43 research outputs found
The LBT Panoramic View on the Recent Star-Formation Activity in IC2574
We present deep imaging of the star-forming dwarf galaxy IC2574 in the M81
group taken with the Large Binocular Telescope in order to study in detail the
recent star-formation history of this galaxy and to constrain the stellar
feedback on its HI gas. We identify the star-forming areas in the galaxy by
removing a smooth disk component from the optical images. We construct
pixel-by-pixel maps of stellar age and stellar mass surface density in these
regions by comparing their observed colors with simple stellar populations
synthesized with STARBURST99. We find that an older burst occurred about 100
Myr ago within the inner 4 kpc and that a younger burst happened in the last 10
Myr mostly at galactocentric radii between 4 and 8 kpc. We analyze the stellar
populations residing in the known HI holes of IC2574. Our results indicate
that, even at the remarkable photometric depth of the LBT data, there is no
clear one-to-one association between the observed HI holes and the most recent
bursts of star formation in IC2574. The stellar populations formed during the
younger burst are usually located at the periphery of the HI holes and are seen
to be younger than the holes dynamical age. The kinetic energy of the holes
expansion is found to be on average 10% of the total stellar energy released by
the stellar winds and supernova explosions of the young stellar populations
within the holes. With the help of control apertures distributed across the
galaxy we estimate that the kinetic energy stored in the HI gas in the form of
its local velocity dispersion is about 35% of the total stellar energy.Comment: 16 pages, 14 figures, accepted for publication in Ap
Mergers and Mass Accretion Rates in Galaxy Assembly: The Millennium Simulation Compared to Observations of z~2 Galaxies
Recent observations of UV-/optically selected, massive star forming galaxies
at z~2 indicate that the baryonic mass assembly and star formation history is
dominated by continuous rapid accretion of gas and internal secular evolution,
rather than by major mergers. We use the Millennium Simulation to build new
halo merger trees, and extract halo merger fractions and mass accretion rates.
We find that even for halos not undergoing major mergers the mass accretion
rates are plausibly sufficient to account for the high star formation rates
observed in z~2 disks. On the other hand, the fraction of major mergers in the
Millennium Simulation is sufficient to account for the number counts of
submillimeter galaxies (SMGs), in support of observational evidence that these
are major mergers. When following the fate of these two populations in the
Millennium Simulation to z=0, we find that subsequent mergers are not frequent
enough to convert all z~2 turbulent disks into elliptical galaxies at z=0.
Similarly, mergers cannot transform the compact SMGs/red sequence galaxies at
z~2 into observed massive cluster ellipticals at z=0. We argue therefore, that
secular and internal evolution must play an important role in the evolution of
a significant fraction of z~2 UV-/optically and submillimeter selected galaxy
populations.Comment: 5 pages, 4 figures, Accepted for publication in Ap
The Blue Straggler population in the globular cluster M53 (NGC5024): a combined HST, LBT, CFHT study
We used a proper combination of multiband high-resolution and wide field
multi-wavelength observations collected at three different telescopes (HST, LBT
and CFHT) to probe Blue Straggler Star (BSS) populations in the globular
cluster M53. Almost 200 BSS have been identified over the entire cluster
extension. The radial distribution of these stars has been found to be bimodal
(similarly to that of several other clusters) with a prominent dip at ~60'' (~2
r_c) from the cluster center. This value turns out to be a factor of two
smaller than the radius of avoidance (r_avoid, the radius within which all the
stars of ~1.2 M_sun have sunk to the core because of dynamical friction effects
in an Hubble time). While in most of the clusters with a bimodal BSS radial
distribution, r_avoid has been found to be located in the region of the
observed minimum, this is the second case (after NGC6388) where this
discrepancy is noted. This evidence suggests that in a few clusters the
dynamical friction seems to be somehow less efficient than expected.
We have also used this data base to construct the radial star density profile
of the cluster: this is the most extended and accurate radial profile ever
published for this cluster, including detailed star counts in the very inner
region. The star density profile is reproduced by a standard King Model with an
extended core (~25'') and a modest value of the concentration parameter
(c=1.58). A deviation from the model is noted in the most external region of
the cluster (at r>6.5' from the center). This feature needs to be further
investigated in order to address the possible presence of a tidal tail in this
cluster.Comment: 25 pages, 9 figures, accepted for publication on Ap
From rings to bulges: evidence for rapid secular galaxy evolution at z~2 from integral field spectroscopy in the SINS survey
We present Ha integral field spectroscopy of well resolved, UV/optically
selected z~2 star-forming galaxies as part of the SINS survey with SINFONI on
the ESO VLT. Our laser guide star adaptive optics and good seeing data show the
presence of turbulent rotating star forming rings/disks, plus central
bulge/inner disk components, whose mass fractions relative to total dynamical
mass appears to scale with [NII]/Ha flux ratio and star formation age. We
propose that the buildup of the central disks and bulges of massive galaxies at
z~2 can be driven by the early secular evolution of gas-rich proto-disks. High
redshift disks exhibit large random motions. This turbulence may in part be
stirred up by the release of gravitational energy in the rapid cold accretion
flows along the filaments of the cosmic web. As a result dynamical friction and
viscous processes proceed on a time scale of <1 Gyr, at least an order of
magnitude faster than in z~0 disk galaxies. Early secular evolution thus drives
gas and stars into the central regions and can build up exponential disks and
massive bulges, even without major mergers. Secular evolution along with
increased efficiency of star formation at high surface densities may also help
to account for the short time scales of the stellar buildup observed in massive
galaxies at z~2.Comment: accepted Astrophysical Journal, main July 8 200
Go Long, Go Deep: Finding Optical Jet Breaks for Swift-Era GRBs with the LBT
Using the 8.4m Large Binocular Telescope, we observed six GRB afterglows from
2.8 hours to 30.8 days after the burst triggers to systematically probe the
late time behaviors of afterglows including jet breaks, flares, and supernova
bumps. We detected five afterglows with Sloan r' magnitudes ranging from
23.0-26.3 mag. The depth of our observations allows us to extend the temporal
baseline for measuring jet breaks by another decade in time scale. We detected
two jet breaks and a third candidate, all of which are not detectable without
deep, late time optical observations. In the other three cases, we do not
detect the jet breaks either because of contamination from the host galaxy
light, the presence of a supernova bump, or the intrinsic faintness of the
optical afterglow.
This suggests that the basic picture that GRBs are collimated is still valid
and that the apparent lack of Swift jet breaks is due to poorly sampled
afterglow light curves, particularly at late times.Comment: Accepted by ApJ Letters, 14 pages, 2 figure
Black Hole Mass Estimates Based on CIV are Consistent with Those Based on the Balmer Lines
Using a sample of high-redshift lensed quasars from the CASTLES project with
observed-frame ultraviolet or optical and near-infrared spectra, we have
searched for possible biases between supermassive black hole (BH) mass
estimates based on the CIV, Halpha and Hbeta broad emission lines. Our sample
is based upon that of Greene, Peng & Ludwig, expanded with new near-IR
spectroscopic observations, consistently analyzed high S/N optical spectra, and
consistent continuum luminosity estimates at 5100A. We find that BH mass
estimates based on the FWHM of CIV show a systematic offset with respect to
those obtained from the line dispersion, sigma_l, of the same emission line,
but not with those obtained from the FWHM of Halpha and Hbeta. The magnitude of
the offset depends on the treatment of the HeII and FeII emission blended with
CIV, but there is little scatter for any fixed measurement prescription. While
we otherwise find no systematic offsets between CIV and Balmer line mass
estimates, we do find that the residuals between them are strongly correlated
with the ratio of the UV and optical continuum luminosities. Removing this
dependency reduces the scatter between the UV- and optical-based BH mass
estimates by a factor of approximately 2, from roughly 0.35 to 0.18 dex. The
dispersion is smallest when comparing the CIV sigma_l mass estimate, after
removing the offset from the FWHM estimates, and either Balmer line mass
estimate. The correlation with the continuum slope is likely due to a
combination of reddening, host contamination and object-dependent SED shapes.
When we add additional heterogeneous measurements from the literature, the
results are unchanged.Comment: Accepted for publication in The Astrophysical Journal. 37 text pages
+ 8 tables + 23 figures. Updated with comments by the referee and with a
expanded discussion on literature data including new observation
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
The SINS survey of z~2 galaxy kinematics: properties of the giant star forming clumps
We have studied the properties of giant star forming clumps in five z~2
star-forming disks with deep SINFONI AO spectroscopy at the ESO VLT. The clumps
reside in disk regions where the Toomre Q-parameter is below unity, consistent
with their being bound and having formed from gravitational instability. Broad
H{\alpha}/[NII] line wings demonstrate that the clumps are launching sites of
powerful outflows. The inferred outflow rates are comparable to or exceed the
star formation rates, in one case by a factor of eight. Typical clumps may lose
a fraction of their original gas by feedback in a few hundred million years,
allowing them to migrate into the center. The most active clumps may lose much
of their mass and disrupt in the disk. The clumps leave a modest imprint on the
gas kinematics. Velocity gradients across the clumps are 10-40 km/s/kpc,
similar to the galactic rotation gradients. Given beam smearing and clump
sizes, these gradients may be consistent with significant rotational support in
typical clumps. Extreme clumps may not be rotationally supported; either they
are not virialized, or they are predominantly pressure supported. The velocity
dispersion is spatially rather constant and increases only weakly with star
formation surface density. The large velocity dispersions may be driven by the
release of gravitational energy, either at the outer disk/accreting streams
interface, and/or by the clump migration within the disk. Spatial variations in
the inferred gas phase oxygen abundance are broadly consistent with inside-out
growing disks, and/or with inward migration of the clumps.Comment: accepted Astrophys. Journal, February 9, 201
LBT/ARGOS adaptive optics observations of z lensed galaxies
Gravitationally lensed systems allow a detailed view of galaxies at high
redshift. High spatial- and spectral-resolution measurements of arc-like
structures can offer unique constraints on the physical and dynamical
properties of high-z systems. We present near-infrared spectra centred on the
gravitational arcs of six known z ~ 2 lensed star-forming galaxies of stellar
masses of 10^9-10^11 Msun and star formation rate (SFR) in the range between 10
and 400 Msun/yr. Ground layer adaptive optics (AO)-assisted observations are
obtained at the Large Binocular Telescope (LBT) with the LUCI spectrographs
during the commissioning of the ARGOS facility. We used MOS masks with curved
slits to follow the extended arched structures and study the diagnostic
emission lines. Combining spatially resolved kinematic properties across the
arc-like morphologies, emission line diagnostics and archival information, we
distinguish between merging and rotationally supported systems, and reveal the
possible presence of ejected gas. For galaxies that have evidence for outflows,
we derive outflow energetics and mass-loading factors compatible with those
observed for stellar winds in local and high-z galaxies. We also use flux ratio
diagnostics to derive gas-phase metallicities. The low signal-to-noise ratio in
the faint H and nitrogen lines allows us to derive an upper limit of ~
0.15 dex for the spatial variations in metallicity along the slit for the
lensed galaxy J1038. Analysed near-infrared spectra presented here represent
the first scientific demonstration of performing AO-assisted multi-object
spectroscopy with narrow curved-shape slits. The increased angular and spectral
resolution, combined with the binocular operation mode with the 8.4-m-wide eyes
of LBT, will allow the characterisation of kinematic and chemical properties of
a large sample of galaxies at high-z in the near future.Comment: 18 pages, 13 figures, accepted for publication in A&
NEBULAR EXCITATION IN z ∼ 2 STAR-FORMING GALAXIES FROM THE SINS AND LUCI SURVEYS: THE INFLUENCE OF SHOCKS AND AGN ∗
Based on high-resolution, spatially resolved data of 10 z ∼ 2 star-forming galaxies from th