14 research outputs found
The young stellar population of NGC 4214 as observed with HST. I. Data and methods
We present the data and methods that we have used to perform a detailed
UV-optical study of the nearby dwarf starburst galaxy NGC 4214 using
multifilter HST/WFPC2+STIS photometry. We explain the process followed to
obtain high-quality photometry and astrometry of the stellar and cluster
populations of this galaxy. We describe the procedure used to transform
magnitudes and colors into physical parameters using spectral energy
distributions. The data show the existence of both young and old stellar
populations that can be resolved at the distance of NGC 4214 (2.94 Mpc) and we
perform a general description of those populations.Comment: 33 pages, 9 figures, and 8 table
Tip of the Red Giant Branch Distances to NGC 4214, UGC 685, and UGC 5456
We have used WFPC2 VRI observations to calculate the distances to three
nearby galaxies, NGC 4214, UGC 685, and UGC 5456 using the tip of the red giant
branch method. Our values for NGC 4214 (2.94 +/- 0.18 Mpc) and UGC 685 (4.79
+/- 0.30 Mpc) are the most precise measurementes of the distances to these
objects ever made. For UGC 5456 the data do not allow us to reach a decisive
conclusion since there are two possible solutions, one leading towards a short
distance around 3.8 Mpc and another one towards a long distance of 5.6 Mpc or
more.Comment: To appear in the March 2002 issue of the Astronomical Journal. 19
pages, including 4 tables and 8 figures. Due to the size limitations of the
astro-ph service, a reduced resolution version of figures 1-3 is provided
here. For a full resolution version, please go to http://www.stsci.edu/~jmai
The young stellar population of NGC 4214 as observed with HST. II. Results
We present the results of a detailed UV-optical study of the nearby dwarf
starburst galaxy NGC 4214 using multifilter HST/WFPC2+STIS photometry. The
stellar extinction is found to be quite patchy, with some areas having values
of E(4405-5495)< 0.1 mag and others, associated with star forming regions, much
more heavily obscured, a result which is consistent with previous studies of
the nebular extinction. We determined the ratio of blue-to-red supergiants and
found it to be consistent with theoretical models for the metallicity of the
SMC. The stellar IMF of the field in the range 20-100 solar masses is found to
be steeper than Salpeter. A number of massive clusters and associations with
ages between a few and 200 million years are detected and their properties are
discussed.Comment: 49 pages, 12 figures, 6 table
HST/WFPC2 and VLA Observations of the Ionized Gas in the Dwarf Starburst Galaxy NGC 4214
We present new H alpha and [O III] 5007 narrow band images of the
starbursting dwarf galaxy NGC 4214, obtained with the WFPC2 onboard HST,
together with VLA observations of the same galaxy. The HST images resolve
features down to physical scales of 2-5 pc, revealing several young (<10 Myr)
star forming complexes of various ionized gas morphologies (compact knots,
complete or fragmentary shells) and sizes (10-200 pc). Our results are
consistent with a uniform set of evolutionary trends: The youngest, smaller,
filled regions that presumably are those just emerging from dense star forming
clouds, tend to be of high excitation and are highly obscured. Evolved, larger
shell-like regions have lower excitation and are less extincted due of the
action of stellar winds and supernovae. In at least one case we find evidence
for induced star formation which has led to a two-stage starburst. Age
estimates based on W(H alpha) measurements do not agree with those inferred
from wind-driven shell models of expanding H II regions. The most likely
explanation for this effect is the existence of a 2 Myr delay in the formation
of superbubbles caused by the pressure exerted by the high density medium in
which massive stars are born. We report the detection of a supernova remnant
embedded in one of the two large H II complexes of NGC 4214. The dust in NGC
4214 is not located in a foreground screen but is physically associated with
the warm ionized gas.Comment: 41 pages, including 9 figures and 7 tables. To appear in the November
issue of the Astronomical Journa
NGC 604, the Scaled OB Association (SOBA) Prototype. I: Spatial Distribution of the Different Gas Phases and Attenuation by Dust
We have analyzed HST and ground-based data to characterize the different gas
phases and their interaction with the MYC in NGC 604, a GHR in M33. The warm
ionized gas is made out of two components: a high-excitation, high-surface
brightness H II surface located at the faces of the molecular clouds directly
exposed to the ionizing radiation of the central SOBA; and a low-excitation,
low-surface brightness halo that extends to much larger distances from the
ionizing stars. The cavities created by the winds and SN explosions are filled
with X-ray-emitting coronal gas. The nebular lines emitted by the warm gas
experience a variable attenuation as a consequence of the dust distribution,
which is patchy in the plane of the sky and with clouds interspersed among
emission-line sources in the same line of sight. The optical depth at H alpha
as measured from the ratio of the thermal radio continuum to H alpha shows a
very good correlation with the total CO (1-0) column, indicating that most of
the dust resides in the cold molecular phase. The optical depth at H alpha as
measured from the ratio of H alpha to H beta also correlates with the CO
emission but not as strongly as in the previous case. We analyze the difference
between those two measurements and we find that <=11% of the H II gas is hidden
behind large-optical-depth molecular clouds. We detect two candidate compact H
II regions embedded inside the molecular cloud; both are within short distance
of WR/Of stars and one of them is located within 16 pc of a RSG. We estimate
the age of the main stellar generation in NGC 604 to be approx. 3 Myr from the
ionization structure of the H II region. The size of the main cavity is smaller
than the one predicted by extrapolating from single-star wind-blown bubbles.Comment: 50 pages, 15 figures. To appear in the September issue of the
Astronomical Journal. Some of the figures in this version have a very low
resolution due to the absurd limitations (for 2004) imposed on file size by
astro-ph. A full-resolution version of the figures is available at
http://www.stsci.edu/~jmai
Continuum subtracting Lyman-alpha images: Low redshift studies using the Solar Blind Channel of HST/ACS
[ABRIDGED] We are undertaking an imaging study of local star-forming galaxies
in the Lyman-alpha (Lya) emission line using the Solar Blind Channel (SBC) of
the ACS onboard the HST. Observations have been obtained in Lya and H-alpha
(Ha) and 6 continuum filters between ~1500AA and the I-band. Previously (Hayes
et al 2005) we demonstrated that the production of Lya line-only images in the
SBC-only data-set is non-trivial and that supporting data is a requirement. We
here develop various methods of continuum subtraction and assess their relative
performance using a variety of spectral energy distributions (SED) as input. We
conclude that simple assumptions about the behavior of the ultraviolet
continuum consistently lead to results that are wildly erroneous, and determine
that an SED fitting approach is essential. Moreover, a single component stellar
or stellar+nebular spectrum is not always sufficient for realistic template
SEDs and, in order to successfully recover the input observables, care must be
taken to control contributions of nebular gas and underlying stellar
populations. Independent measurements of the metallicity must first be
obtained, but details of the initial mass function play only a small role. We
identify the need to bin together pixels in our data to obtain signal-to-noise
(S/N) of ~10 in each band before processing. At S/N=10 we can recover Lya
fluxes accurate to ~30% for Lya lines with intrinsic equivalent width [W(Lya)]
of 10AA. This accuracy improves to ~10% for W(Lya)=100AA. We describe the image
processing applied to the observations presented in \"Ostlin et al. (2009) and
the associated data-release. We also present simulations for an observing
strategy for an alternative low-redshift SBC Lya imaging campaign using
adjacent combinations of long-pass filters at slightly higher redshift.Comment: Accepted by the Astronomical Journa
Open Clusters IC 4665 and Cr 359 and a Probable Birthplace of the Pulsar PSR B1929+10
Based on the epicyclic approximation, we have simulated the motion of the
young open star clusters IC 4665 and Collinder 359. The separation between the
cluster centers is shown to have been minimal 7 Myr ago, 36 pc. We have
established a close evolutionary connection between IC 4665 and the
Scorpius-Centaurus association -- the separation between the centers of these
structures was pc 15 Myr ago. In addition, the center of IC 4665
at this time was near two well-known regions of coronal gas: the Local Bubble
and the North Polar Spur. The star HIP 86768 is shown to be one of the
candidates for a binary (in the past) with the pulsar PSR B1929+10. At the
model radial velocity of the pulsar km s, a close
encounter of this pair occurs in the vicinity of IC 4665 at a time of -1.1 Myr.
At the same time, using currently available data for the pulsar B1929+10 at its
model radial velocity km s, we show that the hypothesis
of Hoogerwerf et al. (2001) about the breakup of the Oph--B1929+10
binary in the vicinity of Upper Scorpius (US) about 0.9 Myr ago is more
plausible.Comment: 19 pages, 8 figure
Carina OB Stars: X-ray Signatures of Wind Shocks and Magnetic Fields
The Chandra Carina Complex contains 200 known O- and B type stars. The
Chandra survey detected 68 of the 70 O stars and 61 of 127 known B0-B3 stars.
We have assembled a publicly available optical/X-ray database to identify OB
stars that depart from the canonical Lx/Lbol relation, or whose average X-ray
temperatures exceed 1 keV. Among the single O stars with high kT we identify
two candidate magnetically confined wind shock sources: Tr16-22, O8.5 V, and LS
1865, O8.5 V((f)). The O4 III(fc) star HD 93250 exhibits strong, hard, variable
X-rays, suggesting it may be a massive binary with a period of >30 days. The
visual O2 If* binary HD 93129A shows soft 0.6 keV and hard 1.9 keV emission
components, suggesting embedded wind shocks close to the O2 If* Aa primary, and
colliding wind shocks between Aa and Ab. Of the 11 known O-type spectroscopic
binaries, the long orbital-period systems HD 93343, HD 93403 and QZ Car have
higher shock temperatures than short-period systems such as HD 93205 and FO 15.
Although the X-rays from most B stars may be produced in the coronae of unseen,
low-mass pre-main-sequence companions, a dozen B stars with high Lx cannot be
explained by a distribution of unseen companions. One of these, SS73 24 in the
Treasure Chest cluster, is a new candidate Herbig Be star.Comment: To be published in a special issue of the Astrophysical Journal
Supplement on the Chandra Carina Complex Projec