High‐Resolution Mid‐Infrared Spectroscopy of Ultraluminous Infrared Galaxies (original) (raw)
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The Astrophysical Journal, 2007
We present spectra taken with the Infrared Spectrograph on Spitzer covering the 5-38 μm region of the 10 ultraluminous infrared galaxies (ULIRGs) found in the IRAS Bright Galaxy Sample (BGS). There is a factor of 50 spread in the rest-frame 5.5-60 μm spectral slopes, and the 9.7 μm silicate optical depths range from at least τ9.7<=0.4 (AV~8) to τ9.7>=4.2 (AV>=78). There is evidence for water ice and hydrocarbon absorption and C2H2 and HCN absorption features in 4 and possibly 6 of the 10 BGS ULIRGs, indicating shielded molecular clouds and a warm, dense ISM. We have detected [Ne V] emission in 3 of the 10 BGS ULIRGs, at flux levels of 5-18×10-14 ergs cm-2 s-1 and [Ne V] 14.3/[Ne II] 12.8 line flux ratios of 0.12-0.85. The remaining BGS ULIRGs have limits on their [Ne V]/[Ne II]line flux ratios, which range from <=0.15 to <=0.01. Among the BGS ULIRGs, the AGN fractions implied by either the [Ne V]/[Ne II] or [O IV]/[Ne II] line flux ratios (or their upper limits) are significantly lower than implied by the MIR slope or strength of the 6.2 μm PAH EQW feature. There is evidence for hot (T>300 K) dust in five of the BGS ULIRGs, with the fraction of hot dust to total dust luminosity ranging from ~1% to 23%, before correcting for extinction. When integrated over the IRAC-8, IRS blue peak-up, and MIPS-24 filter bandpasses, the IRS spectra imply very blue colors for some ULIRGs at z~1.3. The large range in diagnostic parameters among the nearest ULIRGs suggests that matching survey results to a small number of templates may lead to biased results about the fraction of luminous dusty starbursts and AGNs at high z. Based on observations obtained with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407.
American Astronomical Society Meeting Abstracts #204, 2004
We present spectra taken with the Infrared Spectrograph on Spitzer covering the 5Y38 m region of the 10 ultraluminous infrared galaxies (ULIRGs) found in the IRAS Bright Galaxy Sample (BGS). There is a factor of 50 spread in the rest-frame 5.5Y60 m spectral slopes, and the 9.7 m silicate optical depths range from at least 9:7 0:4 (A V $ 8) to 9:7 ! 4:2 (A V ! 78). There is evidence for water ice and hydrocarbon absorption and C 2 H 2 and HCN absorption features in 4 and possibly 6 of the 10 BGS ULIRGs, indicating shielded molecular clouds and a warm, dense ISM. We have detected [Ne v] emission in 3 of the 10 BGS ULIRGs, at flux levels of 5Y18 ; 10 À14 ergs cm À2 s À1 and ½Ne v 14:3/½Ne ii 12:8 line flux ratios of 0.12Y0.85. The remaining BGS ULIRGs have limits on their ½Ne v/½Ne iiline flux ratios, which range from 0.15 to 0.01. Among the BGS ULIRGs, the AGN fractions implied by either the ½Ne v/½Ne ii or ½O iv/½Ne ii line flux ratios (or their upper limits) are significantly lower than implied by the MIR slope or strength of the 6.2 m PAH EQW feature. There is evidence for hot (T > 300 K) dust in five of the BGS ULIRGs, with the fraction of hot dust to total dust luminosity ranging from 11% to 23%, before correcting for extinction. When integrated over the IRAC-8, IRS blue peak-up, and MIPS-24 filter bandpasses, the IRS spectra imply very blue colors for some ULIRGs at z 1 1:3. The large range in diagnostic parameters among the nearest ULIRGs suggests that matching survey results to a small number of templates may lead to biased results about the fraction of luminous dusty starbursts and AGNs at high z.
Unveiling the nature of Ultraluminous Infrared Galaxies with 3-4 μm spectroscopy
Monthly Notices of the Royal Astronomical Society, 2006
We present the results of L-band spectroscopical observations of local bright Ultraluminous Infrared Galaxies (ULIRGs), performed with ISAAC at the VLT. The excellent sensitivity of the telescope and of the instrument provided spectra of unprecedented quality for this class of objects, which allowed a detailed study of the AGN/starburst contribution to the energy output, and of the composition of the circumnuclear absorber. We discuss the L-band spectral features of seven single sources, and the statistical properties of a complete sample of 15 sources obtained combining our observations with other published 3-4 µm spectra. Our main results are: 1. When a spectral indicator suggesting the presence of an AGN (low equivalent width of the 3.3 µm emission line, steep λ − f λ spectrum, presence of an absorption feature at 3.4 µm) is found, the AGN is always confirmed by independent analysis at other wavelengths. Conversely, in all known AGNs at least one of the above indicators is present. 2. Two new diagnostic diagrams are proposed combining the above indicators, in which starbursts and AGNs are clearly and completely separated. 3. The above diagnostic techniques are possible with spectra of relatively low quality, which can be obtained for several tens of ULIRGs with currently available telescopes. This makes L-band spectroscopy the current best tool to disentangle AGNs and starbursts contributions in ULIRGs. 4. The L-band properties of ULIRGs are heterogeneous. However, we show that all the spectral differences among ULIRGs can be reproduced starting from pure intrinsic AGN an starburst spectra and varying two parameters: the amount of dust extinction of the AGN component, and the relative AGN/starburst contribution to the bolometric luminosity. 5. Using the above decomposition model, we show that AGNs in ULIRGs have a low dust-to-gas ratio and a dust extinction curve different from Galactic. 6. The estimate of the presence and contribution of AGN in a complete sample show that AGN are hosted by ∼ 2/3 of ULIRGs, but their energetic contribution is relevant (> 30% of the bolometric luminosity) only in ∼ 20% of the sample.
The Astrophysical Journal, 2006
We present infrared L-band (3-4 m) nuclear spectra of a large sample of nearby ultraluminous infrared galaxies (ULIRGs). ULIRGs classified optically as non-Seyfert galaxies (LINERs, H ii regions, and unclassified) are our main targets. Using the 3.3 m polycyclic aromatic hydrocarbon (PAH) emission and absorption features at 3.1 m due to ice-covered dust and at 3.4 m produced by bare carbonaceous dust, we search for signatures of powerful AGNs deeply buried along virtually all lines of sight. The 3.3 m PAH emission, the signatures of starbursts, is detected in all but two non-Seyfert ULIRGs, but the estimated starburst magnitudes can account for only a small fraction of the infrared luminosities. Three LINER ULIRGs show spectra typical of almost pure buried AGNs, namely, strong absorption features with very small equivalent width PAH emission. Besides these three sources, 14 LINER and three H ii ULIRGs' nuclei show strong absorption features whose absolute optical depths suggest an energy source more centrally concentrated than the surrounding dust, such as a buried AGN. In total, 17 out of 27 (63%) LINER and 3 out of 13 (23%) H ii ULIRGs' nuclei show some degree of evidence for powerful buried AGNs, suggesting that powerful buried AGNs may be more common in LINER ULIRGs than in H ii ULIRGs. The evidence of AGNs is found in non-Seyfert ULIRGs with both warm and cool far-infrared colors. These spectra are compared with those of 15 ULIRGs' nuclei with optical Seyfert signatures taken for comparison. The overall spectral properties suggest that the total amount of dust around buried AGNs in non-Seyfert ULIRGs is systematically larger than that around AGNs in Seyfert 2 ULIRGs. We argue that the optical (non)detectability of Seyfert signatures in ULIRGs is highly dependent on how deeply buried the AGNs are, and that it is essential to properly evaluate the energetic importance of buried AGNs in non-Seyfert ULIRGs.
SpitzerMid‐Infrared Spectroscopy of Infrared Luminous Galaxies atz∼ 2. I. The Spectra
The Astrophysical Journal, 2007
We present the mid-infrared (MIR) spectra obtained with the Spitzer InfraRed Spectrograph (IRS) for a sample of 52 sources, selected as infrared luminous, z ∼ > 1 candidates in the Extragalactic First Look Survey (XFLS). The sample selection criteria are f 24µm ∼ > 0.9mJy, νf ν (24µm)/νf ν (8µm) ∼ > 3.16 and νf ν (24µm)/νf ν (0.7µm) ∼ > 10. Of the 52 spectra, 47 (90%) produced measurable redshifts based solely on the mid-IR spectral features, with the majority (35/47 = 74%) at 1.5 ∼ < z ∼ < 3.2. Keck spectroscopy of a sub-sample (17/47) agrees with the mid-IR redshift measurements. The observed spectra fall crudely into three categories-(1) 33% (17/52) have strong PAH emission, and are probably powered by star formation with total IR luminosity roughly a factor of 5 higher than the local starburst ULIRGs. (2) 33% (17/52) have only deep silicate absorption at 9.8 µm, indicative of deeply embedded dusty systems. The energetic nature of the heating sources in these systems can not be determined by these data alone. (3) The remainder 34% are mid-IR continuum dominated systems with either weak PAH emission and/or silicate absorption. This third of the sample are probably AGNs. We derived monochromatic, rest-frame 5.8 µm, continuum luminosities (νL ν), ranging from 10 10.3 −10 12.6 L ⊙. Our spectra have MIR slope α 5−15µm ∼ > 2.1, much redder than the median value of 1.3 for the optically selected, Palomar-Green (PG) quasars. From the silicate absorption feature, we estimate that roughly two-thirds of the sample have optical depth τ 9.8µm > 1. Their L 1600Å and L IR suggest that our sample is among the most luminous and most dust enshrouded systems of its epoch. Our study has revealed a significant population of dust enshrouded galaxies at z ∼ 2, whose enormous energy output, comparable to that of quasars, is generated by AGN as well as starburst. This IR luminous population has very little overlap with sub-mm and UV-selected populations.
The Astrophysical Journal Supplement Series, 2013
The Great Observatories All-Sky LIRG Survey (GOALS) is a comprehensive, multiwavelength study of luminous infrared galaxies (LIRGs) in the local universe. Here we present low resolution Spitzer IRS spectra covering 5-38 µm and provide a basic analysis of the mid-IR spectral properties observed for nearby LIRGs. In a companion paper, we discuss detailed fits to the spectra and compare the LIRGs to other classes of galaxies. The GOALS sample of 244 nuclei in 180 luminous (10 11 ≤ L IR /L < 10 12 ) and 22 ultraluminous (L IR /L ≥ 10 12 ) IR galaxies represents a complete subset of the IRAS Revised Bright Galaxy Sample and covers a range of merger stages, morphologies and spectral types. The majority (>60%) of the GOALS LIRGs have high 6.2 µm PAH equivalent widths (EQW 6.2µm > 0.4 µm) and low levels of silicate absorption (s 9.7µm > -1.0). There is a general trend among the U/LIRGs for both silicate depth and mid-infrared (MIR) slope to increase with increasing L IR . U/LIRGs in the late to final stages of a merger also have, on average, steeper MIR slopes and higher levels of dust obscuration. Together, these trends suggest that as gas & dust is funneled towards the center of a coalescing merger, the nuclei become more compact and more obscured. As a result, the dust temperature increases leading also to a steeper MIR slope. The sources that depart from these correlations have very low PAH equivalent width (EQW 6.2µm < 0.1 µm) consistent with their emission being dominated by an AGN in the MIR. These extremely low PAH equivalent width sources separate into two distinct types: relatively unobscured sources with a very hot dust component (and thus very shallow MIR slopes) and heavily dust obscured nuclei with a steep temperature gradient. The most heavily dust obscured sources are also the most compact in their MIR emission, suggesting that the obscuring (cool) dust is associated with the outer regions of the starburst and not simply a measure of the dust along the line of sight through a large, dusty disk. A marked decline is seen for the fraction of high EQW (star formation dominated) sources as the merger progresses. The decline is accompanied by an increase in the fraction of composite sources while the fraction of sources where an AGN dominates the MIR emission remains low. When compared to the MIR spectra of submillimeter galaxies (SMGs) at z∼2, both the average GOALS LIRG and ULIRG spectra are more absorbed at 9.7 µm and the average GOALS LIRG has more PAH emission. However, when the AGN contributions to both the local GOALS LIRGs and the high-z SMGs are removed, the average local starbursting LIRG closely resembles the starburst-dominated SMGs.
IRS Spectra of Two Ultraluminous Infrared Galaxies at z = 1.3
The Astrophysical Journal, 2006
We present low-resolution (64 < R < 124) mid-infrared (8-38 µm) spectra of two z ≈ 1.3 ultraluminous infrared galaxies with L 8−1000µm ≈ 10 13 L ⊙ : MIPS J142824.0+352619 and SST24 J142827.19+354127.71. The spectra were taken with the Infrared Spectrograph (IRS ) on board the Spitzer Space Telescope. Both objects were discovered in a Spitzer /MIPS survey of the Boötes field of the NOAO Deep Wide-Field Survey (NDWFS).
The Astrophysical Journal, 2013
We present the data and our analysis of mid-infrared atomic fine-structure emission lines detected in Spitzer/IRS high-resolution spectra of 202 local Luminous Infrared Galaxies (LIRGS) observed as part of the Great Observatories All-sky LIRG Survey (GOALS). We readily detect emission lines of [SIV], [NeII], [NeV], [NeIII], [SIII] 18.7 µm , [OIV], [FeII], [SIII] 33.5 µm , and [SiII]. More than 75% of these galaxies are classified as starburst dominated sources in the mid-infrared, based on the [NeV]/[NeII] line flux ratios and equivalent width of the 6.2µm polycyclic aromatic hydrocarbon feature. We compare ratios of the emission line fluxes to those predicted from stellar photo-ionization and shockionization models to constrain the physical and chemical properties of the gas in the starburst LIRG nuclei. Comparing the [SIV]/[NeII] and [NeIII]/[NeII] line ratios to the Starburst99-Mappings III models with an instantaneous burst history, the emission line ratios suggest that the nuclear starbursts in our LIRGs have ages of 1 − 4.5 Myr, metallicities of 1 − 2 Z ⊙ , and ionization parameters of 2 − 8 × 10 7 cm s −1 . Based on the [SIII] 33.5 µm /[SIII] 18.7 µm ratios, the electron density in LIRG nuclei is typically one to a few hundred cm −3 , with a median electron density of ∼ 300 cm −3 , for those sources above the low density limit for these lines. We also find that strong shocks are likely present in 10 starburst dominated sources of our sample. A significant fraction of the GOALS sources (80) have resolved neon emission line profiles (FWHM ≥ 600 km s −1 ) and five show clear differences in the velocities of the [NeIII] or [NeV] emission lines, relative to [NeII], of more than 200 km s −1 . Furthermore, six starburst and five AGN dominated LIRGs show a clear trend of increasing line width with ionization potential, suggesting the possibility of a compact energy source and stratified ISM in their nuclei. We confirm a strong correlation between the sum of the [NeII] 12.8 µm and [NeIII] 15.5 µm emission, as well as [SIII] 33.5 µm , with both the infrared luminosity and the 24 µm warm dust emission measured from the spectra, consistent with all three lines tracing ongoing star formation. Finally, we find no correlation between the hardness of the radiation field or the emission line width and the ratio of the total infrared to 8 µm emission (IR8), a measure of the strength of the starburst and the distance of the LIRGs from the star-forming main-sequence. This may be a function of the fact that the infrared luminosity and the mid-infrared fine-structure lines are sensitive to different timescales over the starburst, or that IR8 is more sensitive to the geometry of the region emitting the warm dust than the radiation field producing the HII region emission.
Ultraluminous Infrared Galaxies
Springer Praxis Books
Ever since their discovery in the 1970's, UltraLuminous InfraRed Galaxies (ULIRGs; classically Lir > 10 12 L⊙) have fascinated astronomers with their immense luminosities, and frustrated them due to their singularly opaque nature, almost in equal measure. Over the last decade, however, comprehensive observations from the X-ray through to the radio have produced a consensus picture of local ULIRGs, showing that they are mergers between gas rich galaxies, where the interaction triggers some combination of dust-enshrouded starburst and AGN activity, with the starburst usually dominating. Very recent results have thrown ULIRGs even further to the fore. Originally they were thought of as little more than a local oddity, but the latest IR surveys have shown that ULIRGs are vastly more numerous at high redshift, and tantalizing suggestions of physical differences between high and low redshift ULIRGs hint at differences in their formation modes and local environment. In this review we look at recent progress on understanding the physics and evolution of local ULIRGs, the contribution of high redshift ULIRGs to the cosmic infrared background and the global history of star formation, and the role of ULIRGs as diagnostics of the formation of massive galaxies and large-scale structures.