Showing papers by "Dieter Lutz published in 2000"

The Composition and Distribution of Dust along the Line of Sight toward the Galactic Center

Abstract: We discuss the composition of dust and ice along the line of sight to the Galactic center (GC) based on analysis of mid-infrared spectra (2.4-13 μm) from the Short Wavelength Spectrometer on the Infrared Space Observatory (ISO). We have analyzed dust absorption features arising in the molecular cloud material and the diffuse interstellar medium along the lines of sight toward Sgr A* and the Quintuplet sources, GCS 3 and GCS 4. It is evident from the depth of the 3.0 μm H2O and the 4.27 μm CO2 ice features that there is more molecular cloud material along the line of sight toward Sgr A* than toward GCS 3 and GCS 4. In fact, Sgr A* has a rich infrared ice spectrum with evidence for the presence of solid CH4, NH3, and possibly HCOOH. Hydrocarbon dust in the diffuse interstellar medium along the line of sight to the GC is characterized by absorption features centered at 3.4, 6.85, and 7.25 μm. Ground-based studies have identified the 3.4 μm feature with aliphatic hydrocarbons, and ISO has given us the first meaningful observations of the corresponding modes at longer wavelengths. The integrated strengths of these three features suggest that hydrogenated amorphous carbon is their carrier. We attribute an absorption feature centered at 3.28 μm in the GCS 3 spectrum to the C–H stretch in aromatic hydrocarbons. This feature is not detected, and its C–C stretch counterpart appears to be weaker, in the Sgr A* spectrum. A key question now is whether or not aromatics are a widespread component of the diffuse interstellar medium, analogous to aliphatic hydrocarbons.

The Composition and Distribution of Dust Along the Line of Sight Towards the Galactic Center

Abstract: We discuss the composition of dust and ice along the line of sight to the Galactic Center (GC) based on analysis of mid-infrared spectra (24-13 micron) from the Short Wavelength Spectrometer on the Infrared Space Observatory (ISO) We have analyzed dust absorption features arising in the molecular cloud material and the diffuse interstellar medium along the lines of sight toward Sagittarius A* and the Quintuplet sources, GCS3 and GCS4 It is evident from the depth of the 30 micron H2O and the 427 micron CO2 ice features that there is more molecular cloud material along the line of sight toward Sgr A* than GCS3 and 4 In fact, Sgr A* has a rich infrared ice spectrum with evidence for the presence of solid CH4, NH3, and possibly HCOOH Hydrocarbon dust in the diffuse interstellar medium along the line of sight to the GC is characterized by absorption features centered at 34, 685, and 725 micron Ground-based studies have identified the 34 micron feature with aliphatic hydrocarbons, and ISO has given us the first meaningful observations of the corresponding modes at longer wavelengths The integrated strengths of these three features suggest that hydrogenated amorphous carbon is their carrier We attribute an absorption feature centered at 328 micron in the GCS3 spectrum to the C-H stretch in aromatic hydrocarbons This feature is not detected, and its C-C stretch counterpart appears to be weaker, in the SgrA* spectrum One of the key questions which now arises is whether aromatics are a widespread component of the diffuse interstellar medium, analogous to aliphatic hydrocarbons

Massive Star Formation and Evolution in Starburst Galaxies: Mid-infrared Spectroscopy with the ISO Short Wavelength Spectrometer*

Abstract: We present new Infrared Space Observatory Short Wavelength Spectrometer data for a sample of 27 starburst galaxies, and with these data we examine the issues of formation and evolution of the most massive stars in starburst galaxies. Using starburst models which incorporate time evolution, new stellar atmosphere models for massive stars, and a starburst model geometry derived from observations of the prototypical starburst M82, we model the integrated mid-infrared line ratio (Ne III)(15.6 km)/(Ne II)(12.8 km). This line ratio is sensitive to the hardness of the stellar energy distribution and therefore to the most massive stars present. We conclude from our models, with consideration of recent determinations of the stellar census in local, high-mass star-forming regions, that the (Ne III)/(Ne II) ratios we measure are consistent with the formation of massive (D50¨100 stars in most starbursts. In this framework, M _ ) the low nebular excitation inferred from the measured line ratios can be attributed to aging eUects. By including estimates of the ratio of infrared-to-Lyman continuum luminosity for the galaxies in our sample, we further —nd that most starbursts are relatively short-lived (106¨107 yr), only a few O star lifetimes. We discuss a possible cause of such short events: the eUectiveness of stellar winds and super- novae in destroying the starburst environment. Subject headings: galaxies: starburstinfrared: galaxiesstars: atmospheresstars: evolution ¨ stars: formationtechniques: spectroscopic

Massive star formation and evolution in starburst galaxies: mid-infrared spectroscopy with ISO-SWS 1

Abstract: We present new ISO-SWS data for a sample of 27 starburst galaxies, and with these data examine the issues of formation and evolution of the most massive stars in starburst galaxies. Using starburst models which incorporate time evolution, new stellar atmosphere models for massive stars, and a starburst model geometry derived from observations of the prototypical starburst M82, we model the integrated mid-infrared line ratio [Ne III](15.6µm)/[Ne II](12.8µm). This line ratio is sensitive to the hardness of the stellar energy distribution and therefore to the most massive stars present. We conclude from our models, with consideration of recent determinations of the stellar census in local, high-mass star forming regions, that the [Ne III]/[Ne II] ratios we measure are consistent with the formation of massive (∼ 50-100M⊙) stars in most starbursts. In this framework, the low nebular excitation inferred from the measured line ratios can be attributed to aging effects. By including estimates of the ratio of infrared-to-Lyman continuum luminosity for the galaxies in our sample, we further find that most starbursts are relatively short-lived (10 6 -10 7 years), only a few O-star lifetimes. We discuss a possible cause of such short events: the effectiveness of stellar winds and supernovae in destroying the starburst environment. Subject headings: galaxies:starburst—infrared:galaxies—techniques:spectroscopic—stars:formation, evolution, and atmospheres

ISO-SWS Spectroscopy of NGC 1068

Abstract: We present Infrared Space Observatory-Short Wave Spectrometer (ISO-SWS) spectroscopy of NGC 1068 for the complete wavelength range 2.4-45 μm at resolving power ~1500. Selected subranges have been observed at higher sensitivity and full resolving power ~2000. We detect a total of 36 emission lines and derive upper limits for 13 additional transitions. Most of the observed transitions are fine-structure and recombination lines originating in the narrow-line region (NLR) and the inner part of the extended emission-line region. We compare the line profiles of optical lines and reddening-insensitive infrared lines to constrain the dynamical structure and extinction properties of the NLR. The most likely explanation of the considerable differences found is a combination of two effects: (1) The spatial structure of the NGC 1068 NLR is a combination of a highly ionized outflow cone and lower excitation extended emission. (2) Parts of the NLR, mainly in the receding part at velocities above systemic, are subject to extinction that is significantly suppressing optical emission from these clouds. Line asymmetries and net blueshifts remain, however, even for infrared fine-structure lines suffering very little obscuration. This may be either due to an intrinsic asymmetry of the NLR, as perhaps also suggested by the asymmetric radio continuum emission or due to a very high column density obscuring component which is hiding part of the NLR even from infrared view. We present detections and limits for 11 rotational and rovibrational emission lines of molecular hydrogen (H2). They arise in a dense molecular medium at temperatures of a few hundred kelvins that is most likely closely related to the warm and dense components seen in the near-infrared H2 rovibrational transitions, and in millimeter-wave tracers (CO, HCN) of molecular gas. Any emission of the putative parsec-scale molecular torus is likely overwhelmed by this larger scale emission. In companion papers we use the SWS data to derive the spectral energy distribution emitted by the active nucleus of NGC 1068, to put limits on infrared emission from the obscured broad-line region, and discuss the continuum and its features in conjunction with SWS spectra of other galaxies.

Infrared Spectroscopy of NGC 1068: Probing the Obscured Ionizing AGN Continuum

Abstract: The ISO-SWS 2.5-45 μm infrared spectroscopic observations of the nucleus of the Seyfert 2 galaxy NGC 1068 (see companion paper) are combined with a compilation of UV to IR narrow emission line data to determine the spectral energy distribution (SED) of the obscured extreme-UV continuum that photoionizes the narrow-line-emitting gas in the active galactic nucleus. We search a large grid of gas-cloud models and SEDs for the combination that best reproduces the observed line fluxes and narrow-line region (NLR) geometry. Our best-fit model reproduces the observed line fluxes to better than a factor of 2 on average and is in general agreement with the observed NLR geometry. It has two gas components that are consistent with a clumpy distribution of dense outflowing gas in the center and a more extended distribution of less dense and more clumpy gas farther out that has no net outflow. The best-fit SED has a deep trough at ~4 rydbergs, which is consistent with an intrinsic Big Blue Bump that is partially absorbed by ~6 × 1019 cm-2 of neutral hydrogen interior to the NLR.

Dust Emission from the Lensed Lyman Break Galaxy cB58

Abstract: We detect 1.2mm continuum emission from dust in the gravitationally lensed Lyman break galaxy MS 1512+36-cB58. Our detected flux is surprisingly low: relative to local starburst galaxies, cB58 appears to produce somewhat less far-IR emission than its UV reddening predicts. After comparing several different estimates of the source's dust content, we conclude that the apparent discrepancy is most likely related to uncertainty in its UV spectral slope. Alternate scenarios to account for a far-IR "deficit" which rely on a high dust temperature or differential magnification are less satisfactory. Our result underscores one of the risks inherent in characterizing the cosmic star formation history from rest-UV data alone.

Modelling the Infrared continuum of Centaurus A

Abstract: We present ISOPHOT_S, ISOSWS and 8 to 13 micron ground based observations of Centaurus A that show prominent PAH and silicate features. These and other data are used to construct a model for the infrared continuum. We find that in a nuclear sized aperture (~4 arcsec, ~60 pc) the SED is characteristic of emission from a starburst and AGN torus; in larger apertures an additional component of cirrus emission is required. Based on our model, the torus diameter is estimated to be 3.6 pc and the best fitting inclination angle of the torus is 45 degrees. This result has implications for the detectability of tori in low power AGN and in particular for the use of the IRAS 60/25 micron flux ratio as an indicator of the torus inclination.

A Search for Broad Infrared Recombination Lines in NGC 1068

Abstract: We report infrared spectroscopy of the prototypical Seyfert 2 galaxy NGC 1068, aiming at detection of broad components of hydrogen recombination lines that originate in the obscured broad-line region. Using the Short Wavelength Spectrometer on board the Infrared Space Observatory, we have observed for the first time the regions of Bβ 2.626 μm and Pfα 7.460 μm, and present improved data for Bα 4.052 μm. No significant broad components are detected, implying an equivalent visual extinction to the broad-line region of at least 50 mag and an obscuring column density of at least 1023 cm-2. While consistent with a highly obscured broad-line region, as required by the classical unified scenario, these limits are not yet significant enough to discriminate strongly between different torus models or to constrain properties of the gas causing the very large X-ray obscuration. We discuss the systematic limitations of infrared broad-line region searches and suggest that Bα may often be the most favorable transition for future searches.

ISO-SWS spectra of galaxies: continuum and features

Abstract: We present an inventory of mid-infrared spectral features detected in high resolution 2.4 to 45um ISO-SWS spectra of the galaxies M82, NGC253, Circinus, NGC1068, and the 30Doradus region of the LMC. We discuss their identifications and highlight possible relations between these features and the physical state of the interstellar medium in galaxies. In the 5--13 um region, emission from unidentified infrared bands (UIBs), usually ascribed to aromatic molecules, and apparent silicate absorption dominate the spectrum. We demonstrate the danger of overestimating the amount of silicate absorption by comparing the spectrum of M82 to the (absorption free) spectrum of the reflection nebula NGC7023. No strong silicate absorption is present in M82. The (very small grain) dust continuum under the UIB emission in our starburst templates can be modeled by a simple power law, starting at wavelengths between 8 and 9um. The spectra are valuable templates for future mid-infrared missions. We smooth our data to simulate low resolution spectra as obtained with ISOCAM-CVF, ISOPHOT-S, and in the future with the low resolution mode of SIRTF-IRS, and use our high spectral resolution information to highlight possible identification problems at low resolving power that are caused by coincidences of lines and features.

ISO-SWS spectra of galaxies: Continuum and features ?

Abstract: We present an inventory of mid-infrared spec- tral features detected in high resolution (R1500) ISO-SWS 2.4-45m spectra of the galaxies M 82, NGC 253, Circinus, NGC 1068, and a position in the 30 Doradus region of the Large Magellanic Cloud. We discuss their identifications and high- light possible relations between these features and the physical state of the interstellar medium in galaxies. The spectral fea- tures vary considerably from source to source in presence and relative strength. Emission features are largely absent in the intense radiation field close to an AGN. Compared to normal infrared-selected starbursts, they also seem to be weaker in a low metallicity, intensely star forming environment. The large number of features beyond 13m is remarkable. Some of the features have - to our knowledge - not been reported before in astronomical objects. In the 5-13m region, emission from unidentified infrared bands (UIBs), usually ascribed to aromatic molecules, and ap- parent silicate absorption dominate the spectrum. The density of features makes it difficult to determine the continuum, partic- ularly in ground-based data of limited wavelength coverage. In fact the apparent depth of the 9.7m silicate absorption may be overestimated in the presence of UIB emission, as we demon- strate by comparing the spectrum of M 82 to the (absorption free) spectrum of the reflection nebula NGC 7023. No strong sil- icate absorption is present in M 82. The (very small grain) dust continuum under the UIB emission in our starburst templates can be modeled by a simple power law, starting at wavelengths between 8 and 9m. We find broad H2O-ice absorption features at 3.0 mi n M8 2 and NGC 253. Their optical depths (relative to the visual ex- tinction) indicate that the lines of sight towards these galaxies have similar properties as the line of sight towards the Galactic Center. The active galaxy NGC 1068 exhibits a clearly different spectrum of absorption features, indicating different physical conditions in the obscuring regions of this AGN compared to the starburst templates. Send offprint requests to: sturm@mpe.mpg.de ? Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with partici- pation of ISAS and NASA. The spectra are valuable templates for future mid-infrared missions. We smooth our data to simulate low resolution spec- tra as obtained with ISOCAM-CVF, ISOPHOT-S, and in the future with the low resolution mode of SIRTF-IRS, and use our high spectral resolution information to highlight possible iden- tification problems at low resolving power that are caused by coincidences of lines and features. The spectra are available in electronic form from the authors.

Mid-infrared ISO spectroscopy of NGC 4945

Abstract: We have observed the central region of the nearby starburst galaxy NGC 4945 with the mid-infrared spectrom- eters SWS and PHT-S aboard ISO. We do not find any evidence for the existence of the powerful AGN, inferred from hard X-ray observations. The upper limits on our AGN tracers (Nev)14.32 m&24.3m and (Nevi) 7.65 m im- ply an AV >160 towards the NLR, assuming the NLR to be of equal strength as in the Circinus galaxy. Other pos- sibilities are discussed. The starburst excitation indicators (Neiii)15.56 m/(Neii)12.81 m and Lbol/Llyc suggest that the starburst in the central region is at least 5 10 6 yrs old, and that it accounts for at least half of the nuclear bolometric luminosity. The starburst might well power the entire bolometric luminos- ity, but the available constraints are also consistent with an up to 50% contribution of the embedded AGN. With PHT-S, at a resolution of 90, we detect strong absorption features of water ice, and, for the first time in an external galaxy, of CO 2 and CO. The same PHT-S spectrum also reveals strong emission from the family of PAH features. Finally, we have observed and de- tected several pure rotational and ro-vibrational H2 lines, two of which, the (0-0) S(0) & S(1) lines, allow us to determine the ex- citation temperature (160 K) and warm H2 mass (2.410 7 M). The low excitation temperature shows Orion-like shocks not to be representative for the entire emission of the central region of the galaxy and fairly normal PDRs to be perhaps more typical.

The Nature of Ultra-Luminous Infrared Galaxies

Abstract: We review investigations with SWS, ISOPHOT-S, and ISOCAM-CVF of several samples of ultra-luminous infrared galaxies (ULIRGs) drawn from IRAS catalogs. The mid-IR data indicate that the majority of the local ULIRG population is predominantly powered by star formation but that AGN activity plays a signi.cant role in many sources and dominates the most luminous ones. AGN and starburst activity is present in all phases of the ULIRG/merger evolution. There is no obvious trend for AGNs to be more predominant, or the molecular content to be lower in the late ULIRG phases. Star formation activity in ULIRGs likely occurs in short (107 years) phases terminated by the disruptive effects of superwinds and supernovae. Highly obscured AGNs dominating the total energy output may be hidden to the mid-infrared observations in a few cases but are not characteristic of the average ULIRG.

Infrared spectroscopy of NGC 1068: Probing the obscured ionizing AGN continuum

Abstract: The ISO-SWS 2.5-45 um infrared spectroscopic observations of the nucleus of the Seyfert 2 galaxy NGC 1068 (see companion paper) are combined with a compilation of UV to IR narrow emission line data to determine the spectral energy distribution (SED) of the obscured extreme-UV continuum that photoionizes the narrow line emitting gas in the active galactic nucleus. We search a large grid of gas cloud models and SEDs for the combination that best reproduces the observed line fluxes and NLR geometry. Our best fit model reproduces the observed line fluxes to better than a factor of 2 on average and is in general agreement with the observed NLR geometry. It has two gas components that are consistent with a clumpy distribution of dense outflowing gas in the center and a more extended distribution of less dense and more clumpy gas farther out that has no net outflow. The best fit SED has a deep trough at ~4 Ryd, which is consistent with an intrinsic Big Blue Bump that is partially absorbed by ~6x10^19 cm^-2 of neutral hydrogen interior to the NLR.

ISO Spectroscopy of Active Galactic Nuclei

Abstract: Spectroscopy in the mid-infrared spectral range provides a number of unique diagnostics of active galactic nuclei which can now be used thanks to ISO’s sensitivity and full wavelength coverage. Mid-infrared aromatic emission features are absent close to the AGN but can be strong on larger host galaxy scales, in particular if there is intense circumnuclear star formation. The rich.ne structure and coronal line spectrum can be used to reconstruct the otherwise unobservable extreme ultraviolet emission of the AGN. We find evidence for ‘bigblue bump’ emission from the accretion disk but also for neutral absorbers located inside the narrow line region. The ability of mid-infrared emission to penetrate dust allows to probe the structure of dusty narrow line regions, and to search for the obscured broad line region in Seyfert 2 galaxies.

Searching for AGN signatures in Mid-IR Spectra: The case of NGC1068. The case of the prototypical Seyfert 2 galaxy NGC 1068

Abstract: We present mid-IR observations of the prototypical Seyfert 2 galaxy NGC 1068 obtained with ISOCAM between 5 and 16micron. The proximity of this galaxy coupled with the spectro-imaging capabilities of the instrument allow us to separate the mid-IR emission of the active galactic nucleus (AGN) from the distinctly different emission of the circumnuclear star forming regions. The Unidentified Infrared Bands (UIBs), which trace the starburst contribution very well, are not detected in the spectrum of the AGN region where their carriers could be destroyed. Moreover, the featureless continuum of the AGN exhibits a strong hot dust component below 10micron not observed in the starburst regions. Those two distinct mid-IR spectral properties, as well as the presence of high excitation ionic lines such as [NeVI](7.7micron) and [NeV](14.3micron) in the AGN spectrum, provide us with very powerful complementary tools to disentangle AGNs from starbursts. The effects of high extinction on the mid-IR identification of AGNs are also discussed.

Morphology and Evolution of Ultraluminous IR Galaxies

Abstract: Ultraluminous infrared galaxies (ULIRGs) are probably the local analogues of a major mode of galaxy formation in the early universe. Here we give a brief progress report on our ongoing programme to study the nature and evolution of ULIRGs. Our near-IR imaging of a large sample of these highly disturbed merger systems provides a data base of morphological parameters like tidal features or projected separation of the nuclei. Together with ISO Mid-IR spectroscopy these morphological parameters allow us to explore the merger dynamics and evolution of ULIRGs as they may progress from starburst to buried AGN to exposed QSO. We find that the fraction of ULIRGs predominantly powered by AGNs increases with luminosity above ~ 3 x 1012 L ⊙, but that there is no obvious trend for ULIRGs to be more AGN-like with more advanced merger phase.

Searching for AGN signatures in Mid-IR Spectra: The case of NGC1068

Abstract: We present mid-IR observations of the prototypical Seyfert 2 galaxy NGC 1068 obtained with ISOCAM between 5 and 16micron. The proximity of this galaxy coupled with the spectro-imaging capabilities of the instrument allow us to separate the mid-IR emission of the active galactic nucleus (AGN) from the distinctly different emission of the circumnuclear star forming regions. The Unidentified Infrared Bands (UIBs), which trace the starburst contribution very well, are not detected in the spectrum of the AGN region where their carriers could be destroyed. Moreover, the featureless continuum of the AGN exhibits a strong hot dust component below 10micron not observed in the starburst regions. Those two distinct mid-IR spectral properties, as well as the presence of high excitation ionic lines such as [NeVI](7.7micron) and [NeV](14.3micron) in the AGN spectrum, provide us with very powerful complementary tools to disentangle AGNs from starbursts. The effects of high extinction on the mid-IR identification of AGNs are also discussed.