Typical Starburst Research Articles

Exploring the physics behind the non-thermal emission from star-forming galaxies detected in γ rays

Context. Star-forming galaxies emit non-thermal radiation from radio to γ rays. Observations show that their radio and γ-ray luminosities scale with their star formation rates, supporting the hypothesis that non-thermal radiation is emitted by cosmic rays produced by their stellar populations. However, the nature of the main cosmic-ray transport processes that shape the emission in these galaxies is still poorly understood, especially at low star formation rates. Aims. Our aim is to investigate the main mechanisms of global cosmic-ray transport and cooling in star-forming galaxies. The way they contribute to shaping the relations between non-thermal luminosities and star formation rates could shed light onto their nature, and allow us to quantify their relative importance at different star formation rates. Methods. We developed a model to compute the cosmic-ray populations of star-forming galaxies, taking into account their production, transport, and cooling. The model is parametrised only through global galaxy properties, and describes the non-thermal emission in radio (at 1.4 GHz and 150 MHz) and γ rays (in the 0.1−100 GeV band). We focused on the role of diffusive and advective transport by galactic winds, either driven by turbulent or thermal instabilities. We compared model predictions to observations, for which we compiled a homogeneous set of luminosities in these radio bands, and updated those available in γ rays. Results. Our model reproduces reasonably well the observed relations between the γ-ray or 1.4 GHz radio luminosities and the star formation rate, assuming a single power-law scaling of the magnetic field (with index β = 0.3) and winds blowing either at Alfvenic speeds (∼tens of km s−1, for ≲5 M⊙ yr−1) or typical starburst wind velocities (∼hundreds of km s−1, for ≳5 M⊙ yr−1). Escape of cosmic rays is negligible for ≳30 M⊙ yr−1. A constant ionisation fraction of the interstellar medium fails to reproduce the 150 MHz radio luminosity throughout the whole star formation rate range. Conclusions. Our results reinforce the idea that galaxies with high star formation rates are cosmic-ray calorimeters, and that the main mechanism driving proton escape is diffusion, whereas electron escape also proceeds via wind advection. They also suggest that these winds should be cosmic-ray or thermally driven at low and intermediate star formation rates, respectively. Our results globally support that magneto-hydrodynamic turbulence is responsible for the dependence of the magnetic field strength on the star formation rate and that the ionisation fraction is strongly disfavoured to be constant throughout the whole range of star formation rates.

AzTEC survey of the central molecular zone: increasing spectral index of dust with density

ABSTRACT The central molecular zone (CMZ) of our Galaxy hosts an extreme environment analogous to that found in typical starburst galaxies in the distant Universe. In order to understand dust properties in environments like our CMZ, we present results from a joint spectral energy distribution analysis of our AzTEC/Large Millimeter Telescope survey, together with existing Herschel far-IR data on the CMZ, from a wavelength range of 160 $\mu {\rm m}$ to 1.1 mm. We include global foreground and background contributions in a novel Bayesian modelling that incorporates the point spread functions of the different maps, which enables the full utilization of our high-resolution (10.5 arcsec) map at 1.1 mm and reveals unprecedentedly detailed information on the spatial distribution of dusty gas across the CMZ. There is a remarkable trend of increasing dust spectral index β, from 2.0 to 2.4, towards dense peaks in the CMZ, indicating a deficiency of large grains or a fundamental change in dust optical properties. This environmental dependence of β could have a significant impact on the determination of dust temperature in other studies. Depending on how the optical properties of dust deviate from the conventional model, dust temperatures could be underestimated by $10\!-\!50{{\ \rm per\ cent}}$ in particularly dense regions.

A Multi-Frequency Study of the Milky Way-Like Spiral Galaxy NGC 6744

AbstractWe present a multi-frequency study of the intermediate spiral SAB(r)bc type galaxy NGC 6744, using available data from the Chandra X-Ray telescope, radio continuum data from the Australia Telescope Compact Array and Murchison Widefield Array, and Wide-field Infrared Survey Explorer infrared observations. We identify 117 X-ray sources and 280 radio sources. Of these, we find nine sources in common between the X-ray and radio catalogues, one of which is a faint central black hole with a bolometric radio luminosity similar to the Milky Way’s central black hole. We classify 5 objects as supernova remnant (SNR) candidates, 2 objects as likely SNRs, 17 as H ii regions, 1 source as an AGN; the remaining 255 radio sources are categorised as background objects and one X-ray source is classified as a foreground star. We find the star-formation rate (SFR) of NGC 6744 to be in the range 2.8–4.7 M⊙~yr − 1 signifying the galaxy is still actively forming stars. The specific SFR of NGC 6744 is greater than that of late-type spirals such as the Milky Way, but considerably less that that of a typical starburst galaxy.

SOURCE-PLANE RECONSTRUCTION OF THE GIANT GRAVITATIONAL ARC IN A2667: A CANDIDATE WOLF-RAYET GALAXY ATz∼ 1

We present a new analysis of HST, Spitzer telescope imaging and VLT imaging and spectroscopic data of a bright lensed galaxy at $z$=1.0334 in the lensing cluster Abell~2667. Using this high-resolution imaging we present an updated lens model that allows us to fully understand the lensing geometry and reconstruct the lensed galaxy in the source plane. This giant arc gives a unique opportunity to peer into the structure of a high-redshift disk galaxy. We find that the lensed galaxy of Abell 2667 is a typical spiral galaxy with morphology similar to the structure of its counterparts at higher redshift $z\sim 2$. The surface brightness of the reconstructed source galaxy in the z$_{850}$ band reveals the central surface brightness $I(0)=20.28\pm0.22$ mag arcsec$^{-2}$ and the characteristic radius $r_s=2.01\pm0.16$ kpc at redshift $z \sim 1$. The morphological reconstruction in different bands shows obvious negative radial color gradients for this galaxy. Moreover, the redder central bulge tends to contain a metal-rich stellar population, rather than being heavily reddened by dust due to high and patchy obscuration. We analyze the VIMOS/IFU spectroscopic data and find that, in the given wavelength range ($\sim 1800-3200$ \AA), the combined arc spectrum of the source galaxy is characterized by a strong continuum emission with strong UV absorption lines (FeII and MgII) and shows the features of a typical starburst Wolf-Rayet galaxy NGC5253. More specifically, we have measured the EWs of FeII and MgII lines in the Abell 2667 spectrum, and obtained similar values for the same wavelength interval of the NGC5253 spectrum. Marginal evidence for CIII] 1909 emission at the edge of the grism range further confirms our expectation.

Evolution of multiple supernova remnants

(Abridged) Heating of the interstellar medium by multiple supernovae (SNe) explosions is at the heart of producing galaxy-scale outflows. We use hydrodynamical simulations to study the efficiency of multiple SNe in heating the interstellar medium (ISM) and filling the volume with gas of high temperatures. We argue that it is important for SNe remnants to have a large filling factor {\it and} a large heating efficiency. For this, they have to be clustered in space and time, and keep exploding until the hot gas percolates through the whole region, in order to compensate for the radiative loss. In the case of a limited number of SNe, we find that although the filling factor can be large, the heating efficiency declines after reaching a large value. In the case of a continuous series of SNe, the hot gas ($T \ge 3 \times 10^6$ K) can percolate through the whole region after the total volume filling factor reaches a threshold of $\sim 0.3$. The efficiency of heating the gas to X-ray temperatures can be $\ge 0.1$ after this percolation epoch, which occurs after a period of $\approx 10$ Myr for a typical starburst SNe rate density of $\nu_{\rm SN} \approx 10^{-9}$ pc$^{-3}$ yr$^{-1}$ and gas density of $n\approx 10$ cm$^{-3}$ in starburst nuclei regions. This matches the recent observations of a time delay of similar order between the onset of star formation and galactic outflows. The efficiency to heat gas up to X-ray temperatures ($\ge 10^{6.5}$ K) roughly scales as $\nu_{\rm SN}^{0.2} n^{-0.6}$. For a typical SNe rate density and gas density in starburst nuclei, the heating efficiency is $\sim 0.15$, also consistent with previous interpretations from X-ray observations. We discuss the implications of our results with regard to observational diagnostics of ionic ratios and emission measures in starburst nuclei regions.

THE IMACS CLUSTER BUILDING SURVEY. V. FURTHER EVIDENCE FOR STARBURST RECYCLING FROM QUANTITATIVE GALAXY MORPHOLOGIES

Using $J$ and $K_s$ band imaging obtained as part of the IMACS Cluster Building Survey (ICBS) we measure S\'ersic indices for 2160 field and cluster galaxies at $0.31 < z < 0.54$. Using both mass- and magnitude-limited samples, we compare the distributions for spectroscopically determined passive, continuously starforming, starburst, and poststarburst systems and show that previously established spatial and statistical connections between these types extend to their gross morphologies. Outside of cluster cores, we find close structural ties between starburst and continuously starforming, as well as poststarburst and passive types, but not between starbursts and poststarbursts. These results independently support two conclusions presented in Paper II of this ICBS series (Dressler et al.): 1) most starbursts are the product of a non-disruptive triggering mechanism that is insensitive to global environment, such as minor-mergers; 2) starbursts and poststarbursts generally represent transient phases in the lives of "normal" starforming and quiescent galaxies, respectively, originating from and returning to these systems in closed "recycling" loops. In this picture, spectroscopically identified poststarbursts constitute a minority of all recently terminated starbursts, largely ruling-out the typical starburst as a quenching event in all but the densest environments.

Spitz Nevi

Controversy exists regarding strategies for diagnosis and management of Spitz nevi, a type of melanocytic neoplasm that most often develops in children. To determine the beliefs, behaviors, and experiences of pediatric dermatologists with regard to Spitz nevi. Anonymous web-based survey. Private and academic dermatology practices. Respondents included 175 pediatric dermatologists from the United States and around the world, representing a 51.1% response rate (175 of 342). Analyses were limited to the 144 respondents whose practices included at least 50% children (younger than 18 years). Assessment of the following with regard to Spitz nevi: frequency of diagnosis, general beliefs, techniques used for evaluation (eg, dermoscopy and biopsy), management strategies, and observed outcomes. Collectively, respondents had seen approximately 20 000 Spitz nevi; 67.6% (96 of 142) had diagnosed at least 6 Spitz nevi yearly, whereas 90.1% (128 of 142) had diagnosed no more than 2 prepubertal melanomas in the past 5 years. Ninety-six percent of respondents (95.8%; 136 of 142) categorized typical Spitz nevi as benign. Eighty percent of respondents (79.6%; 113 of 142) used dermatoscopy, and 96.5% (137 of 142) avoided partial biopsies of Spitz nevi. In children with a suspected Spitz nevus, clinical follow-up was chosen by 49.3% (69 of 140) of respondents for a small, stable nonpigmented lesion and by 29.7% (41 of 138) for a pigmented lesion with a typical starburst pattern seen via dermatoscopy. Predictors of clinical follow-up of the latter lesion included believing that Spitz nevi are not melanoma precursors (P = .04). Forty-seven percent (62 of 132) of respondents had observed involution of Spitz nevi. No deaths had resulted from the approximately 10 000 Spitz nevi or atypical spitzoid neoplasms seen by the 91 respondents with academic or hospital-based practices. The results of our survey support conservative management of Spitz nevi in children, with clinical follow-up representing an option for typical lesions. This represents an important difference from strategies used for management of these lesions in adults.

Nuclear mid-infrared properties of nearby low-luminosity AGN

We present ground-based high-spatial resolution mid-infrared (MIR) observations of 20 nearby low-luminosity AGN (LLAGN) with VLT/VISIR and the preliminary analysis of a new sample of 10 low-luminosity Seyferts observed with Gemini/Michelle. LLAGN are of great interest because these objects are the most common among active galaxies, especially in the nearby universe. Studying them in great detail makes it possible to investigate the AGN evolution over cosmic timescale. Indeed, many LLAGN likely represent the final stage of an AGN's lifetime. We show that even at low luminosities and accretion rates nuclear unresolved MIR emission is present in most objects. Compared to lower spatial resolution Spitzer/IRS spectra, the high-resolution MIR photometry exhibits significantly lower fluxes and different PAH emission feature properties in many cases. By using scaled Spitzer/IRS spectra of typical starburst galaxies, we show that the star formation contribution to the 12 μm emission is minor in the central parsecs of most LLAGN. Therefore, the observed MIR emission in the VISIR and Michelle data is most likely emitted by the AGN itself, which, for higher luminosity AGN, is interpreted as thermal emission from a dusty torus. Furthermore, the 12 /amemission of the LLAGN is strongly correlated with the absorption corrected 2-10 keV luminosity and the MIR- X-ray correlation found previously for AGN is extended to a range from 1040 to 1045 erg/s. This correlation is independent of the object type, and in particular the low-luminosity Seyferts observed with Michelle fall exactly on the power-law fit valid for brighter AGN. In addition, no dependency of the MIR-X-ray ratio on the accretion rate is found. These results are consistent with the unification model being applicable even in the probed low-luminosity regime.

Gamma-rays from pulsar wind nebulae in starburst galaxies

Recently, gamma-ray emission at TeV energies has been detected from the starburst galaxies NGC253 (Acero et al., 2009) [1] and M82 (Acciari et al., 2009) [2]. It has been claimed that pion production due to cosmic rays accelerated in supernova remnants interacting with the interstellar gas is responsible for the observed gamma rays. Here, we show that the gamma-ray pulsar wind nebulae left behind by the supernovae contribute to the TeV luminosity in a major way. A single pulsar wind nebula produces about ten times the total luminosity of the Sun at energies above 1TeV during a lifetime of 105 years. A large number of 3×104 pulsar wind nebulae expected in a typical starburst galaxy at a distance of 4 Mpc can readily produce the observed TeV gamma rays.

GALACTIC H2CO DENSITOMETRY. I. PILOT SURVEY OF ULTRACOMPACT H II REGIONS AND METHODOLOGY

We present a pilot survey of 21 lines of sight towards UCHII regions in the H2CO 1-1 (6cm) and 2-2 (2cm) transitions, using the H2CO centimeter lines as a molecular gas densitometer. Using Arecibo and Green Bank beam-matched observations, we measure the density of 51 detected H2CO line pairs and present upper limits on density for an additional 24 detected 1-1 lines. We analyze the systematic uncertainties in the H2CO densitometer, achieving H2 density measurements with accuracies ~ 0.1-0.3 dex. The densities measured are not correlated with distance, implying that it is possible to make accurate density measurements throughout the galaxy without a distance bias. We confirm that ultracompact HII regions are associated with gas at densities n(H2)~10^5cm^-3. The densities measured in line-of-sight molecular clouds suggest that they consist of low volume filling factor (f~10^-2) gas at high (n(H2)>10^4cm^-3) density, which is inconsistent with purely supersonic turbulence and requires high-density clumping greater than typically observed in gravoturbulent simulations. We observe complex line morphologies that indicate density variations with velocity around UCHII regions, and we classify a subset of the UCHII molecular envelopes as collapsing or expanding. We compare these measurements to Bolocam Galactic Plane Survey 1.1mm observations, and note that most UCHII regions have 1.1mm emission consisting of significant (5-70%) free-free emission and are therefore not necessarily dominated by optically thin dust emission. A comparison of our data with the Mangum et al. starburst sample shows that the area filling factor of dense (n(H2)~10^5cm^-3) molecular gas in typical starburst galaxies is <~0.01, but in extreme starburst galaxies like Arp 220, is ~0.1, suggesting that Arp 220 is physically similar to an oversized UCHII region.

The properties of the interstellar medium within a star-forming galaxy at z= 2.3

We present an analysis of the molecular and atomic gas emission in the rest-frame far-infrared and sub-millimetre, from the lensed z=2.3 sub-millimetre galaxy SMM J2135-0102. We obtain very high signal-to-noise detections of 11 transitions from 3 species and limits on a further 20 transitions from 9 species. We use the 12CO, [CI] and HCN line strengths to investigate the gas mass, kinematic structure and interstellar medium (ISM) chemistry, and find strong evidence for a two-phase medium comprising a hot, dense, luminous component and an underlying extended cool, low-excitation massive component. Employing photo-dissociation region models we show that on average the molecular gas is exposed to a UV radiation field that is ~1000 x more intense than the Milky Way, with star-forming regions having a characteristic density of n~10^4 /cm^3. These conditions are similar to those found in local ULIRGs and in the central regions of typical starburst galaxies, even though the star formation rate is far higher in this system. The 12CO spectral line energy distribution and line profiles give strong evidence that the system comprises multiple kinematic components with different conditions, including temperature, and line ratios suggestive of high cosmic ray flux within clouds. We show that, when integrated over the galaxy, the gas and star-formation surface densities appear to follow the Kennicutt-Schmidt relation, although when compared to high-resolution sub-mm imaging, our data suggest that this relation breaks down on scales of <100pc. By virtue of the lens amplification, these observations uncover a wealth of information on the star formation and ISM at z~2.3 at a level of detail that has only recently become possible at z<0.1, and show the potential physical properties that will be studied in unlensed galaxies when ALMA is in full operation. (Abridged).

PROBING THE EXCITATION OF EXTREME STARBURSTS: HIGH-RESOLUTION MID-INFRARED SPECTROSCOPY OF BLUE COMPACT DWARFS

We present an analysis of the mid-infrared emission lines for a sample of 12 low metallicity Blue Compact Dwarf (BCD) galaxies based on high resolution observations obtained with Infrared Spectrograph on board the {\rm Spitzer} Space Telescope. We compare our sample with a local sample of typical starburst galaxies and active galactic nuclei (AGNs), to study the ionization field of starbursts over a broad range of physical parameters and examine its difference from the one produced by AGN. The high-ionization line [OIV]25.89$\mu$m is detected in most of the BCDs, starbursts, and AGNs in our sample. We propose a diagnostic diagram of the line ratios [OIV]25.89$\mu$m/[SIII]33.48$\mu$m as a function of [NeIII]15.56$\mu$m/[NeII]12.81$\mu$m which can be useful in identifying the principal excitation mechanism in a galaxy. Galaxies in this diagram split naturally into two branches. Classic AGNs as well as starburst galaxies with an AGN component populate the upper branch, with stronger AGNs displaying higher [NeIII]/[NeII] ratios. BCDs and pure starbursts are located in the lower branch. We find that overall the placement of galaxies on this diagram correlates well with their corresponding locations in the log([NII]/H$\alpha$) vs. log([OIII]/H$\beta$) diagnostic diagram, which has been widely used in the optical. The two diagrams provide consistent classifications of the excitation mechanism in a galaxy. On the other hand, the diagram of [NeIII]15.56$\mu$m/[NeII]12.81$\mu$m vs. [SIV]10.51$\mu$m/[SIII]18.71$\mu$m is not as efficient in separating AGNs from BCDs and pure starbursts. (abridged)

Traumatized Recurrent Melanocytic Naevus with Typical Starburst Pattern on Dermoscopy

This article does not have an abstract.

X-Ray Study of Discrete Sources in NGC 1808 with Chandra

Abstract We analyzed discrete sources in a typical starburst galaxy, NGC 1808, with Chandra in order to study the relation between X-ray and star-forming activities. We detected thirty-eight X-ray sources with an X-ray luminosity ($L_{\rm X}$) of $\gt$ 10$^{37}$ erg s$^{-1}$ in the central 5$^{\prime}\times$5$^{\prime}$ (13.4 kpc $\times$ 13.4 kpc) region. About 90% of the hard X-rays previously detected with ASCA came from these discrete sources. Among the detected sources, sixteen sources had an $L_{\rm X}$ of $\gt$ 10$^{38}$ erg s$^{-1}$. The number of luminous sources is nearly equal to that of M 82, although their concentration into the central star-forming region is lower than that of M 82. There are several soft X-ray sources with a temperature of 0.6 keV in the central star-forming region. The X-ray properties of the soft sources are similar to those of a H II region, or a supernova remnant. The existence of both luminous sources and soft X-ray sources indicates the star-forming activity of NGC 1808. However, we detected only a few candidates of high-mass X-ray binaries that would be produced by enhanced star-forming activity. This may be due to the starburst age of NGC 1808.

Dust in the Extremely Metal‐Poor Blue Compact Dwarf Galaxy I Zw 18: TheSpitzerMid‐infrared View

I Zw 18, a blue compact dwarf (BCD) galaxy with the second lowest metallicity measured in a star-forming object, has been observed with all three instruments on board the Spitzer Space Telescope. We present the deepest 5-36 μm mid-infrared (mid-IR) spectrum of this galaxy as yet obtained, as well as 3.6-70 μm imaging results. As with SBS 0335-052E, another BCD with similar metallicity, I Zw 18 shows no detectable emission from polycyclic aromatic hydrocarbons (PAHs). However, the continuum emission, from 15 to 70 μm, of I Zw 18 has a much steeper slope, more characteristic of a typical starburst galaxy of solar abundance. The neon abundance as measured from the infrared fine-structure lines is ~1/23 Z☉, and the sulfur abundance is ~1/35 Z☉, generally consistent with the nebular oxygen abundance of 1/30 Z☉ derived from optical lines. This suggests that the extinction to the infrared-emitting regions of this galaxy is low, also in agreement with the optical Balmer line ratios.

Mid‐Infrared Properties of Low‐Metallicity Blue Compact Dwarf Galaxies from theSpitzerInfrared Spectrograph

We present a Spitzer-based mid-infrared (MIR) study of a large sample of blue compact dwarfs (BCDs) using the InfraredSpectrograph(IRS),includingthefirstMIRspectrumof IZw18,thearchetypefortheBCDclassandamong the most metal-poor galaxies known. We show the spectra of polycyclic aromatic hydrocarbon (PAH) emission in a low-metallicity environment. We find that the equivalent widths (EWs) of PAHs at 6.2, 7.7, 8.6, and 11.2 � ma re generally weaker in BCDs than in typical starburst galaxies and that the fine-structure line ratio, [Ne iii]/[Ne ii], has a weak anticorrelation with the PAH EW. A much stronger anticorrelation is shown between the PAH EW and the product of the [Ne iii]/[Ne ii] ratio and the UV luminosity density divided by the metallicity. We conclude that the PAH EWin metal-poor high-excitation environments is determined by a combination of PAH formation and destruction effects. Subject headings: dust, extinction — galaxies: abundances — galaxies: dwarf — galaxies: starburst — infrared: galaxies

ISM gas removal from starburst galaxies and the premature death of star clusters

Recent observational studies of the age distribution of star clusters in nearby merging galaxies and starburst (SB) galaxies indicate a premature death of the young clusters. The fate of an evolving star cluster crucially depends of its gas content. This behaves like a glue that helps to keep the star system gravitationally bound. In SB systems where the rate of supernovae (SNe) explosions is elevated one should expect an efficient heating of the gas and its complete removal which could then favor the rapid dissociation of the evolving star clusters. Based on a contemporaneous study of the dynamical evolution of the interstellar gas in SB environments (Melioli & de Gouveia Dal Pino 2004, A&A, 424, 817) where it has been considered also the presence of dense clouds that may inhibit the heating efficiency of the interstellar gas by the SNe, we have here computed the timescales for gas removal from young clusters embedded in these systems and found that they are consistent with the very short timescales for cluster dissolution which are inferred from the observational studies above. Our results indicate that typical SB proto-clusters should start to disperse after less than 5 Myr. For a given total gas mass content, this result is nearly insensitive to the initial star formation efficiency.

Mid-infrared spectral evidence for a luminous dust enshrouded source in Arp 220

We have re-analyzed the 6–12 μm ISO spectrum of the ultra-luminous infrared galaxy Arp 220 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 NGC 4418 and M 83 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 μm flux and likely dominates the luminosity. Of particular significance relative to previous studies of Arp 220 is the fact that the emission feature at 7.7 μm 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. ([CITE]) in their higher spatial resolution 10 μm 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 Arp 220 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 Arp 220 to be the most highly obscured AGN known.

First Mid‐Infrared Spectrum of a Faint High‐ z Galaxy: Observations of CFRS 14.1157 with the Infrared Spectrograph on the Spitzer Space Telescope

The unprecedented sensitivity of the Infrared Spectrograph on the Spitzer Space Telescope allows for the first time the measurement of mid-infrared spectra from 14 to 38 μm of faint high-z galaxies. This unique capability is demonstrated with observations of sources having 16 μm fluxes of 3.6 mJy (CFRS 14.1157) and 0.35 mJy (CFRS 14.9025). A spectral-fitting technique is illustrated that determines the redshift by fitting emission and absorption features characteristic of nearby galaxies to the spectrum of an unknown source. For CFRS 14.1157, the measured redshift is z = 1.00 ± 0.20, in agreement with the published result of z = 1.15. The spectrum is dominated by emission from an active galactic nucleus, similar to the nucleus of NGC 1068, rather than a typical starburst with strong polycyclic aromatic hydrocarbon emission like M82. Such spectra will be crucial in characterizing the nature of newly discovered distant galaxies, which are too faint for optical follow-up.

3.1 Micron H2O Ice Absorption in LINER‐Type Ultraluminous Infrared Galaxies with Cool Far‐Infrared Colors: The Centrally Concentrated Nature of Their Deeply Buried Energy Sources

Ground-based 2.8-4.1 ?m slit spectra of the nuclei of seven ultraluminous infrared galaxies (ULIRGs) that are classified optically as LINERs and have cool far-infrared colors are presented. All the nuclei show 3.3 ?m polycyclic aromatic hydrocarbon (PAH) emission, with equivalent widths that are systematically lower than those in starburst galaxies. Strong 3.1 ?m H2O ice absorption, with optical depth greater than 0.6, is also detected in five nuclei, and 3.4 ?m carbonaceous dust absorption is detected clearly in one of the five nuclei. It is quantitatively demonstrated that the large optical depths of the H2O ice absorption in the five sources, and the 3.4 ?m absorption in one source, are incompatible with a geometry in which the energy sources are spatially mixed with dust and molecular gas, as is expected for a typical starburst, but instead require that a large amount of nuclear dust (including ice-covered grains) and molecular gas be distributed in a screen in front of the 3-4 ?m continuum-emitting sources. This geometrical requirement can naturally be met if the energy sources are more centrally concentrated than the nuclear dust and molecular gas. The low equivalent widths of the PAH emission compared to starbursts and the central concentration of the nuclear energy sources in these five ULIRGs are best explained by the presence of energetically important active galactic nuclei deeply buried in dust and molecular gas.