21 research outputs found
Hierarchical Triggering of Star Formation by Superbubbles in W3/W4
It is generally believed that expanding superbubbles and mechanical feedback
from massive stars trigger star formation, because there are numerous examples
of superbubbles showing secondary star formation at their edges. However, while
these systems show an age sequence, they do not provide strong evidence of a
causal relationship. The W3/W4 Galactic star-forming complex suggests a
three-generation hierarchy: the supergiant shell structures correspond to the
oldest generation; these triggered the formation of IC 1795 in W3, the
progenitor of a molecular superbubble; which in turn triggered the current
star-forming episodes in the embedded regions W3-North, W3-Main, and W3-OH. We
present UBV photometry and spectroscopic classifications for IC 1795, which
show an age of 3 - 5 Myr. This age is intermediate between the reported 6 - 20
Myr age of the supergiant shell system, and the extremely young ages (10^4 -
10^5 yr) for the embedded knots of ultracompact HII regions, W3-North, W3-Main,
and W3-OH. Thus, an age sequence is indeed confirmed for the entire W3/W4
hierarchical system. This therefore provides some of the first convincing
evidence that superbubble action and mechanical feedback are indeed a
triggering mechanism for star formation.Comment: 10 pages, 6 figures; accepted to the Astronomical Journal. Figure 2
included in this submission as JPE
Solar abundances of rock-forming elements, extreme oxygen and hydrogen in a young polluted white dwarf
The Teff = 20â800 K white dwarf WD 1536+520 is shown to have broadly solar abundances of the major rock-forming elements O, Mg, Al, Si, Ca, and Fe, together with a strong relative depletion in the volatile elements C and S. In addition to the highest metal abundances observed to date, including logâ(O/He) = â3.4, the helium-dominated atmosphere has an exceptional hydrogen abundance at logâ(H/He) = â1.7. Within the uncertainties, the metal-to-metal ratios are consistent with the accretion of an H2O-rich and rocky parent body, an interpretation supported by the anomalously high trace hydrogen. The mixed atmosphere yields unusually short diffusion time-scales for a helium atmosphere white dwarf, of no more than a few hundred years, and equivalent to those in a much cooler, hydrogen-rich star. The overall heavy element abundances of the disrupted parent body deviate modestly from a bulk Earth pattern, and suggest the deposition of some core-like material. The total inferred accretion rate is 4.2 Ă 109 g sâ1, and at least four times higher than for any white dwarf with a comparable diffusion time-scale. Notably, when accretion is exhausted in this system, both metals and hydrogen will become undetectable within roughly 300 Myr, thus supporting a scenario where the trace hydrogen is related to the ongoing accretion of planetary debris
ALMA 1.3 mm Survey of Lensed Submillimeter Galaxies Selected by Herschel: Discovery of Spatially Extended SMGs and Implications
We present an ALMA 1.3 mm (Band 6) continuum survey of lensed submillimeter galaxies (SMGs) at z = 1.0 to âŒ3.2 with an angular resolution of âŒ0farcs2. These galaxies were uncovered by the Herschel Lensing Survey and feature exceptionally bright far-infrared continuum emission (Speak âł 90 mJy) owing to their lensing magnification. We detect 29 sources in 20 fields of massive galaxy clusters with ALMA. Using both the Spitzer/IRAC (3.6/4.5 ÎŒm) and ALMA data, we have successfully modeled the surface brightness profiles of 26 sources in the rest-frame near- and far-infrared. Similar to previous studies, we find the median dust-to-stellar continuum size ratio to be small (Re,dust/Re,star = 0.38 ± 0.14) for the observed SMGs, indicating that star formation is centrally concentrated. This is, however, not the case for two spatially extended main-sequence SMGs with a low surface brightness at 1.3 mm (âČ0.1 mJy arcsecâ2), in which the star formation is distributed over the entire galaxy (Re,dust/Re,star > 1). As a whole, our SMG sample shows a tight anticorrelation between (Re,dust/Re,star) and far-infrared surface brightness (ÎŁIR) over a factor of â1000 in ÎŁIR. This indicates that SMGs with less vigorous star formation (i.e., lower ÎŁIR) lack central starburst and are likely to retain a broader spatial distribution of star formation over the whole galaxies (i.e., larger Re,dust/Re,star). The same trend can be reproduced with cosmological simulations as a result of central starburst and potentially subsequent "inside-out" quenching, which likely accounts for the emergence of compact quiescent galaxies at z ⌠2
Extensive Lensing Survey of Optical and Near-infrared Dark Objects (El Sonido): HST H-faint Galaxies behind 101 Lensing Clusters
We present a Spitzer/IRAC survey of H-faint (H160 âł 26.4, < 5Ï) sources in 101 lensing cluster fields. Across a CANDELS/Wide-like survey area of âŒ648 arcmin2 (effectively âŒ221 arcmin2 in the source plane), we have securely discovered 53 sources in the IRAC Channel-2 band (CH2, 4.5 ÎŒm; median CH2 = 22.46 ± 0.11 AB mag) that lack robust HST/WFC3-IR F160W counterparts. The most remarkable source in our sample, namely ES-009 in the field of Abell 2813, is the brightest H-faint galaxy at 4.5 ÎŒm known so far (CH2 = 20.48 ± 0.03 AB mag). We show that the H-faint sources in our sample are massive (median Mstar = 1010.3±0.3 Mâ), star-forming (median star formation rate Mâ yrâ1), and dust-obscured (AV = 2.6 ± 0.3) galaxies around a median photometric redshift of z = 3.9 ± 0.4. The stellar continua of 14 H-faint galaxies can be resolved in the CH2 band, suggesting a median circularized effective radius (Re,circ; lensing corrected) of 1.9 ± 0.2 kpc and <1.5 kpc for the resolved and whole samples, respectively. This is consistent with the sizes of massive unobscured galaxies at z ⌠4, indicating that H-faint galaxies represent the dusty tail of the distribution of a wider galaxy population. Comparing with the ALMA dust continuum sizes of similar galaxies reported previously, we conclude that the heavy dust obscuration in H-faint galaxies is related to the compactness of both stellar and dust continua (Re,circ ⌠1 kpc). These H-faint galaxies make up % of the galaxies in the stellar-mass range of 1010 â 1011.2 Mâ at z = 3 ⌠5, contributing to % of the cosmic star formation rate density in this epoch and likely tracing the early phase of massive galaxy formatio
The Survey for Ionization in Neutral-Gas Galaxies: III. Diffuse, Warm Ionized Medium and Escape of Ionizing Radiation
We use the first data release from the SINGG H-alpha survey of HI-selected
galaxies to study the quantitative behavior of the diffuse, warm ionized medium
(WIM) across the range of properties represented by these 109 galaxies. The
mean fraction f_WIM of diffuse ionized gas in this sample is 0.59+/- 0.19,
slightly higher than found in previous samples. Since lower surface-brightness
galaxies tend to have higher f_WIM, we believe that most of this difference is
due to selection effects favoring large, optically-bright, nearby galaxies with
high star-formation rates. As found in previous studies, there is no
appreciable correlation with Hubble type or total star-formation rate. However,
we find that starburst galaxies, defined here by an H-alpha surface brightness
> 2.5x 10^39 erg s^-1 kpc^-2 within the H-alpha half-light radius, do show much
lower fractions of diffuse H-alpha emission. The cause apparently is not
dominated by a lower fraction of field OB stars. However, it is qualitatively
consistent with an expected escape of ionizing radiation above a threshold
star-formation rate, predicted from our model in which the ISM is shredded by
pressure-driven supernova feedback. The HI gas fractions in the starburst
galaxies are also lower, suggesting that the starbursts are consuming and
ionizing all the gas, and thus promoting regions of density-bounded ionization.
If true, these effects imply that some amount of Lyman continuum radiation is
escaping from most starburst galaxies, and that WIM properties and outflows
from mechanical feedback are likely to be pressure-driven. However, in view of
previous studies showing that the escape fraction of ionizing radiation is
generally low, it is likely that other factors also drive the low fractions of
diffuse ionized gas in starbursts.Comment: 16 pages plus separate 2-page PostScript table. Accepted to the
Astrophysical Journa
Mid-Infrared Determination of Total Infrared Luminosity and Star Formation Rates of Local and High-Redshift Galaxies
We demonstrate estimating the total infrared luminosity, LIR, and star
formation rates (SFRs) of star-forming galaxies at redshift 0 < z < 2.8 from
single-band 24 micron observations, using local spectral energy distributions
(SED) templates without introducing additional free parameters. Our method is
based on characterizing the SEDs of galaxies as a function of their LIR surface
density, which is motivated by the indications that the majority of IR luminous
star-forming galaxies at 1 < z < 3 have extended star-forming regions, in
contrast to the strongly nuclear concentrated, merger-induced starbursts in
local luminous and ultraluminous IR galaxies. We validate our procedure for
estimating LIR by comparing the resulting LIRs with those measured from far-IR
observations at 0 < z < 2.8. AGNs were excluded using X-ray and 3.6-8.0 micron
observations, which are generally available in deep cosmological survey fields.
The Gaussian fits to the distribution of the discrepancies between the LIR
measurements from single-band 24 micron and Herschel observations have sigma <
0.1 dex, with ~10% of objects disagreeing by more than 0.2 dex. Since the 24
micron estimates are based on SEDs for extended galaxies, this agreement
suggests that ~90% of IR galaxies at high z are indeed much more physically
extended than local counterparts of similar LIR, consistent with recent
independent studies of the fractions of galaxies forming stars in the
main-sequence and starburst modes, respectively. Because we have not introduced
empirical corrections to enhance these estimates, in principle, our method
should be applicable to lower luminosity galaxies. This will enable use of the
21 micron band of the Mid-Infrared Instrument (MIRI) on board the JWST to
provide an extremely sensitive tracer of obscured SFR in individual
star-forming galaxies across the peak of the cosmic star formation history.Comment: 17 pages, 7 figures, ApJ accepte