7,035 research outputs found
The Nature of Starbursts: I. The Star Formation Histories of Eighteen Nearby Starburst Dwarf Galaxies
We use archival HST observations of resolved stellar populations to derive
the star formation histories (SFHs) of 18 nearby starburst dwarf galaxies. In
this first paper we present the observations, color-magnitude diagrams, and the
SFHs of the 18 starburst galaxies, based on a homogeneous approach to the data
reduction, differential extinction, and treatment of photometric completeness.
We adopt a star formation rate (SFR) threshold normalized to the average SFR of
the individual system as a metric for classifying starbursts in SFHs derived
from resolved stellar populations. This choice facilitates finding not only
currently bursting galaxies but also "fossil" bursts increasing the sample size
of starburst galaxies in the nearby (D<8 Mpc) universe. Thirteen of the
eighteen galaxies are experiencing ongoing bursts and five galaxies show fossil
bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a
burst are sustained for hundreds of Myr with variations on small timescales. A
long >100 Myr temporal baseline is thus fundamental to any starburst definition
or identification method. The longer lived bursts rule out rapid
"self-quenching" of starbursts on global scales. The bursting galaxies' gas
consumption timescales are shorter than the Hubble time for all but one galaxy
confirming the short-lived nature of starbursts based on fuel limitations.
Additionally, we find the strength of the H{\alpha} emission usually correlates
with the CMD based SFR during the last 4-10 Myr. However, in four cases, the
H{\alpha} emission is significantly less than what is expected for models of
starbursts; the discrepancy is due to the SFR changing on timescales of a few
Myr. The inherently short timescale of the H{\alpha} emission limits
identifying galaxies as starbursts based on the current characteristics which
may or may not be representative of the recent SFH of a galaxy.Comment: 53 pages, 11 figure
Revealing the Young Starburst in Haro 3 with Radio and Infrared Imaging
The Wolf-Rayet galaxy Haro 3 (Mrk 35, NGC 3353) was observed at the near-IR
and radio wavelengths as part of ongoing program to study the earliest stages
of starbursts. These observations confirm that the current episode of star
formation is dominated by a single region (region A). While there are knots of
recent (~10 Myr) star formation outside of region A, the sources of ionizing
radiation as observed in both radio and Br-gamma observations are almost
exclusively associated with region A. The derived ionizing flux implies a star
formation rate of ~0.6 solar masses per year localized within a radius of ~0.1
kpc. A comparison with observations from HST indicates that one or more of the
star clusters in region A are optically obscured. The star clusters in region A
have ages at least as young as ~5 Myr, and possibly as young as ~0.1 Myr. The
star cluster that appears to be the youngest also exhibits a near-IR excess in
its colors, possibly indicating natal dust in very close proximity to the
ionizing stars. The difference between optical- and radio-determined ionizing
fluxes as well as the near-IR colors indicate an average extinction value of
A_V ~2.5 in region A. The total stellar mass associated with the current
starburst in region A is inferred from both the near-IR and radio observations
to be ~10^6 solar masses. The other main stellar concentrations observed in the
near-IR (Regions B1 and B2) are somewhat older than region A, with ages ~8-10
Myr, and the near-IR observations indicate they have stellar masses of ~8x10^4
and ~2x10^4 solar masses, respectively.Comment: 17 pages, 3 figures, to appear in AJ August 200
Chandra Observations of Extended X-ray Emission in Arp 220
We resolve the extended X-ray emission from the prototypical ultraluminous
infrared galaxy Arp 220. Extended, faint edge-brightened, soft X-ray lobes
outside the optical galaxy are observed to a distance of 10 to 15 kpc on each
side of the nuclear region. Bright plumes inside the optical isophotes coincide
with the optical line emission and extend 11 kpc from end to end across the
nucleus. The data for the plumes cannot be fit by a single temperature plasma,
and display a range of temperatures from 0.2 to 1 keV. The plumes emerge from
bright, diffuse circumnuclear emission in the inner 3 kpc centered on the
Halpha peak, which is displaced from the radio nuclei. There is a close
morphological correspondence between the Halpha and soft X-ray emission on all
spatial scales. We interpret the plumes as a starburst-driven superwind, and
discuss two interpretations of the emission from the lobes in the context of
simulations of the merger dynamics of Arp 220.Comment: Accepted for publication in ApJ; see also astro-ph/0208477 (Paper 1
ALMA Observations of Warm Dense Gas in NGC 1614 --- Breaking of Star Formation Law in the Central kpc
We present ALMA Cycle-0 observations of the CO (6-5) line emission and of the
435um dust continuum emission in the central kpc of NGC 1614, a local luminous
infrared galaxy (LIRG) at a distance of 67.8 Mpc (1 arcsec = 329 pc). The CO
emission is well resolved by the ALMA beam (0".26 x 0".20) into a
circum-nuclear ring, with an integrated flux of f_{CO(6-5)} = 898 (+-153) Jy
km/s, which is 63(+-12)% of the total CO(6-5) flux measured by Herschel. The
molecular ring, located between 100pc < r < 350pc from the nucleus, looks
clumpy and includes seven unresolved (or marginally resolved) knots with median
velocity dispersion of 40 km/s. These knots are associated with strong star
formation regions with \Sigma_{SFR} 100 M_\sun/yr/kpc^{2} and \Sigma_{Gas}
1.0E4 M_\sun/pc^{2}. The non-detections of the nucleus in both the CO (6-5)
line emission and the 435um continuum rule out, with relatively high
confidence, a Compton-thick AGN in NGC 1614. Comparisons with radio continuum
emission show a strong deviation from an expected local correlation between
\Sigma_{Gas} and \Sigma_{SFR}, indicating a breakdown of the Kennicutt-Schmidt
law on the linear scale of 100 pc.Comment: 13 pages, 10 figures; accepted by Ap
The Luminous Starburst Ring in NGC 7771: Sequential Star Formation?
Only two of the twenty highly luminous starburst galaxies analyzed by Smith
et al. exhibit circumnuclear rings of star formation. These galaxies provide a
link between 10^11 L_sun systems and classical, less-luminous ringed systems.
We report the discovery of a near-infrared counterpart to the nuclear ring of
radio emission in NGC 7771. A displacement between the ~10 radio bright clumps
and the ~10 near-infrared bright clumps indicates the presence of multiple
generations of star formation. The estimated thermal emission from each radio
source is equivalent to that of ~35000 O6 stars. Each near-infrared bright knot
contains ~5000 red supergiants, on average. The stellar mass of each knot is
estimated to be ~10^7 M_sun. The implied time-averaged star formation rate is
\~40 M_sun per yr. Several similarities are found between the properties of
this system and other ringed and non-ringed starbursts. Morphological
differences between NGC 7771 and the starburst + Seyfert 1 galaxy NGC 7469
suggest that NGC 7771 may not be old enough to fuel an AGN, or may not be
capable of fueling an AGN. Alternatively, the differences may be unrelated to
the presence or absence of an AGN and may simply reflect the possibility that
star formation in rings is episodic.Comment: accepted for publication in The Astrophysical Journal (10 January
1999); 48 pages including 13 figures; AAS LaTe
Constraints on the presence of water megamaser emission in z~2.5 ultraluminous infrared starburst galaxies
We present Expanded Very Large Array and Arecibo observations of two lensed
submm galaxies at z~2.5, in order to search for redshifted 22.235 GHz water
megamaser emission. Both SMM J14011+0252 and SMM J16359+6612 have
multi-wavelength characteristics consistent with ongoing starburst activity, as
well as CO line emission indicating the presence of warm molecular gas. Our
observations do not reveal any evidence for H2O megamaser emission in either
target, while the lensing allows us to obtain deep limits to the H_2O line
luminosities, L(H2O) < 7470 Lsun (3-sigma) in the case of SMM J14011+0252, and
L(H2O) < 1893 Lsun for SMM J16359+6612, assuming linewidths of 80 km/s. Our
search for, and subsequent non-detection of H2O megamaser emission in two
strongly lensed starburst galaxies, rich in gas and dust, suggests that such
megamaser emission is not likely to be common within the unlensed population of
high-redshift starburst galaxies. We use the recent detection of strong H2O
megamaser emission in the lensed quasar, MG J0414+0534 at z = 2.64 to make
predictions for future EVLA C-band surveys of H2O megamaser emission in submm
galaxies hosting AGN.Comment: AJ accepte
The Evolutionary Status of Isolated Dwarf Irregular Galaxies II. Star Formation Histories and Gas Depletion
The results of UBV and H alpha imaging of a large sample of isolated dwarf
irregular galaxies are interpreted in the context of composite stellar
population models. The observed optical colors are best fit by composite
stellar populations which have had approximately constant star formation rates
for at least 10 Gyr. The galaxies span a range of central surface brightness,
from 20.5 to 25.0 mag arcsec^{-2}; there is no correlation between surface
brightness and star formation history. Although the current star formation
rates are low, it is possible to reproduce the observed luminosities without a
major starburst episode. The derived gas depletion timescales are long,
typically ~20 Gyr. These results indicate that dwarf irregular galaxies will be
able to continue with their slow, but constant, star formation activity for at
least another Hubble time.
The sample of isolated dIs is compared to a sample of star bursting dwarf
galaxies taken from the literature. The star bursting dwarf galaxies have many
similar properties; the main difference between these two types of gas-rich
dwarf galaxies is that the current star formation is concentrated in the center
of the star bursting systems while it is much more distributed in the quiescent
dIs. This results in pronounced color gradients for the starbursting dwarf
galaxies, while the majority of the quiescent dwarf irregular galaxies have
minor or non-existent color gradients. Thus, the combination of low current
star formation rates, blue colors, and the lack of significant color gradients
indicates that star formation percolates slowly across the disk of normal dwarf
galaxies in a quasi-continuous manner.Comment: 16 pages, uses emulateapj, to appear in The Astronomical Journal
(April 2001
High Sensitivity Array Observations of the z=1.87 Sub-Millimeter Galaxy GOODS 850-3
We present sensitive phase-referenced VLBI results on the radio continuum
emission from the z=1.87 luminous submillimeter galaxy (SMG) GOODS 850-3. The
observations were carried out at 1.4 GHz using the High Sensitivity Array
(HSA). Our sensitive tapered VLBI image of GOODS 850-3 at 0.47 x 0.34 arcsec
(3.9 x 2.9 kpc) resolution shows a marginally resolved continuum structure with
a peak flux density of 148 \pm 38 uJy/beam, and a total flux density of 168 \pm
73 uJy, consistent with previous VLA and MERLIN measurements. The derived
intrinsic brightness temperature is > 5 \pm 2 x 10^3 K. The radio continuum
position of this galaxy coincides with a bright and extended near-infrared
source that nearly disappears in the deep HST optical image, indicating a dusty
source of nearly 9 kpc in diameter. No continuum emission is detected at the
full VLBI resolution (13.2 x 7.2 mas, 111 x 61 pc), with a 4-sigma point source
upper limit of 26 uJy/beam, or an upper limit to the intrinsic brightness
temperature of 4.7 x 10^5 K. The extent of the observed continuum source at 1.4
GHz and the derived brightness temperature limits are consistent with the radio
emission (and thus presumably the far-infrared emission) being powered by a
major starburst in GOODS 850-3, with a star formation rate of ~2500 M_sun/yr.
Moreover, the absence of any continuum emission at the full resolution of the
VLBI observations indicates the lack of a compact radio AGN source in this
z=1.87 SMG.Comment: 19 pages, 4 figures, accepted for publication in A
- …