Shobita Satyapal

The [C II] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies Revisited

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.

Infrared Space Observatory Measurements of a [C II] 158 micron Line Deficit in Ultraluminous Infrared Galaxies

We report measurements of the [C II] 157.74 μm fine-structure line in a sample of seven ultraluminous infrared galaxies (ULIGs) (LIR > 1012 L☉) with the Long Wavelength Spectrometer on the Infrared Space Observatory. The [C II] line is an important coolant in galaxies and arises in interstellar gas exposed to far-ultraviolet photons (hν≥11.26 eV); in ULIGs, this radiation stems from the bursts of star formation and/or from the active galactic nuclei that power the tremendous infrared luminosity. The [C II] 158 μm line is detected in four of the seven ULIGs; the absolute line flux (about a few times 10-20 W cm-2) represents some of the faintest extragalactic[C II] emission yet observed. Relative to the far-infrared continuum, the [C II] flux from the observed ULIGs is ~10% of that seen from nearby normal and starburst galaxies. We discuss possible causes for the [C II] deficit, namely (1) self-absorbed or optically thick [C II] emission, (2) saturation of the [C II] emission in photodissociated gas with high gas density n (3 × 103 cm-3) or with a high ratio of incident UV flux G0 to n (G0/n 10 cm3), or (3) the presence of a soft ultraviolet radiation field caused, for example, by a stellar population deficient in massive main-sequence stars. As nearby examples of colliding galaxies, ULIGs may resemble high-redshift protogalaxies in both morphology and spectral behavior. If true, the suggested [C II] deficit in ULIGs poses limitations on the detection rate of high-z sources and on the usefulness of [C II] as an eventual tracer of protogalaxies.

The Intrinsic Properties of the Stellar Clusters in the M82 Starburst Complex: Propagating Star Formation?

Near-Infrared spectroscopy combined with high spatial resolution imaging have been used in this work to probe the central 500 pc of M82. Imaging observations in the 2.36 μm CO band head are added to our previously published near-infrared hydrogen recombination line imaging, near-infrared broadband imaging, and 3.29 μm dust feature imaging observations, in order to study the nature of the starburst stellar population. A starburst model is constructed and compared with the observations of the stellar clusters in the starburst complex. Our analysis implies that the typical age for the starburst clusters is 107 yr. In addition, our high spatial resolution observations indicate that there is an age dispersion within the starburst complex that is correlated with projected distance from the center of the galaxy. The inferred age dispersion is 6 × 106 yr. If the starburst in M82 is propagating outward from the center, this age dispersion corresponds to a velocity of propagation, originating in the center, of ~50 km s-1. Our quantitative analysis also reveals that a Salpeter initial mass function, extending from 0.1 to 100 M☉, can fit the observed properties of M82 without using up more than 30% of the total dynamical mass in the starburst.

ISO LWS spectroscopy of M82: A unified evolutionary model

We present the first complete far-infrared spectrum (43-197 μm) of M82, the brightest infrared galaxy in the sky, taken with the Long Wavelength Spectrometer of the Infrared Space Observatory (ISO). We detected seven fine structure emission lines, [O I] 63 and 145 μm, [O III] 52 and 88 μm , [N II] 122 μm, [N III] 57 μm, and [C II] 158 μm, and fitted their ratios to a combination starburst and photodissociation region (PDR) model. The best fit is obtained with H II regions with n = 250 cm-3, an ionization parameter of 10-3.5, and PDRs with n = 103.3 cm-3 and a far-ultraviolet flux of G0 = 102.8. We applied both continuous and instantaneous starburst models, with our best fit being a 3-5 Myr old instantaneous burst model with a 100 M⊙ cutoff. We also detected the ground-state rotational line of OH in absorption at 119.4 μm. No excited level OH transitions are apparent, indicating that the OH is almost entirely in its ground state with a column density ∼4 × 1014 cm-2. The spectral energy distribution over the long-wavelength spectrometer wavelength range is well fitted with a 48 K dust temperature and an optical depth, τDust ∝ λ-1.

The Polycyclic Aromatic Hydrocarbon Emission Deficit in Low-Metallicity Galaxies—A Spitzer View

Archival observations of 18 starburst galaxies that span a wide range in metallicity reveal for the first time a correlation between the ratio of emission-line fluxes of [Fe II] at 26 ?m and [Ne II] at 12.8 ?m and the 7.7 ?m PAH strength, with the [Fe II]/[Ne II] flux ratio decreasing with increasing PAH strength. We also find a strong correlation between the [Fe II]/[Ne II] flux ratio and the host galaxy metallicity, with the flux ratio decreasing with increasing metallicity. Since [Fe II] emission has been linked primarily to supernova shocks, we attribute the high [Fe II]/[Ne II] ratios in low-metallicity galaxies to enhanced supernova activity. We consider this to be a dominant mechanism for PAH destruction, rather than grain destruction in photoionized regions surrounding young massive stars. We also consider whether the extreme youth of the low-metallicity galaxies is responsible for the lack of PAH emission.

Spitzer Uncovers Active Galactic Nuclei Missed by Optical Surveys in Seven Late-Type Galaxies

We conducted a high resolution mid-infrared spectroscopic investigation using Spitzer of 32 latetype (Sbc or later) galaxies that show no definitive signatures of Active Galactic Nuclei (AGN) in their optical spectra in order to search for low luminosity and/or embedded AGN. These observations reveal the presence of the high ionization [NeV] 14µm and/or 24µm line in 7 sources, providing strong evidence for AGNs in these galaxies. Taking into account the variable sensitivity of our observations, we find that the AGN detection rate based on mid-infrared diagnostics in optically normal late-type galaxies is � 30%, implying an AGN detection rate in late-type galaxies that is possibly 4 times larger than what optical spectroscopic observations alone suggest. We demonstrate using photoionization models with both an input AGN and an extreme EUV-bright starburst ionizing radiation field that the observed mid-infrared line ratios in our 7 AGN candidates cannot be replicated unless an AGN contribution, in some cases as little as 10% of the total galaxy luminosity, is included. These models show that when the fraction of the total luminosity due to the AGN is low, optical diagnostics are insensitive to the presence of the AGN. In this regime of parameter space, the mid-infrared diagnostics offer a powerful tool for uncovering AGN missed by optical spectroscopy. The AGN bolometric luminosities in our sample inferred using our [NeV] line luminosities range from � 3×10 41 ergs s −1 to � 2×10 43 ergs s −1 . Assuming that the AGN is radiating at the Eddington limit, this range corresponds to a lower mass limit for the black hole that ranges from � 3×10 3 M⊙ to as high as � 1.5×10 5 M⊙. These lower mass limits however do not put a strain on the well-known relationship between the black hole mass and the host galaxy’s stellar velocity dispersion established in predominantly early-type galaxies. Our findings add to the growing evidence that black holes do form and grow in low-bulge environments and that they are significantly more common than optical studies indicate.

A Chandra Snapshot Survey of Infrared-bright LINERs: A Possible Link Between Star Formation, Active Galactic Nucleus Fueling, and Mass Accretion

We present results from a high-resolution X-ray imaging study of nearby LINERs observed by ACIS on board Chandra. This study complements and extends previous X-ray studies of LINERs, focusing on the underexplored population of nearby dust-enshrouded infrared-bright LINERs. The sample consists of 15 IR-bright LINERs (LFIR/LB > 3), with distances that range from 11 to 26 Mpc. Combining our sample with previous Chandra studies, we find that ~51% (28/55) of the LINERs display compact hard X-ray cores. The nuclear 2-10 keV luminosities of the galaxies in this expanded sample range from ~2 × 1038 to ~2 × 1044 ergs s-1. We find that the most extreme IR-faint LINERs are exclusively active galactic nuclei (AGNs). The fraction of LINERs containing AGNs appears to decrease with IR brightness and increase again at the highest values of LFIR/LB. We find that of the 24 LINERs showing compact nuclear hard X-ray cores in the expanded sample that were observed at Hα wavelengths, only eight actually show evidence of a broad line. Similarly, of the 14 LINERs showing compact nuclear hard X-ray cores with corresponding radio observations, only eight display a compact flat spectrum radio core. These findings emphasize the need for high-resolution X-ray imaging observations in the study of IR-bright LINERs. Finally, we find an intriguing trend in the Eddington ratio versus LFIR and LFIR/LB for the AGN-LINERs in the expanded sample that extends over 7 orders of magnitude in L/LEdd. This correlation may imply a link between black hole growth, as measured by the Eddington ratio, and the star formation rate, as measured by the far-IR luminosity and IR-brightness ratio. If the far-IR luminosity is an indicator of the molecular gas content in our sample of LINERs, our results may further indicate that the mass accretion rate scales with the host galaxys fuel supply. We discuss the potential implications of our results in the framework of black hole growth and AGN fueling in low-luminosity AGNs.

Mid-Infrared Fine-Structure Line Ratios in Active Galactic Nuclei Observed with the Spitzer IRS: Evidence for Extinction by the Torus

We present the first systematic investigation of the [Ne V] (14 μm/24 μm) and [S III] (18 μm/33 μm) infrared line flux ratios, traditionally used to estimate the density of the ionized gas, in a sample of 41 type 1 and type 2 active galactic nuclei (AGNs) observed with the Infrared Spectrograph on board Spitzer. The majority of galaxies with both [Ne V] lines detected have observed [Ne V] line flux ratios consistent with or below the theoretical low-density limit, based on calculations using currently available collision strengths and ignoring absorption and stimulated emission. We find that type 2 AGNs have lower line flux ratios than type 1 AGNs and that all of the galaxies with line flux ratios below the low-density limit are type 2 AGNs. We argue that differential infrared extinction to the [Ne V] emitting region due to dust in the obscuring torus is responsible for the ratios below the low-density limit and we suggest that the ratio may be a tracer of the inclination angle of the torus to our line of sight. Because the temperature of the gas, the amount of extinction, and the effect of absorption and stimulated emission on the line ratios are all unknown, we are not able to determine the electron densities associated with the [Ne V] line flux ratios for the objects in our sample. We also find that the [S III] emission from the galaxies in our sample is extended and originates primarily in star-forming regions. Since the emission from low-ionization species is extended, any analysis using line flux ratios from such species obtained from slits of different sizes is invalid for most nearby galaxies.

The Discovery of an Active Galactic Nucleus in the Late-Type Galaxy NGC 3621: Spitzer Spectroscopic Observations

We report the discovery of an active galactic nucleus (AGN) in the nearby SAd galaxy NGC 3621 using Spitzer high spectral resolution observations. These observations reveal the presence of [Ne V] 14 and 24 ?m emission that is centrally concentrated and peaks at the position of the near-infrared nucleus. Using the [Ne V] line luminosity, we estimate that the nuclear bolometric luminosity of the AGN is ~5 ? 1041 ergs s-1, which, based on the Eddington limit, corresponds to a lower mass limit of the black hole of ~4 ? 103 M?. Using an order-of-magnitude estimate for the bulge mass based on the Hubble type of the galaxy, we find that this lower mass limit does not put a strain on the well-known relationship between the black hole mass and the host galaxys stellar velocity dispersion established in predominantly early-type galaxies. Multiwavelength follow-up observations of NGC 3621 are required to obtain more precise estimates of the bulge mass, black hole mass, accretion rate, and nuclear bolometric luminosity. The discovery reported here adds to the growing evidence that a black hole can form and grow in a galaxy with no or minimal bulge.

THE LINK BETWEEN STAR FORMATION AND ACCRETION IN LINERs: A COMPARISON WITH OTHER ACTIVE GALACTIC NUCLEUS SUBCLASSES

We present archival high-resolution X-ray imaging observations of 25 nearby LINERs observed by ACIS on board Chandra. This sample builds on our previously published proprietary and archival X-ray observations and includes the complete set of LINERs with published black hole masses and FIR luminosities that have been observed by Chandra. Of the 82 LINERs observed by Chandra, 41 (50%) display hard nuclear cores consistent with an AGN. The nuclear 2-10 keV luminosities of these AGN-LINERs range from ~2 × 1038 to ~1 × 1044 ergs s-1. Reinforcing our previous work, we find a significant correlation between the Eddington ratio, Lbol/LEdd, and the FIR luminosity, LFIR, as well as the IR brightness ratio, LFIR/LB, in the host galaxy of AGN-LINERs that extends over 7 orders of magnitude in Lbol/LEdd. Combining our AGN-LINER sample with galaxies from other AGN subclasses, we find that this correlation is reinforced in a sample of 129 AGNs, extending over almost 9 orders of magnitude in Lbol/LEdd. Using archival and previously published observations of the 6.2 μm PAH feature from ISO, we find that it is unlikely that dust heating by the AGN dominates the FIR luminosity in our sample of AGNs. Our results may therefore imply a fundamental link between the mass accretion rate (), as measured by the Eddington ratio, and the star formation rate (SFR), as measured by the FIR luminosity. Apart from the overall correlation, we find that the different AGN subclasses occupy distinct regions in the LFIR and Lbol/LEdd plane. Assuming a constant radiative efficiency for accretion, our results may imply a variation in the SFR/ ratio as a function of AGN activity level, a result that may have significant consequences for our understanding of galaxy formation and black hole growth.