74 research outputs found
Spatially resolved dust emission of extremely metal poor galaxies
We present infrared (IR) spectral energy distributions (SEDs) of individual
star-forming regions in four extremely metal poor (EMP) galaxies with
metallicity Z around Zsun/10 as observed by the Herschel Space Observatory.
With the good wavelength coverage of the SED, it is found that these EMP
star-forming regions show distinct SED shapes as compared to those of grand
design Spirals and higher metallicity dwarfs: they have on average much higher
f70um/f160um ratios at a given f160um/f250um ratio; single modified black-body
(MBB) fittings to the SED at \lambda >= 100 um still reveal higher dust
temperatures and lower emissivity indices compared to that of Spirals, while
two MBB fittings to the full SED with a fixed emissivity index (beta = 2) show
that even at 100 um about half of the emission comes from warm (50 K) dust, in
contrast to the cold (~20 K) dust component. Our spatially resolved images
further reveal that the far-IR colors including f70um/f160um, f160um/f250um and
f250um/f350um are all related to the surface densities of young stars as traced
by far-UV, 24 um and SFRs, but not to the stellar mass surface densities. This
suggests that the dust emitting at wavelengths from 70 um to 350 um is
primarily heated by radiation from young stars.Comment: 9 pages, 7 figures, 7 tables, accepted for publication in MNRA
A Joint Model of the X-Ray and Infrared Extragalactic Backgrounds. I. Model Construction and First Results
We present an extragalactic population model of the cosmic background light to interpret the rich high-quality survey data in the X-ray and IR bands. The model incorporates star formation and supermassive black hole (SMBH) accretion in a co-evolution scenario to fit simultaneously 617 data points of number counts, redshift distributions, and local luminosity functions (LFs) with 19 free parameters. The model has four main components, the total IR LF, the SMBH accretion energy fraction in the IR band, the star formation spectral energy distribution (SED), and the unobscured SMBH SED extinguished with a H I column density distribution. As a result of the observational uncertainties about the star formation and SMBH SEDs, we present several variants of the model. The best-fit reduced χ2 reaches as small as 2.7-2.9 of which a significant amount (>0.8) is contributed by cosmic variances or caveats associated with data. Compared to previous models, the unique result of this model is to constrain the SMBH energy fraction in the IR band that is found to increase with the IR luminosity but decrease with redshift up to z ~ 1.5; this result is separately verified using aromatic feature equivalent-width data. The joint modeling of X-ray and mid-IR data allows for improved constraints on the obscured active galactic nucleus (AGN), especially the Compton-thick AGN population. All variants of the model require that Compton-thick AGN fractions decrease with the SMBH luminosity but increase with redshift while the type 1 AGN fraction has the reverse trend
The Weak Carbon Monoxide Emission In An Extremely Metal Poor Galaxy, Sextans A
Carbon monoxide (CO) is one of the primary coolants of gas and an easily
accessible tracer of molecular gas in spiral galaxies but it is unclear if CO
plays a similar role in metal poor dwarfs. We carried out a deep observation
with IRAM 30 m to search for CO emission by targeting the brightest far-IR peak
in a nearby extremely metal poor galaxy, Sextans A, with 7% Solar metallicity.
A weak CO J=1-0 emission is seen, which is already faint enough to place a
strong constraint on the conversion factor (a_CO) from the CO luminosity to the
molecular gas mass that is derived from the spatially resolved dust mass map.
The a_CO is at least seven hundred times the Milky Way value. This indicates
that CO emission is exceedingly weak in extremely metal poor galaxies,
challenging its role as a coolant in these galaxies.Comment: 4 pages, 1 table, 4 figures. ApJL in pres
Inefficient Star Formation In Extremely Metal Poor Galaxies
The first galaxies contain stars born out of gas with little or no metals.
The lack of metals is expected to inhibit efficient gas cooling and star
formation but this effect has yet to be observed in galaxies with oxygen
abundance relative to hydrogen below a tenth of that of the Sun. Extremely
metal poor nearby galaxies may be our best local laboratories for studying in
detail the conditions that prevailed in low metallicity galaxies at early
epochs. Carbon Monoxide (CO) emission is unreliable as tracers of gas at low
metallicities, and while dust has been used to trace gas in low-metallicity
galaxies, low-spatial resolution in the far-infrared has typically led to large
uncertainties. Here we report spatially-resolved infrared observations of two
galaxies with oxygen abundances below 10 per cent solar, and show that stars
form very inefficiently in seven star-forming clumps of these galaxies. The
star formation efficiencies are more than ten times lower than found in normal,
metal rich galaxies today, suggesting that star formation may have been very
inefficient in the early Universe.Comment: Author's version (10 pages, 4 figures). Published in Natur
The Arecibo Legacy Fast ALFA Survey: VIII. HI Source Catalog of the Anti-Virgo Region at dec = +25 deg
We present a fourth catalog of HI sources from the Arecibo Legacy Fast ALFA
(ALFALFA) Survey. We report 541 detections over 136 deg2, within the region of
the sky having 22h < R.A. < 03h and 24 deg < Dec. < 26 deg . This complements a
previous catalog in the region 26 deg < Dec. < 28 deg (Saintonge et al. 2008).
We present here the detections falling into three classes: (a) extragalactic
sources with S/N > 6.5, where the reliability of the catalog is better than
95%; (b) extragalactic sources 5.0 < S/N < 6.5 and a previously measured
optical redshift that corroborates our detection; or (c) High Velocity Clouds
(HVCs), or subcomponents of such clouds, in the periphery of the Milky Way. Of
the 541 objects presented here, 90 are associated with High Velocity Clouds,
while the remaining 451 are identified as extragalactic objects. Optical
counterparts have been matched with all but one of the extragalactic objects.Comment: 26 pages, 5 figures, 1 table, accepted for publication in
Astrophysical Journal Supplement Serie
Review: far-infrared instrumentation and technological development for the next decade
Far-infrared astronomy has advanced rapidly since its inception in the late 1950s, driven by a maturing technology base and an expanding community of researchers. This advancement has shown that observations at far-infrared wavelengths are important in nearly all areas of astrophysics, from the search for habitable planets and the origin of life to the earliest stages of galaxy assembly in the first few hundred million years of cosmic history. The combination of a still-developing portfolio of technologies, particularly in the field of detectors, and a widening ensemble of platforms within which these technologies can be deployed, means that far-infrared astronomy holds the potential for paradigm-shifting advances over the next decade. We examine the current and future far-infrared observing platforms, including ground-based, suborbital, and space-based facilities, and discuss the technology development pathways that will enable and enhance these platforms to best address the challenges facing far-infrared astronomy in the 21st century
The Role of Star Formation and an AGN in Dust Heating of z = 0.3–2.8 Galaxies. I. Evolution with Redshift and Luminosity
We characterize infrared spectral energy distributions of 343 (ultra)luminous infrared galaxies from z = 0.3–2.8. We diagnose the presence of an active galactic nucleus (AGN) by decomposing individual Spitzer mid-IR spectroscopy into emission from star formation and an AGN-powered continuum; we classify sources as star-forming galaxies (SFGs), AGNs, or composites. Composites comprise 30% of our sample and are prevalent at faint and bright S_(24), making them an important source of IR AGN emission. We combine spectroscopy with multiwavelength photometry, including Herschel imaging, to create three libraries of publicly available templates (2–1000 μm). We fit the far-IR emission using a two-temperature modified blackbody to measure cold and warm dust temperatures (T_c and T_w). We find that T_c does not depend on mid-IR classification, while T_w shows a notable increase as the AGN grows more luminous. We measure a quadratic relationship between mid-IR AGN emission and total AGN contribution to LIR. AGNs, composites, and SFGs separate in S_8/S_(3.6) and S_(250)/S_(24), providing a useful diagnostic for estimating relative amounts of these sources. We estimate that >40% of IR-selected samples host an AGN, even at faint selection thresholds (S_(24) > 100 μJy). Our decomposition technique and color diagnostics are relevant given upcoming observations with the James Webb Space Telescope
The FUV to Near-IR Morphologies of Luminous Infrared Galaxies in the GOALS Sample
We compare the morphologies of a sample of 20 LIRGs from the Great
Observatories All-sky LIRG Survey (GOALS) in the FUV, B, I and H bands, using
the Gini (G) and M20 parameters to quantitatively estimate the distribution and
concentration of flux as a function of wavelength. HST images provide an
average spatial resolution of ~80 pc. While our LIRGs can be reliably
classified as mergers across the entire range of wavelengths studied here,
there is a clear shift toward more negative M20 (more bulge-dominated) and a
less significant decrease in G values at longer wavelengths. We find no
correlation between the derived FUV G-M20 parameters and the global measures of
the IR to FUV flux ratio, IRX. Given the fine resolution in our HST data, this
suggests either that the UV morphology and IRX are correlated on very small
scales, or that the regions emitting the bulk of the IR emission emit almost no
FUV light. We use our multi-wavelength data to simulate how merging LIRGs would
appear from z~0.5-3 in deep optical and near-infrared images such as the HUDF,
and use these simulations to measure the G-M20 at these redshifts. Our
simulations indicate a noticeable decrease in G, which flattens at z >= 2 by as
much as 40%, resulting in mis-classifying our LIRGs as disk-like, even in the
rest-frame FUV. The higher redshift values of M20 for the GOALS sources do not
appear to change more than about 10% from the values at z~0. The change in
G-M20 is caused by the surface brightness dimming of extended tidal features
and asymmetries, and also the decreased spatial resolution which reduced the
number of individual clumps identified. This effect, seen as early as z~0.5,
could easily lead to an underestimate of the number of merging galaxies at
high-redshift in the rest-frame FUV.Comment: Accepted for publication in the Astronomical Journal. The total page
count is 15 pages with 13 figures and 1 Tabl
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