Molecular Hydrogen Excitation in Ultraluminous Infrared Galaxies (original) (raw)
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The Astrophysical Journal, 2004
The central regions of many interacting and early-type spiral galaxies are actively forming stars. This process affects the physical and chemical properties of the local interstellar medium as well as the evolution of the galaxies. We observed near-infrared H 2 emission lines: v = 1-0 S(1), 3-2 S(3), 1-0 S(0), and 2-1 S(1) from the central ∼ 1 kpc regions of the archetypical starburst galaxies, M82 and NGC 253, and the less dramatic but still vigorously star-forming galaxies, NGC 6946 and IC 342. Like the far-infrared continuum luminosity, the near-infrared H 2 emission luminosity can directly trace the amount of star formation activity because the H 2 emission lines arise from the interaction between hot and young stars and nearby neutral clouds. The observed H 2 line ratios show that both thermal and non-thermal excittion are responsible for the emission lines, but that the great majority of the near-infrared H 2 line emission in these galaxies arises from energy states excited by ultraviolet fluorescence. The derived physical conditions, e.g., far-ultraviolet radiation field and gas density, from [C II] and [O I] lines and farinfrared continuum observations when used as inputs to photodissociation models, also explain the luminosity of the observed H 2 1-0 S(1) line. The ratio of the H 2 1-0 S(1) line to far-IR continuum luminosity is remarkably constant over a broad range of galaxy luminosities; L H2 /L F IR ≃ 10 −5 , in normal late-type galaxies (including the Galactic center), in nearby starburst galaxies, and in luminous IR galaxies (LIRGs: L F IR > 10 11 L ⊙). Examining this constant ratio in the context of photodissociation region models, we conclude that it implies that the strength of the incident UV field on typical molecular clouds follows the gas density at the cloud surface.
Molecular Hydrogen Excitation around Active Galactic Nuclei
The Astrophysical Journal, 2005
We report R ∼ 3000 Very Large Telescope ISAAC K-band spectroscopy of the nuclei (i.e. central 100-300 pc) of nine galaxies hosting an active galactic nucleus. For five of these we also present spectra of the circumnuclear region out to 1 kpc. We have measured a number of molecular hydrogen lines in the ν = 1 − 0, 2 − 1, and 3 − 2 vibrational transitions, as well as the Brγ and He i recombination lines, and the Na i stellar absorption feature. Although only three of the galaxies are classified as type 1 Seyferts in the literature, broad Brγ (FWHM 1000 km s −1) is seen in seven of the objects. The ν = 1 − 0 emission appears thermalised at temperatures T ∼ 1000 K. However, the ν = 2 − 1 and ν = 3 − 2 emission show evidence of being radiatively excited by far-ultraviolet photons. The photo-dissociation region models that fit the data best are, as for the ultraluminous infrared galaxies in Davies et al. (2003), those for which the H 2 emission arises in dense clouds illuminated by intense FUV radiation. The Na i stellar absorption line is clearly seen in six of the nuclear spectra of these AGN, indicating the presence of a significant population of late type stars. It is possible that these stars are a result of the same episode of star formation that gave rise to the stars heating the PDRs. It seems unlikely that the AGN is the dominant source of excitation for the near infrared H 2 emission: in two of the nuclear spectra H 2 was not detected at all, and in general we find no evidence of suppression of the 2-1 S(3) line, which may occur in X-ray irradiated gas. Our data do not reveal any significant difference between the nuclear and circumnuclear line ratios, suggesting that the physical conditions of the dominant excitation mechanism are similar both near the AGN and in the larger scale environment around it, and that star formation is an important process even in the central 100 pc acround AGN.
Infrared Molecular Starburst Fingerprints in Deeply Obscured (Ultra)Luminous Infrared Galaxy Nuclei
The Astrophysical Journal, 2007
High-resolution spectra of the Spitzer Space Telescope show vibration-rotation absorption bands of gaseous C 2 H 2 , HCN, and CO 2 molecules toward a sample of deeply obscured (U )LIRG nuclei. The observed bands reveal the presence of dense (n k 10 7 cm À3 ), warm (T ex ¼ 200Y700 K) molecular gas with high column densities of these molecules ranging from a few 10 15 to 10 17 cm À2 . Abundances relative to H 2 , inferred from the silicate optical depth, range from $10 À7 to 10 À6 and show no correlation with temperature. Theoretical studies show that the high abundances of both C 2 H 2 and HCN exclude an X-ray dominated region (XDR) associated with the toroid surrounding an AGN as the origin of this dense warm molecular gas. Galactic massive protostars in the so-called hot-core phase have similar physical characteristics with comparable high abundances of C 2 H 2 , HCN, and CO 2 in the hot phase. However, the abundances of C 2 H 2 and HCN and the C 2 H 2 /CO 2 and HCN/CO 2 ratios are much higher toward the (U)LIRGs in the cooler (T ex P 400 K) phase. We suggest that the warm dense molecular gas revealed by the mid-IR absorption lines is associated with a phase of deeply embedded star formation, where the extreme pressures and densities of the nuclear starburst environment have inhibited the expansion of H ii regions and the global disruption of the starforming molecular cloud cores and have ''trapped'' the star formation process in an ''extended'' hot-core phase. 11 The IRS SH slit width is 4.7 00 , equal to the size of the point-spread function at 19.5 m.
The Astrophysical Journal, 2010
We present a high spatial (diffraction-limited) resolution (∼ 0.3 ′′ ) mid-infrared (MIR) spectroscopic study of the nuclei and star-forming regions of 4 local luminous infrared galaxies (LIRGs) using T-ReCS on the Gemini South telescope. We investigate the spatial variations of the features seen in the N -band spectra of LIRGs on scales of ∼ 100 pc, which allow us to resolve their nuclear regions and separate the AGN emission from that of the star formation (SF). We compare (qualitatively and quantitatively) our Gemini T-ReCS nuclear and integrated spectra of LIRGs with those obtained with Spitzer IRS. Star-forming regions and AGNs show distinct features in the MIR spectra, and we spatially separate these, which is not possible using the Spitzer data. The 9.7 µm silicate absorption feature is weaker in the nuclei of the LIRGs than in the surrounding regions. This is probably due to the either clumpy or compact environment of the central AGN or young, nuclear starburst. We find that the [Ne ii]12.81 µm luminosity surface density is tightly and directly correlated with that of Paα for the LIRG star-forming regions (slope of 1.00±0.02). Although the 11.3 µm PAH feature shows also a trend with Paα, this is not common for all the regions and the slope is significantly lower. We also find that the [Ne ii]12.81 µm/Paα ratio does not depend on the Paα equivalent width (EW), i.e., on the age of the ionizing stellar populations, suggesting that, on the scales probed here, the [Ne ii]12.81 µm emission line is a good tracer of the SF activity in LIRGs. On the other hand, the 11.3 µm PAH/Paα ratio increases for smaller values of the Paα EW (increasing ages), indicating that the 11.3 µm PAH feature can also be excited by older stars than those responsible for the Paα emission. Finally, more data are needed in order to address the different physical processes (age of the stellar populations, hardness and intensity of the radiation field, mass of the star-forming regions) affecting the energetics of the PAH features in a statistical way. Additional high spatial resolution observations are essential to investigate the star formation in local LIRGs at the smallest scales and to probe ultimately whether they share the same physical properties as high-z LIRGs, ULIRGs and submillimiter galaxies and therefore belong to the same galaxy population.
NIR spectroscopy of luminous infrared galaxies and the hydrogen recombination photon deficit
Astronomy and Astrophysics, 2005
We report on near-infrared medium-resolution spectroscopy of a sample of luminous and ultraluminous infrared galaxies (LIRGs-ULIRGs), carried out with SOFI at the ESO 3.5 m New Technology Telescope. Because of wavelength dependence of the attenuation, the detection of the Paα or Brγ line in the Ks band should provide relevant constraints on SFR and the contribution of an AGN. We find, however, that the intensities of the Paα and Brγ lines, even corrected for slit losses, are on average only 10% and 40%, respectively, of that expected from a normal starburst of similar bolometric luminosity. The corresponding star formation rates, after correcting for the attenuation derived from the NIR-optical emission line ratios, are 14% and 60% of that expected if the far infrared luminosity were entirely powered by the starburst. This confirms the existence of a recombination photon deficit, particularly in the case of the Paα line, already found in the Brγ line in other infrared galaxies of similar luminosity. In discussing the possible causes of the discrepancy, we find unlikely that it is due to the presence of an AGN, though two objects show evidence of broadening of the Paα line and of the presence of coronal line emission. In fact, from our own observations and data collected from the literature we argue that the studied galaxies appear to be predominantly powered by a nuclear starburst. Two scenarios compatible with the present data are that either there exists a highly attenuated nuclear star forming region, and/or that a significant fraction (80%) of the ionizing photons are absorbed by dust within the HII regions. We suggest that observations in the Brα spectral region could constitute a powerful tool to disentangle these two possibilities.
Dense gas in luminous infrared galaxies
Astronomy and Astrophysics, 2008
Aims. Molecules that trace the high-density regions of the interstellar medium have been observed in (ultra-)luminous (far-)infrared galaxies, in order to initiate multiple-molecule multiple-transition studies to evaluate the physical and chemical environment of the nuclear medium and its response to the ongoing nuclear activity. Methods. The HCN(1−0), HNC(1−0), HCO + (1−0), CN(1−0) and CN(2−1), CO(2−1), and CS(3−2) transitions were observed in sources covering three decades of infrared luminosity including sources with known OH megamaser activity. The data for the molecules that trace the high-density regions have been augmented with data available in the literature. Results. The integrated emissions of high-density tracer molecules show a strong relation to the far-infrared luminosity. Ratios of integrated line luminosities have been used for a first order diagnosis of the integrated molecular environment of the evolving nuclear starbursts. Diagnostic diagrams display significant differentiation among the sources that relate to initial conditions and the radiative excitation environment. Initial differentiation has been introduced between the FUV radiation field in photon-dominated-regions and the X-ray field in X-ray-dominated-regions. The galaxies displaying OH megamaser activity have line ratios typical of photon-dominated regions.
The Astrophysical Journal, 2009
We analyze a sample of galaxies chosen to have F 24µm > 0.5mJy and satisfy a certain IRAC color criterion. IRS spectra yield redshifts, spectral types, and PAH luminosities, to which we add broadband photometry from optical through IRAC wavelengths, MIPS from 24-160 µm, 1.1 millimeter, and radio at 1.4 GHz. Stellar population modeling and IRS spectra together demonstrate that the double criteria used to select this sample have efficiently isolated massive star-forming galaxies at z ∼ 1.9. This is the first starburst-dominated ULIRG sample at high redshift with total infrared luminosity measured directly from FIR and millimeter photometry, and as such gives us the first accurate view of broadband SEDs for starburst galaxies at extremely high luminosity and at all wavelengths. Similar broadband data are assembled for three other galaxy samples -local starburst galaxies, local AGN/ULIRGS, and a second 24µm-luminous z ∼ 2 sample dominated by AGN. L P AH /L IR for the new z ∼ 2 starburst sample is the highest ever seen, some three times higher than in local starbursts, whereas in AGNs this ratio is depressed below the starburst trend, often severely. Several pieces of evidence imply that AGNs exist in this starburst dominated sample, except two of which even host very strong AGN, while they still have very strong PAH emission. The ACS images show most objects have very extended morphologies in the rest-frame UV band, thus extended distribution of PAH molecules. Such an extended distribution prevents further destruction PAH molecules by central AGNs. We conclude that objects in this sample are ULIRGs powered mainly by starburst; and the total infrared luminosity density contributed by this type of objects is 0.9 − 2.6 × 10 7 L ⊙ /Mpc 3 .
Near‐Infrared and Star‐forming Properties of Local Luminous Infrared Galaxies
The Astrophysical Journal, 2006
We use Hubble Space Telescope (HST) NICMOS continuum and Paα observations to study the near-infrared and star-formation properties of a representative sample of 30 local (d ∼ 35 − 75 Mpc) luminous infrared galaxies (LIRGs, infrared [8 − 1000 µm] luminosities of log L IR = 11 − 11.9 [L ⊙ ]). The data provide spatial resolutions of 25 − 50 pc and cover the central ∼ 3.3 − 7.1 kpc regions of these galaxies. About half of the LIRGs show compact (∼ 1 − 2 kpc) Paα emission with a high surface brightness in the form of nuclear emission, rings, and mini-spirals. The rest of the sample show Paα emission along the disk and the spiral arms extending over scales of 3 − 7 kpc and larger. About half of the sample contains HII regions with Hα luminosities significantly higher than those observed in normal galaxies. There is a linear empirical relationship between the mid-IR 24 µm and hydrogen recombination (extinction-corrected Paα) luminosity for these LIRGs, and the HII regions in the central part of M51. This relation holds over more than four decades in luminosity suggesting that the mid-IR emission is a good tracer of the star formation rate (SFR). Analogous to the widely used relation between the SFR and total IR luminosity of Kennicutt (1998), we derive an empirical calibration of the SFR in terms of the monochromatic 24 µm luminosity that can be used for luminous, dusty galaxies.
Arxiv preprint arXiv: …, 2007
Aims. By using the spectral energy distribution (SED) from the near-infrared to the radio of a statistically significant number of luminous infrared galaxies we determine important physical parameters for this population of objects. In particular we constrain the optical depth towards the luminosity source, the star formation rate, the star formation efficiency and the AGN fraction. Methods. We fit the near-infrared to radio spectral energy distributions of a sample of 30 luminous and ultra-luminous infrared galaxies with pure starburst models or models that include both starburst and AGN components. Results. We find that although about half of our sample have best-fit models that include an AGN component, only 30% (9/30) have an AGN which accounts for more than 10% of the infrared luminosity from 8 to 1000 µm, whereas all have an energetically dominant starburst. Our derived AGN fractions are generally in good agreement other measurements based in the mid-infrared line ratios, Ne[V]/Ne [II] and O[IV]/Ne[II] measured by Spitzer IRS, but much lower than those derived from PAH equivalent widths or the mid-infrared spectral slope. Our models determine the mass of dense molecular gas within which active star formation takes place via the extinction required to reproduce the infrared part of the SED. Assuming that this mass is that traced by the HCN molecule, we reproduce the observed linear relation between HCN flux and infrared luminosity found by Gao & Solomon, 2004a. We also find that the star formation efficiency, as defined by the current star formation rate per unit molecular gas mass, falls as the starburst ages.
Anomalous Hydrogen Recombination-Line Ratios in Ultraluminous Infrared Galaxies
arXiv: Astrophysics of Galaxies, 2021
We conducted systematic observations of the H I Brα (4.05 µm) and Brβ (2.63 µm) lines in 52 nearby (z < 0.3) ultraluminous infrared galaxies (ULIRGs) with AKARI. Among 33 ULIRGs wherein the lines are detected, three galaxies show anomalous Brβ/Brα line ratios (∼ 1.0), which are significantly higher than those for case B (0.565). Our observations also show that ULIRGs have a tendency to exhibit higher Brβ/Brα line ratios than those observed in Galactic H II regions. The high Brβ/Brα line ratios cannot be explained by a combination of dust extinction and case B since dust extinction reduces the ratio. We explore possible causes for the high Brβ/Brα line ratios and show that the observed ratios can be explained by a combination of an optically thick Brα line and an optically thin Brβ line. We simulated the H II regions in ULIRGs with the Cloudy code, and our results show that the high Brβ/Brα line ratios can be explained by high-density conditions, wherein the Brα line becomes optically thick. To achieve a column density large enough to make the Brα line optically thick within a single H II region, the gas density must be as high as n ∼ 10 8 cm −3. We therefore propose an ensemble of H II regions, in each of which the Brα line is optically thick, to explain the high Brβ/Brα line ratio.