ranges for clumps identified in rest-UV or line
emission in other studies. The clump sizes and masses in the subset of disks
are broadly consistent with expectations for clump formation via gravitational
instabilities in gas-rich, turbulent disks given the host galaxies' global
properties. By combining the NIC2 data with ACS/F814W imaging available for one
source, and AO-assisted SINFONI Halpha data for another, we infer modest color,
M/L, and stellar age variations within each galaxy. In these two objects, sets
of clumps identified at different wavelengths do not fully overlap;
NIC2-identified clumps tend to be redder/older than ACS- or Halpha-identified
clumps without rest-frame optical counterparts. There is evidence for a
systematic trend of older ages at smaller galactocentric radii among the
clumps, consistent with scenarios where inward migration of clumps transports
material towards the central regions. From constraints on a bulge-like
component at radii <1-3kpc, none of the five disks in our sample appears to
contain a compact massive stellar core, and we do not discern a trend of bulge
stellar mass fraction with stellar age of the galaxy. Further observations are
necessary to probe the build-up of stellar bulges and the role of clumps in
this process.Comment: 29 pages, 11 figures. Revised version accepted for publication in the
Astrophysical Journa
High-Redshift Star-Forming Galaxies: Angular Momentum and Baryon Fraction, Turbulent Pressure Effects and the Origin of Turbulence
The structure of a sample of high-redshift (z=2), rotating galaxies with high
star formation rates and turbulent gas velocities of sigma=40-80 km/s is
investigated. Fitting the observed disk rotational velocities and radii with a
Mo, Mao, White (1998) (MMW) model requires unusually large disk spin parameters
lambda_d>0.1 and disk-to-dark halo mass fraction m_d=0.2, close to the cosmic
baryon fraction. The galaxies segregate into dispersion-dominated systems with
1<vmax/sigma<3, maximum rotational velocities vmax<200 km/s and disk half-light
radii rd=1-3 kpc and rotation-dominated systems with vmax>200 km/s,
vmax/sigma>3 and rd=4-8 kpc. For the dispersion-dominated sample, radial
pressure gradients partly compensate the gravitational force, reducing the
rotational velocities. Including this pressure effect in the MMW model,
dispersion-dominated galaxies can be fitted well with spin parameters lf
lambda_d=0.03-0.05 for high disk mass fractions of m_d=0.2 and with
lambda_d=0.01-0.03 for m_d=0.05. These values are in good agreement with
cosmological expectations. For the rotation-dominated sample however pressure
effects are small and better agreement with theoretically expected disk spin
parameters can only be achieved if the dark halo mass contribution in the
visible disk regime (2-3*rd) is smaller than predicted by the MMW model. We
argue that these galaxies can still be embedded in standard cold dark matter
halos if the halos did not contract adiabatically in response to disk
formation. It is shown that the observed high turbulent gas motions of the
galaxies are consistent with a Toomre instability parameter Q=1 which is equal
to the critical value, expected for gravitational disk instability to be the
major driver of turbulence. The dominant energy source of turbulence is then
the potential energy of the gas in the disk.Comment: 23 pages, 4 figures, ApJ, in pres
An HST/WFC3-IR Morphological Survey of Galaxies at z = 1.5-3.6: II. The Relation between Morphology and Gas-Phase Kinematics
We analyze rest-frame optical morphologies and gas-phase kinematics as traced
by rest-frame far-UV and optical spectra for a sample of 204 star forming
galaxies in the redshift range z ~ 2-3 drawn from the Keck Baryonic Structure
Survey (KBSS). We find that spectroscopic properties and gas-phase kinematics
are closely linked to morphology: compact galaxies with semi-major axis radii r
<~ 2 kpc are substantially more likely than their larger counterparts to
exhibit LyA in emission. Although LyA emission strength varies widely within
galaxies of a given morphological type, all but one of 19 galaxies with LyA
equivalent width W_LyA > 20 Angstroms have compact and/or multiple-component
morphologies with r <= 2.5 kpc. The velocity structure of absorption lines in
the galactic continuum spectra also varies as a function of morphology.
Galaxies of all morphological types drive similarly strong outflows (as traced
by the blue wing of interstellar absorption line features), but the outflows of
larger galaxies are less highly ionized and exhibit larger optical depth at the
systemic redshift that may correspond to a decreasing efficiency of feedback in
evacuating gas from the galaxy. This v ~ 0 km/s gas is responsible both for
shifting the mean absorption line redshift and attenuating W_LyA (via a longer
resonant scattering path) in galaxies with larger rest-optical half light
radii. In contrast to galaxies at lower redshifts, there is no evidence for a
correlation between outflow velocity and inclination, suggesting that outflows
from these puffy and irregular systems may be poorly collimated. (Abbrev.)Comment: 18 pages, 11 figures. Revised version accepted for publication in
ApJ. Version with full-resolution figures is available at
http://di.utoronto.ca/~drlaw/Papers/wfc3_uvspec.pd
3D kinematics through the X-shaped Milky Way bulge
Context. It has recently been discovered that the Galactic bulge is X-shaped, with the two southern arms of the X both crossing the lines of sight at l = 0 and | b| > 4, hence producing a double red clump in the bulge color magnitude diagram. Dynamical models predict the formation of X-shaped bulges as extreme cases of boxy-peanut bulges. However, since X-shaped bulges were known to be present only in external galaxies, models have never been compared to 3D kinematical data for individual stars.
Aims. We study the orbital motion of Galactic bulge stars in the two arms (overdensities) of the X in the southern hemisphere. The goal is to provide observational constraints to bulge formation models that predict the formation of X-shapes through bar dynamical instabilities.
Methods. Radial velocities have been obtained for a sample of 454 bulge giants, roughly equally distributed between the bright and the faint red clump, in a field at (l,b) = (0, â6). Proper motions were derived for all red clump stars in the same field by combining images from two epochs, which were obtained 11 years apart, with WFI at the 2.2âm at La Silla. The observed field contains the globular cluster NGC 6558, whose member stars were used to assess the accuracy of the proper motion measurement. At the same time, as a by-product, we provide the first proper motion measurement of NGC 6558. The proper motions for the spectroscopic subsample are analyzed for a subsample of 352 stars, taking into account the radial velocities and metallicities measured from near-infrared calcium triplet lines.
Results. The radial velocity distribution of stars in the bright red clump, which traces the closer overdensity of bulge stars, shows an excess of stars moving towards the Sun. Similarly, an excess of stars receding from the Sun is seen in the far overdensity, which is traced by faint red clump stars. This is explained by the presence of stars on elongated orbits, which are most likely streaming along the arms of the X-shaped bulge. Proper motions for these stars are consistent with qualitative predictions of dynamical models of peanut-shaped bulges. Surprisingly, stars on elongated orbits have preferentially metal-poor (subsolar) metallicities, while the metal rich ones, in both overdensities, are preferentially found in more axisymmetric orbits. The observed proper motion of NGC 6558 has been measured as (ÎŒlcos â (b),ÎŒb) = (0.30 â ± â 0.14, â0.43 ± 0.13), with a velocity dispersion of (Ïlcos(b),Ïb) = (1.8,1.7) mas/yr. This is the first proper motion measurement for this cluster
Gas Physics, Disk Fragmentation, and Bulge Formation in Young Galaxies
We investigate the evolution of star-forming gas-rich disks, using a 3D
chemodynamical model including a dark halo, stars, and a two-phase interstellar
medium with feedback processes from the stars. We show that galaxy evolution
proceeds along very different routes depending on whether it is the gas disk or
the stellar disk which first becomes unstable, as measured by the respective
Q-parameters. This in turn depends on the uncertain efficiency of energy
dissipation of the cold cloud component from which stars form. When the cold
gas cools efficiently and drives the instability, the galactic disk fragments
and forms a number of massive clumps of stars and gas. The clumps spiral to the
center of the galaxy in a few dynamical times and merge there to form a central
bulge component in a strong starburst. When the kinetic energy of the cold
clouds is dissipated at a lower rate, stars form from the gas in a more
quiescent mode, and an instability only sets in at later times, when the
surface density of the stellar disk has grown sufficiently high. The system
then forms a stellar bar, which channels gas into the center, evolves, and
forms a bulge whose stars are the result of a more extended star formation
history. We investigate the stability of the gas-stellar disks in both regimes,
as well as the star formation rates and element enrichment. We study the
morphology of the evolving disks, calculating spatially resolved colours from
the distribution of stars in age and metallicity, including dust absorption. We
then discuss morphological observations such as clumpy structures and chain
galaxies at high redshift as possible signatures of fragmenting, gas-rich
disks. Finally, we investigate abundance ratio distributions as a means to
distinguish the different scenarios for bulge formation.Comment: 16 pages, Latex, 14 figures, to appear in Astronomy and Astrophysics,
Version with high quality images available at
http://www.astro.unibas.ch/leute/ai.shtm
The Kiloparsec-Scale Kinematics of High-Redshift Star-Forming Galaxies
We present the results of a spectroscopic survey of the kinematic structure
of star-forming galaxies at redshift z ~ 2 - 3 using Keck/OSIRIS integral field
spectroscopy. Our sample is comprised of 12 galaxies between redshifts z ~ 2.0
and 2.5 and one galaxy at z ~ 3.3 which are well detected in either HAlpha or
[O III] emission. These observations were obtained in conjunction with the Keck
laser guide star adaptive optics system, with a typical angular resolution
after spatial smoothing ~ 0.15" (approximately 1 kpc at the redshift of the
target sample). At most five of these 13 galaxies have spatially resolved
velocity gradients consistent with rotation while the remaining galaxies have
relatively featureless or irregular velocity fields. All of our galaxies show
local velocity dispersions ~ 60 - 100 km/s, suggesting that (particularly for
those galaxies with featureless velocity fields) rotation about a preferred
axis may not be the dominant mechanism of physical support. While some galaxies
show evidence for major mergers such evidence is unrelated to the kinematics of
individual components (one of our strongest merger candidates also exhibits
unambiguous rotational structure), refuting a simple bimodal disk/merger
classification scheme. We discuss these data in light of complementary surveys
and extant UV-IR spectroscopy and photometry, concluding that the dynamical
importance of cold gas may be the primary factor governing the observed
kinematics of z ~ 2 galaxies. We conclude by speculating on the importance of
mechanisms for accreting low angular-momentum gas and the early formation of
quasi-spheroidal systems in the young universe.(abridged)Comment: 34 pages, 13 figures. Revised version accepted for publication in the
Astrophysical Journal. Version with full-resolution figures is available at
http://www.astro.ucla.edu/~drlaw/Papers/OSIRIS_data2.pd