Continuum Extinction Research Articles

Infrared Spectroscopic Survey of the Quiescent Medium of Nearby Clouds. II. Ice Formation and Grain Growth in Perseus and Serpens

The properties of dust change during the transition from diffuse to dense clouds as a result of ice formation and dust coagulation, but much is still unclear about this transformation. We present 2–20 μm spectra of 49 field stars behind the Perseus and Serpens Molecular Clouds and establish relationships between the near-infrared continuum extinction (A K) and the depths of the 9.7 μm silicate (τ 9.7) and 3.0 μm H2O ice (τ 3.0) absorption bands. The τ 9.7/A K ratio varies from large, diffuse interstellar medium-like values (∼0.55), to much lower ratios (∼0.26). Above extinctions of A K ∼ 1.2 (A V ∼ 10; Perseus, Lupus, dense cores) and ∼2.0 (A V ∼ 17; Serpens), the τ 9.7/A K ratio is lowest. The τ 9.7/A K reduction from diffuse to dense clouds is consistent with a moderate degree of grain growth (sizes up to ∼0.5 μm), increasing the near-infrared color excess (and thus A K), but not affecting the ice and silicate band profiles. This grain growth process seems to be related to the ice column densities and dense core formation thresholds, highlighting the importance of density. After correction for Serpens foreground extinction, the H2O ice formation threshold is in the range of A K = 0.31–0.40 (A V = 2.6–3.4) for all clouds, and thus grain growth takes place after the ices are formed. Finally, abundant CH3OH ice (∼21% relative to H2O) is reported for 2MASSJ18285266+0028242 (Serpens), a factor of >4 larger than for the other targets.

Physical properties of local star-forming analogues toz ∼ 5 Lyman-break galaxies

Intense, compact, star-forming galaxies are rare in the local Universe but ubiquitous at high redshift. We interpret the 0.1-22 um spectral energy distributions (SED) of a sample of 180 galaxies at 0.05<z<0.25 selected for extremely high surface densities of inferred star formation in the ultraviolet. By comparison with well-established stellar population synthesis models we find that our sample comprises young (~ 60 - 400 Myrs), moderate mass (~ $6\times 10^9$ Msun) star-forming galaxies with little dust extinction (mean stellar continuum extinction $E_\mathrm{cont}$(B-V) ~ 0.1) and find star formation rates of a few tens of Solar masses per year. We use our inferred masses to determine a mean specific star formation rate for this sample of ~ $10^{-9}$ yr$^{-1}$, and compare this to the specific star formation rates in distant Lyman break galaxies (LBGs), and in other low redshift populations. We conclude that our sample's characteristics overlap significantly with those of the z~5 LBG population, making ours the first local analogue population well tuned to match those high redshift galaxies. We consider implications for the origin and evolution of early galaxies.

Dust attenuation inz~ 1 galaxies fromHerscheland 3D-HST Hαmeasurements

We combined the spectroscopic information from the 3D-HST survey with \textit{Herschel} data to characterize the H$\alpha$ dust attenuation properties of a sample of 79 main sequence star-forming galaxies at $z \sim 1$ in the GOODS-S field. The sample was selected in the far-IR, at $\lambda$=100 and/or 160 $\mu$m, and only includes galaxies with a secure H$\alpha$ detection (S/N$>$3). From the low resolution 3D-HST spectra we measured the redshifts and the H$\alpha$ fluxes for the whole sample (a factor of 1/1.2 was applied to the observed fluxes to remove the [NII] contamination). The stellar masses (M$_{\star}$), infrared (L$_{IR}$) and UV luminosities (L$_{UV}$) were derived from the SEDs by fitting multi-band data from GALEX near-UV to SPIRE 500 $\mu$m. We estimated the continuum extinction E$_{star}$(B-V) from both the IRX=L$_{IR}$/L$_{UV}$ ratio and the UV-slope, $\beta$, and found an excellent agreement between the two. The nebular extinction was estimated from comparison of the observed SFR$_{H\alpha}$ and SFR$_{UV}$. We obtained \emph{f}=E$_{star}$(B-V)/E$_{neb}$(B-V)=0.93$\pm$0.06, i.e. higher than the canonical value of \emph{f}=0.44 measured in the local Universe. Our derived dust correction produces good agreement between the H$\alpha$ and IR+UV SFRs for galaxies with SFR$\gtrsim$ 20 M$_{\odot}$/yr and M$_{\star} \gtrsim 5 \times 10^{10}$ M$_{\odot}$, while objects with lower SFR and M$_{\star}$ seem to require a smaller \emph{f}-factor (i.e. higher H$\alpha$ extinction correction). Our results then imply that the nebular extinction for our sample is comparable to that in the optical-UV continuum and suggest that the \emph{f}-factor is a function of both M$_{\star}$ and SFR, in agreement with previous studies.

Influence of physical galaxy properties on Lyαescape in star-forming galaxies

[abridged] Among the different observational techniques used to select high-redshift galaxies, the hydrogen recombination line Lyman-alpha (Lya) is of particular interest as it gives access to the measurement of cosmological quantities such as the star formation rate of distant galaxy populations. However, the interpretation of this line and the calibration of such observables is still subject to serious uncertainties. Therefore, it important to understand under what conditions the Lya line can be used as a reliable star formation diagnostic tool. We use a sample of 24 Lya emitters at z ~ 0.3 with an optical spectroscopic follow-up to calculate the Lya escape fraction and its dependency upon different physical properties. We also examine the reliability of Lya as a star formation rate indicator. We combine these observations with a compilation of Lya emitters selected at z = 0 - 0.3 to assemble a larger sample. The Lya escape fraction depends clearly on the dust extinction following the relation fesc(Lya) = C(Lya) x 10^(-0.4 E(B-V) k(Lya)), but with a shallower slope than previously reported, with k(Lya) ~ 6.67 and C(Lya) = 0.22. However, the correlation does not follow the expected curve for a simple dust attenuation. We explore the various mechanisms than lead to fesc(Lya) values above the continuum extinction curve, i.e. to an enhancement of the Lya output. We also observe that the strength of Lya and the escape fraction appear unrelated to the galaxy metallicity. Regarding the reliability of Lya as a star formation rate (SFR) indicator, we show that the deviation of SFR(Lya) from the true SFR (as traced by the UV continuum) is a function of the observed SFR(UV), which can be seen as the decrease of fesc(Lya) with increasing UV luminosity. Moreover, we observe a redshift-dependence of this relationship revealing the underlying evolution of fesc(Lya) with redshift.

INFRARED SPECTROSCOPIC SURVEY OF THE QUIESCENT MEDIUM OF NEARBY CLOUDS. I. ICE FORMATION AND GRAIN GROWTH IN LUPUS

Infrared photometry and spectroscopy (1-25 um) of background stars reddened by the Lupus molecular cloud complex are used to determine the properties of the grains and the composition of the ices before they are incorporated into circumstellar envelopes and disks. H2O ices form at extinctions of Ak=0.25+/-0.07 mag (Av=2.1+/-0.6). Such a low ice formation threshold is consistent with the absence of nearby hot stars. Overall, the Lupus clouds are in an early chemical phase. The abundance of H2O ice (2.3+/-0.1*10^-5 relative to Nh) is typical for quiescent regions, but lower by a factor of 3-4 compared to dense envelopes of YSOs. The low solid CH3OH abundance (<3-8% relative to H2O) indicates a low gas phase H/CO ratio, which is consistent with the observed incomplete CO freeze out. Furthermore it is found that the grains in Lupus experienced growth by coagulation. The mid-infrared (>5 um) continuum extinction relative to Ak increases as a function of Ak. Most Lupus lines of sight are well fitted with empirically derived extinction curves corresponding to Rv~ 3.5 (Ak=0.71) and Rv~5.0 (Ak=1.47). For lines of sight with Ak>1.0 mag, the tau9.7/Ak ratio is a factor of 2 lower compared to the diffuse medium. Below 1.0 mag, values scatter between the dense and diffuse medium ratios. The absence of a gradual transition between diffuse and dense medium-type dust indicates that local conditions matter in the process that sets the tau9.7/Ak ratio. This process is likely related to grain growth by coagulation, as traced by the A7.4/Ak continuum extinction ratio, but not to ice mantle formation. Conversely, grains acquire ice mantles before the process of coagulation starts.

ICE AND DUST IN THE QUIESCENT MEDIUM OF ISOLATED DENSE CORES

The relation between ices in the envelopes and disks surrounding YSOs and those in the quiescent interstellar medium is investigated. For a sample of 31 stars behind isolated dense cores, ground-based and Spitzer spectra and photometry in the 1-25 um wavelength range are combined. The baseline for the broad and overlapping ice features is modeled, using calculated spectra of giants, H2O ice and silicates. The adopted extinction curve is derived empirically. Its high resolution allows for the separation of continuum and feature extinction. The extinction between 13-25 um is ~50% relative to that at 2.2 um. The strengths of the 6.0 and 6.85 um absorption bands are in line with those of YSOs. Thus, their carriers, which, besides H2O and CH3OH, may include NH4+, HCOOH, H2CO and NH3, are readily formed in the dense core phase, before stars form. The 3.53 um C-H stretching mode of solid CH3OH was discovered. The CH3OH/H2O abundance ratios of 5-12% are larger than upper limits in the Taurus molecular cloud. The initial ice composition, before star formation occurs, therefore depends on the environment. Signs of thermal and energetic processing that were found toward some YSOs are absent in the ices toward background stars. Finally, the peak optical depth of the 9.7 um band of silicates relative to the continuum extinction at 2.2 um is significantly shallower than in the diffuse interstellar medium. This extends the results of Chiar et al. (2007) to a larger sample and higher extinctions.

Exploring the active galactic nucleus and starburst content of local ultraluminous infrared galaxies through 5-8 μm spectroscopy

We present a 5–8 μm analysis of the Spitzer spectra of 71 ultraluminous infrared galaxies (ULIRGs) with redshift z < 0.15, devoted to the study of the role of active galactic nuclei (AGN) and starbursts (SBs) as the power source of the extreme infrared emission. Around ∼5 μm, an AGN is much brighter (by a factor of ≈30) than an SB of equal bolometric luminosity. This allows us to detect the presence of even faint accretion-driven cores inside ULIRGs: signatures of AGN activity are found in ∼70 per cent of our sample (50/71 sources). Through a simple analytical model, we are also able to obtain a quantitative estimate of the AGN/SB contribution to the overall energy output of each source. Although the main fraction of ULIRG luminosity is confirmed to arise from star formation events, the AGN contribution is non-negligible (∼23 per cent) and is shown to increase with luminosity. The existence of a rather heterogeneous pattern in the composition and geometrical structure of the dust among ULIRGs is newly supported by the comparison between individual absorption features and continuum extinction.

A Survey of Seyfert AGN: Nuclear Gas Disks and Direct Black Hole Mass Estimates

AbstractIn a survey of 18 nearby Seyfert nuclei, we find evidence for geometrically thick gas disks on scales of tens of parsecs. Mapping the interstellar medium traced by H2 ν = 1–0 S(1) emission using the infrared integral field spectrometers OSIRIS and SINFONI reveals general disk rotation with an additional significant component of random bulk motion implied by the high local velocity dispersion. The size scale of the typical nuclear gas disk is ~30 pc in radius with a comparable vertical height, and the distribution and kinematics suggest the gas is spatially mixed with the nuclear stellar population. Based on the estimated characteristic gas mass fraction of 10%, the average gas mass within this region is ~107M⊙. This suggests column densities of NH ~ 5 × 1023 cm−2, but the significantly lower densities implied by the stellar continuum extinction indicate that the gas distribution on these scales is dominated by dense clumps. We discuss the feasibility of constraining the masses of the central black holes via modeling of the gas disk kinematics, highlighting the importance of properly accounting for the gas velocity dispersion, and the use of these direct mass estimates to calibrate masses derived from the method of reverberation mapping.

Star formation rate in galaxies from UV, IR, and Hαestimators

Infrared (IR) luminosity of galaxies originating from dust emission can be used as an indicator of the star formation rate (SFR). Inoue et al. (2000, IHK) have derived a formula for the conversion from IR luminosity to SFR by using the following three quantities: the fraction of Lyman continuum luminosity absorbed by gas (f), the fraction of UV luminosity absorbed by dust (epsilon), and the fraction of dust heating from old (>10^8 yr) stellar populations (eta). We develop a method to estimate those three quantities based on the idea that the various way of SFR estimates should return the same SFR. After applying our method to samples of galaxies, the following results are obtained. First, our method is applied to star-forming galaxies, finding that f~0.6, epsilon~0.5, and eta~0.4 as representative values. Next, we apply the method to a starburst sample, which shows larger extinction than the star-forming galaxy sample. With the aid of f, epsilon, and eta, we estimate reliable SFRs. Moreover, the H-alpha luminosity, if the H-alpha extinction is corrected by using the Balmer decrement, is suitable for a statistical analysis of SFR, because the same correction factor for the Lyman continuum extinction is applicable to both normal and starburst galaxies over all the range of SFR. The metallicity dependence of f and epsilon is also tested: Only the latter proves to have a correlation with metallicity. As an extension of our result, we show that all UV, H-alpha, and IR comoving luminosity densities at z=0 give a consistent SFR (~ 3x10^{-2}h M_sun/Mpc^3). Useful formulae for SFR estimate are listed.

Lyman Continuum Extinction by Dust in H [CSC]ii[/CSC] Regions of Galaxies

We examine Lyman continuum extinction (LCE) in H II regions by comparing infrared fluxes of 49 H II regions in the Galaxy, M31, M33, and the LMC with estimated production rates of Lyman continuum photons. A typical fraction of Lyman continuum photons that contribute to hydrogen ionization in the H II regions of three spiral galaxies is $\la$ 50 %. The fraction may become smaller as the metallicity (or dust-to-gas ratio) increases. We examine the LCE effect on estimated star formation rates (SFR) of galaxies. The correction factor for the Galactic dust-to-gas ratio is 2-5.

Effect of Dust Extinction on Estimating the Star Formation Rate of Galaxies: Lyman Continuum Extinction

We reexamine the effect of Lyman continuum (λ ≤ 912 A) extinction (LCE) by dust in H II regions in detail and discuss how it affects the estimation of the global star formation rate (SFR) of galaxies. To clarify the first issue, we establish two independent methods for estimating a parameter of LCE (f), which is defined as the fraction of Lyman continuum photons contributing to hydrogen ionization in an H II region. One of those methods determines f from the set of Lyman continuum flux, electron density, and metallicity. In the framework of this method, as the metallicity and/or the Lyman photon flux increase, f is found to decrease. The other method determines f from the ratio of infrared flux to Lyman continuum flux. Importantly, we show that f 0.5 via both methods in many H II regions of the Galaxy. Thus, it establishes that dust in such H II regions absorbs significant amount of Lyman continuum photons directly. To examine the second issue, we approximate f to a function of only the dust-to-gas mass ratio (i.e., metallicity), assuming a parameter fit for the Galactic H II regions. We find that a characteristic , which is defined as f averaged over a galaxywide scale, is 0.3 for the nearby spiral galaxies. This relatively small indicates that a typical increment factor due to LCE for estimating the global SFR (1/) is large (~3) for the nearby spiral galaxies. Therefore, we conclude that the effect of LCE is not negligible relative to other uncertainties of estimating the SFR of galaxies.

Silicon Nanoparticles: Source of Extended Red Emission?

We have reviewed the characteristics of the extended red emission (ERE) as observed in many dusty astronomical environments, in particular, the diffuse interstellar medium of the Galaxy. The spectral nature and the photon conversion efficiency of the ERE identify the underlying process as highly efficient photoluminescence by an abundant component of interstellar dust. We have compared the photoluminescence properties of a variety of carbon- and silicon-based materials proposed as sources for the ERE with the observationally established constraints. We found that silicon nanoparticles provide the best match to the spectrum and the efficiency requirement of the ERE. If present in interstellar space with an abundance sufficient to explain the intensity of the ERE, silicon nanoparticles will also contribute to the interstellar 9.7 micron Si-O stretch feature in absorption, to the near- and mid-IR nonequilibrium thermal background radiation, and to the continuum extinction in the near- and far-UV. About 36% of the interstellar silicon depleted into the dust phase would be needed in the form of silicon nanoparticles, amounting to less than 5% of the interstellar dust mass. We propose that silicon nanoparticles form through the nucleation of SiO in oxygen-rich stellar mass outflows and that they represent an important small-grain component of the interstellar dust spectrum.

Some Remarks about Nova Spectroscopy and Photometry

view Abstract Citations (8) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Some Remarks about Nova Spectroscopy and Photometry Pistinner, Shlomi ; Shaviv, Giora ; Hauschildt, Peter H. ; Starrfield, Sumner Abstract In this paper we explore the sensitivity of stellar atmosphere modeling of early nova spectra to the input parameters. Our results identify spectral regions where the continuum spectra are insensitive to the input parameters and certain wave bands where they are sensitive to the input parameters. Thus, we identify the best spectral region for determining elemental abundances, effective temperatures, and luminosities. We find in some cases that nova atmospheres have a continuum extinction coefficient that is dominated by scattering. The continuum scattering and the resulting continuum extinction are for these cases nearly gray. We apply and extend the small parameter Chandrasekhar expansion and we use the properties of the gray opacity, to study the effects of small residual absorption in the atmosphere. Two cases are treated: the case of a static extended atmosphere and the case of an extended atmosphere in motion. We solve, self-consistently, the transfer equation for these physical conditions. The assumption of radiative equilibrium allows us to also obtain a self-consistent temperature profile which validates our approximation a posteriori. Using these solutions, we (a) derive conditions for the required signal-to-noise ratio (or the relative accuracy of the observed fluxes) and the required observed frequency range needed to obtain a prescribed accuracy for those nova parameters obtained from a comparison of observed with synthetic spectra; (b) assess and establish the importance of spectroscopic (in contrast to photometric) measurements; (c) show that the use of synthetic spectra is an essential part, in the optically thick phase, of the determination of nova characteristics from the observations; and (d) define a new property continuum similarity and show how and when nova spectra possess this property. We find that the only wavelength region which is not affected by the physical conditions used to arrive at these conclusions is the ultraviolet. Therefore, we conclude that the ultraviolet is the most important wavelength region to use when deriving nova parameters from synthetic spectra. Publication: The Astrophysical Journal Pub Date: October 1995 DOI: 10.1086/176259 Bibcode: 1995ApJ...451..724P Keywords: METHODS: NUMERICAL; RADIATIVE TRANSFER; STARS: NOVAE; CATACLYSMIC VARIABLES full text sources ADS | data products MAST (1)

Interstellar extinction and polarization in the infrared

view Abstract Citations (292) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar Extinction and Polarization in the Infrared Martin, P. G. ; Whittet, D. C. B. Abstract The wavelength dependences of interstellar continuum extinction and polarization in the range 0.35-5 microns are examined. The existence of a universal extinction curve with power law index of about 1.8 extending from the near-IR to at least 5 microns appears to be established for both diffuse and dense cloud dust. The polarization yields evidence for some degree of universality in the 1.6-5 micron regime which may be represented by a power law with index 1.5-2.0, encompassing that for extinction. The form of the polarization curve in the IR seems independent of the wavelength at which the degree of polarization peaks in the optical, implying that variations in that wavelength are caused by changes in the optical properties of the particle at blue-visible rather than IR wavelengths. It is argued that the more significant alterations of the grain size distribution from one environment to another occur for the smaller particles. Publication: The Astrophysical Journal Pub Date: July 1990 DOI: 10.1086/168896 Bibcode: 1990ApJ...357..113M Keywords: Cosmic Dust; Interstellar Extinction; Interstellar Matter; Molecular Clouds; Near Infrared Radiation; Energy Distribution; Particle Size Distribution; Stellar Mass; Astrophysics; INFRARED: GENERAL; INTERSTELLAR: GRAINS; INTERSTELLAR: MATTER; POLARIZATION; SPECTROPHOTOMETRY full text sources ADS | data products SIMBAD (10)

Absorption spectroscopy in sooting flames using a tunable diode laser

The diode laser serves as a source of narrow-linewidth irradiation whose wavelength can be rapidly tuned to perform fast high-resoltuion absorption spectroscopy. The laser beam is passed through the soot-laden post-flame region of a laminar premixed acetylene-air flame, and the wavelength is tuned across the full width of an absorption line of a gaseous component, in this instance, CO. The modulation of laser wavelength provides a simple means of discrimination against the continuum extinction effects of particulates. The result of a single measurement is the basis for a fully resolved profile of one or more CO absorption lines from which may be inferred the CO concentration, the temperature, or the collision halfwidth of the absorption line. The primary limitation of the technique, for some applications, is that it is a line-of-sight measurement. The advantages of tunable diode laser spectroscopy include its sensitivity, simplicity, and high spectral resolution. 1 figure.

Temperature dependence of infrared absorption by the water vapor continuum near 1200 cm^−1

Infrared absorption by the water vapor continuum near 1200 cm(-1) has been measured with a lead-tin-telluride diode laser over a 40.5-m optical path. The measurements were made as a function of temperature from 333 K to 473 K; thus, they overlap and extend previous measurements made at temperatures between 293 K and 388 K. Over the entire temperature range studied here, the continuum extinction coefficient increases quadratically with water-vapor partial pressure as expected for the relatively high partial pressures used in these measurements. At temperatures below 398 K. our measured extinction coefficients agree well with previously reported data. At higher temperatures, however, the extinction coefficient is almost independent of temperature and is substantially larger than predicted by empirical formulas. Values of the self-broadening coefficient for water vapor have been extracted from the experimental data, and a possible interpretation of the results involving both dimer and line-broadening effects is presented.

Absorption and emission in the atmospheric window from 770 to 1,250 cm−1

AbstractCombined absorption and emission spectra have been recorded in the atmospheric window from 770 to 1,250 cm−1 both in London and in the relatively clean air of Ascot, Berks. The absorption spectra have been partially analysed in terms of selective and continuous extinction. The continuous extinction outweighs the selective in both Ascot and London. There is good agreement between the observed selective absorption and existing theoretical and laboratory data upon gases known to be present in the atmosphere.The opacity of the continuum shows an increase towards low frequency, and it is much greater in London than in Ascot; in both places it is positively correlated with water vapour. The average effective emission temperature of the continuum is not significantly different from that of water vapour.The only hypothesis which seems to fit all the observations is that in London the continuum extinction is largely caused by aerosol extinction while in Ascot this is only of minor importance. This requires that the extinction at Ascot be caused by wings of distant lines; this old suggestion of Elsasser is re‐examined and found to be plausible.Attention is drawn to the important contribution which the continuum can make to the heat balance of the atmosphere below 2 km.