Showing papers by "Dieter Lutz published in 2003"

The [C II] 158 micron line deficit in ultraluminous infrared galaxies revisited

Abstract: We present a study of the [C II] 157.74 μm fine-structure line in a sample of 15 ultraluminous infrared (IR) galaxies (IR luminosity LIR ≥ 1012 L☉; ULIRGs) using the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). We confirm the observed order of magnitude deficit (compared to normal and starburst galaxies) in the strength of the [C II] line relative to the far-infrared (FIR) dust continuum emission found in our initial report, but here with a sample that is twice as large. This result suggests that the deficit is a general phenomenon affecting 4 out of 5 ULIRGs. We present an analysis using observations of generally acknowledged photodissociation region (PDR) tracers ([C II], [O I] 63 and 145 μm, and FIR continuum emission), which suggests that a high ultraviolet flux G0 incident on a moderate density n PDR could explain the deficit. However, comparisons with other ULIRG observations, including CO (1-0), [C I] (1-0), and 6.2 μm polycyclic aromatic hydrocarbon (PAH) emission, suggest that high G0/n PDRs alone cannot produce a self-consistent solution that is compatible with all of the observations. We propose that non-PDR contributions to the FIR continuum can explain the apparent [C II] deficiency. Here, unusually high G0 and/or n physical conditions in ULIRGs as compared to those in normal and starburst galaxies are not required to explain the [C II] deficit. Dust-bounded photoionization regions, which generate much of the FIR emission but do not contribute significant [C II] emission, offer one possible physical origin for this additional non-PDR component. Such environments may also contribute to the observed suppression of FIR fine-structure emission from ionized gas and PAHs, as well as the warmer FIR colors found in ULIRGs. The implications for observations at higher redshifts are also revisited.

The [CII] 158 um Line Deficit in Ultraluminous Infrared Galaxies Revisited

Abstract: We present a study of the [CII] 157.74 um fine-structure line in a sample of 15 ultraluminous infrared (IR) galaxies (L_IR>10^12 Lsun; ULIRGs) using the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). We confirm the observed order of magnitude deficit (compared to normal and starburst galaxies) in the strength of the [CII] line relative to the far-IR dust continuum emission found in our initial report (Luhman et al. 1998), but here with a sample that is twice as large. This result suggests that the deficit is a general phenomenon affecting 4/5 ULIRGs. We present an analysis using observations of generally acknowledged photodissociation region (PDR) tracers ([CII], [OI] 63 and 145 um, and FIR continuum emission), which suggests that a high UV flux G_o incident on a moderate density n PDR could explain the deficit. However, comparisons with other ULIRG observations, including CO (1-0), [CI] (1-0), and 6.2 um polycyclic aromatic hydrocarbon (PAH) emission, suggest that high G_o/n PDRs alone cannot produce a self-consistent solution that is compatible with all of the observations. We propose that non-PDR contributions to the FIR continuum can explain the apparent [CII] deficiency. Here, unusually high G_o and/or n physical conditions in ULIRGs as compared to those in normal and starburst galaxies are not required to explain the [CII] deficit. Dust-bounded photoionization regions, which generate much of the FIR emission but do not contribute significant [CII] emission, offer one possible physical origin for this additional non-PDR component. Such environments may also contribute to the observed suppression of FIR fine-structure emission from ionized gas and PAHs, as well as the warmer FIR colors found in ULIRGs. The implications for observations at higher redshifts are also revisited.

Spatially Resolved Millimeter Interferometry of SMM J02399–0136: A Very Massive Galaxy at z = 2.8*

Abstract: We report high-resolution millimeter mapping with the IRAM Plateau de Bure interferometer of rest-frame 335 ?m continuum and CO (3-2) line emission from the z = 2.8 submillimeter galaxy SMM J02399-0136. The continuum emission comes from a ~3'' diameter structure whose elongation is approximately east-west and whose centroid is coincident within the astrometric errors with the brightest X-ray and rest-UV peak (L1). The line data show that this structure is most likely a rapidly rotating disk. Its rotation velocity of ?420 km s-1 implies a total dynamical mass of ?3 ? 1011 sin?2 i h M? within an intrinsic radius of 8 h kpc, most of which is plausibly in the form of stars and gas. SMM J02399-0136 is thus a very massive system, whose formation at z ~ 3 is not easy to understand in current cold dark matter hierarchical merger cosmogonies.

The Nature of Starburst Activity in M82

Abstract: We present new evolutionary synthesis models of M82 based mainly on observations consisting of near-infrared integral field spectroscopy and mid-infrared spectroscopy. The models incorporate stellar evolution, spectral synthesis, and photoionization modeling and are optimized forλ = 1-45 μm observations of starburst galaxies. The data allow us to model the starburst regions on scales as small as 25 pc. We investigate the initial mass function (IMF) of the stars and constrain quantitatively the spatial and temporal evolution of starburst activity in M82. We find a typical decay timescale for individual burst sites of a few million years. The data are consistent with the formation of very massive stars (50-100 M☉) and require a flattening of the starburst IMF below a few solar masses, assuming a Salpeter slope dN/dm ∝ m-2.35 at higher masses. Our results are well matched by a scenario in which the global starburst activity in M82 occurred in two successive episodes each lasting a few million years, peaking about 107 yr and 5 × 106 yr ago. The first episode took place throughout the central regions of M82 and was particularly intense at the nucleus, while the second episode occurred predominantly in a circumnuclear ring and along the stellar bar. We interpret this sequence as resulting from the gravitational interaction between M82 and its neighbor M81, and subsequent bar-driven evolution. The short burst duration on all spatial scales indicates strong negative feedback effects of starburst activity, both locally and globally. Simple energetics considerations suggest that the collective mechanical energy released by massive stars was able to rapidly inhibit star formation after the onset of each episode.

A mid-infrared spectroscopic survey of starburst galaxies: Excitation and abundances

Abstract: We present spectroscopy of mid-infrared emission lines in twelve starburst regions, located in eleven starburst galaxies, for which a significant number of lines between 2.38 and 45µm were observed with the ISO Short Wavelength Spectrometer, with the intention of providing a reference resource for mid-infrared spectra of starburst galaxies. The observation apertures were centred on actively star forming regions, including those which are inaccessible at optical wavelengths due to high levels of obscuration. We use this data set, which includes fine structure and hydrogen recombination lines, to investigate excitation and to derive gas phase abundances of neon, argon, and sulphur of the starburst galaxies. The derived Ne abundances span approximately an order of magnitude, up to values of ∼ 3 times solar. The excitation ratios measured from the Ne and Ar lines correlate well with each other (positively) and with abundances (negatively). Both in excitation and abundance, a separation of objects with visible Wolf-Rayet features (high excitation, low abundance) is noted from those without (low excitation, high abundance). For a given abundance, the starbursts are of relatively lower excitation than a comparative sample of HII regions, possibly due to ageing stellar populations. By considering the abundance ratios of S with Ne and Ar we find that, in our higher metallicity systems, S is relatively underabundant by a factor of ∼ 3. We discuss the origin of this deficit and favour depletion of S onto dust grains as a likely explanation. This weakness of the mid-infrared fine structure lines of sulphur has ramifications for future infrared missions such as SIRTF and Herschel since it indicates that the S lines are less favourable tracers of star formation than is suggested by nebular models which do not consider this effect. In a related paper (Sturm et al. 2002), we combine our results with spectra of Seyfert galaxies in order to derive diagnostic diagrams which can effectively discriminate between the two types of activity in obscured regions on the basis of excitation derived from detected mid-infrared lines.

ISO spectroscopy of star formation and active nuclei in the luminous infrared galaxy NGC 6240

Abstract: We present Infrared Space Observatory mid- and far-infrared spectroscopy of the merging galaxy NGC 6240, an object presenting many aspects of importance for the role of star formation and AGN activity in (ultra)luminous infrared galaxies. The mid-infrared spectrum shows starburst indicators in the form of low excitation fine-structure line emission and aromatic "PAH" features. A strong high excitation (O IV) line is observed which most likely originates in the Narrow Line Region of an optically obscured AGN. NGC 6240 shows extremely powerful emission in the pure rotational lines of molecular hydrogen. We argue that this emission is mainly due to shocks in its turbulent central gas component and its starburst superwind. The total shock cooling in infrared emission lines accounts for0.6% of the bolometric luminosity, mainly through rotational H2 emission and the (O I) 63m line. We analyse several ways of estimating the luminosities of the starburst and the AGN in NGC 6240 and suggest that the contributions to its bolometric luminosity are most likely in the range 50-75% starburst and 25-50% AGN.

Mid-infrared spectral evidence for a luminous dust enshrouded source in Arp220

Abstract: We have re-analyzed the 6-12 micron ISO spectrum of the ultra-luminous infrared galaxy Arp220 with the conclusion that it is not consistent with that of a scaled up version of a typical starburst. Instead, both template fitting with spectra of the galaxies NGC4418 and M83 and with dust models suggest that it is best represented by combinations of a typical starburst component, exhibiting PAH emission features, and a heavily absorbed dust continuum which contributes ~40% of the 6-12 micron flux and likely dominates the luminosity. Of particular significance relative to previous studies of Arp220 is the fact that the emission feature at 7.7 micron comprises both PAH emission and a broader component resulting from ice and silicate absorption against a heavily absorbed continuum. Extinction to the PAH emitting source, however, appears to be relatively low. We tentatively associate the PAH emitting and heavily dust/ice absorbed components with the diffuse emission region and the two compact nuclei respectively identified by Soifer et al. (2002) in their higher spatial resolution 10 micron study. Both the similarity of the absorbed continuum with that of the embedded Galactic protostars and results of the dust models imply that the embedded source(s) in Arp220 could be powered by, albeit extremely dense, starburst activity. Due to the high extinction, it is not possible with the available data to exclude that AGN(s) also contribute some or all of the observed luminosity. In this case, however, the upper limit measured for its hard X-ray emission would require Arp220 to be the most highly obscured AGN known.

Detection of strongly processed ice in the central starburst of NGC 4945

Abstract: The composition of ice grains provides an important tool for the study of the molecular environment of star forming regions. Using ISAAC at the VLT to obtain spectra around 4.65m we have detected for the first time "XCN" and CO ice in an extragalactic environment: the nuclear region of the nearby dusty starburst/AGN galaxy NGC 4945. The profile of the solid CO band reveals the importance of thermal processing of the ice while the prominence of the XCN band attests to the importance of energetic processing of the ice by FUV radiation and/or energetic particles. In analogy to the processing of ices by embedded protostars in our Galaxy, we attribute the processing of the ices in the center of NGC 4945 to ongoing massive star formation. Our M-band spectrum also shows strong HI Pf and H2 0-0 S(9) line emission and gas phase CO absorption lines. The HI, H2, PAH, gas phase CO and the ices seem to be embedded in a rotating molecular disk which is undergoing vigorous star formation. Recently, strong OCN absorption has been detected in the spectrum of the Galactic center star GC: IRS 19. The most likely environment for the OCN absorption is the strongly UV-exposed GC molecular ring. The presence of processed ice in the center of NGC 4945 and our Galactic center leads us to believe that processed ice may be a common characteristic of dense molecular material in star forming galactic nuclei.

Detection of strongly processed ice in the central starburst of NGC4945

Abstract: The composition of ice grains provides an important tool for the study of the molecular environment of star forming regions. Using ISAAC at the VLT to obtain spectra around 4.65 microns we have detected for the first time `XCN' and CO ice in an extragalactic environment: the nuclear region of the nearby dusty starburst/AGN galaxy NGC4945. The profile of the solid CO band reveals the importance of thermal processing of the ice while the prominence of the XCN band attests to the importance of energetic processing of the ice by FUV radiation and/or energetic particles. In analogy to the processing of ices by embedded protostars in our Galaxy, we attribute the processing of the ices in the center of NGC4945 to ongoing massive star formation. Our M-band spectrum also shows strong HI Pfund-beta and H2 0-0 S(9) line emission and gas phase CO absorption lines. The HI, H2, PAH, gas phase CO and the ices seem to be embedded in a rotating molecular disk which is undergoing vigorous star formation. Recently, strong OCN- absorption has been detected in the spectrum of the Galactic center star GC:IRS19. The most likely environment for the OCN- absorption is the strongly UV-exposed GC molecular ring. The presence of processed ice in the center of NGC4945 and our Galactic center leads us to believe that processed ice may be a common characteristic of dense molecular material in star forming galactic nuclei.

The universe in 3D : First observations with SPIFFI, the infrared integral field spectrometer for the VLT

Abstract: 17 S PIFFI (SPECTROMETER FOR Infrared Faint Field Imaging) is the new near-infrared field spectrometer for the VLT, developed at the Max-Planck-Institute for Extraterrestrial Physics (MPE, Eisenhauer et al. 2000). Here we report on the results of its first observing runs as a ‘guest instrument’ at the VLT in the February to April 2003 period. As of 2004 on, SPIFFI will be coupled to an adaptive optics module developed at ESO (Bonnet et al. 2003) to provide the SINFONI (SINgle Faint Object Near-IR Investigation) facility. SPIFFI offers imaging spectroscopy of a contiguous, two-dimensional field of 32 32 spatial pixels in the 1.1 – 2.45 μm wavelength range and a resolving power of 1300-3500. As a result, the instrument delivers a simultaneous, three-dimensional data-cube with two spatial dimensions and one spectral dimension. SPIFFI is the successor to the MPE integral field spectrometer 3D, the world’s first infrared integral field spectrometer developed in the early 1990s. When the new generation of 10242 pixels, near-infrared detectors became available in the mid-1990’s, we started the development of SPIFFI, for an order of magnitude increase in the number of spatial and spectral elements over 3D. Even more importantly, because of its fully cryogenic image slicer and high throughput optics, along with OH airglow suppression and smaller pixels, SPIFFI at the VLT delivers a factor of 20 to 50 improvement in point source sensitivity over 3D. This development attracted the attention of ESO, specifically because of its major advantages over long slit spectroscopy when operated together with adaptive optics. Simultaneous observation of a two-dimensional field is the best way to reach the full diffraction limited resolution in imaging spectroscopy, while in addition minimizing slit losses. The development of such an adaptive optics assisted integral field spectrometer was subsequently recommended by the ESO Scientific and Technical Committee (STC) in 1997, and finally formalized in 2001 with a contract between ESO and MPE. Because the research and development program for SPIFFI had been launched long before the official start of the full project, we took advantage of this head start and brought SPIFFI to the VLT as a guest instrument for seeing limited observations, while integration of the adaptive optics module was starting at ESO-Garching.