H_2 emission arises outside photodissociation regions in ultra-luminous infrared galaxies

Ultra-luminous infrared galaxies are among the most luminous objects in the local universe and are thought to be powered by intense star formation. It has been shown that in these objects the rotational spectral lines of molecular hydrogen observed at mid-infrared wavelengths are not affected by dust obscuration, leaving unresolved the source of excitation of this emission. Here I report an analysis of archival Spitzer Space Telescope data on ultra-luminous infrared galaxies and demonstrate that star formation regions are buried inside optically thick clouds of gas and dust, so that dust obscuration affects star-formation indicators but not molecular hydrogen. I thereby establish that the emission of H_2 is not co-spatial with the buried starburst activity and originates outside the obscured regions. This is rather surprising in light of the standard view that H_2 emission is directly associated with star-formation activity. Instead, I propose that H_2 emission in these objects traces shocks in the surrounding material, which are in turn excited by interactions with nearby galaxies, and that powerful large-scale shocks cooling by means of H_2 emission may be much more common than previously thought. In the early universe, a boost in H_2 emission by this process may speed up the cooling of matter as it collapsed to form the first stars and galaxies and would make these first structures more readily observable.Comment: Main text and supplemental information, 21 pages including 6 figures, 2 table

Infrared Space Observatory long-wavelength spectrometer spectroscopy of star-forming regions in M33

We present Infrared Space Observatory (ISO) Long-Wavelength Spectrometer (LWS) far-infrared (FIR) spectra of the nucleus and six giant H II regions in M33 (NGC 595, IC 142, NGC 592, NGC 604, NGC 588, and IC 133). The seven fine-structure lines observed in the FIR are used to model the H II and photodissociation regions (PDRs). There is no observed trend in the FIR properties, observed with the LWS, as a function of galactic radius or metallicity. The cold neutral medium (CNM) is the main reservoir for the atomic gas, containing between 60% and 95% of the gas. The FIRLWS spectral energy distribution can be fitted with a single-temperature graybody spectrum with a temperature in the range 35 K less than or equal to T less than or equal to 49 K. The [C II] 158 mum line flux is 0.2%-0.7% FIRLWS, which is typical of values seen (0.1%-1% FIR) in the nuclei of star-forming galaxies. The [C II]/FIRLWS ratio peaks at the nucleus and is fairly constant across the rest of the sample. Massive star formation is traced by the intensity of the [O III] 88 mum line. The emission from the observed FIR lines that arise solely from H II regions can be modeled as a single component with a given oxygen and nitrogen abundance, effective temperature, density, and ionizing flux. There is no need for an extended low-density component (ELDWIM). Apart from NGC 604 and NGC 595, the fractional [C II] emission that arises from the H II regions and/or PDRs is not well constrained, but typically 5%-50% arises in the H II regions, 10%-35% from the CNM, and the bulk of the emission (40%-90%) in the PDRs. The average PDR in this sample has a gas density [n] similar to 10(3.1) cm(-3), an average incident far-ultraviolet flux (in units of the local interstellar value) [G(0)] = 10(2.4), a gas temperature T similar to 200 K, and an A(V) similar to 10 through the clouds. NGC 604 has 40% of the atomic gas residing in the PDRs, while the rest have a much smaller fraction, similar to5%-15%. The PDRs are similar to those found in other star-forming galaxies such as Centaurus A. [G(0)] is at the lower end of the range observed in samples of spiral and starburst galaxies (2.2 less than or equal to log G(0) less than or equal to 5), and [log(n)] sits comfortably in the middle of the observed range (1.8 less than or equal to log n less than or equal to 4.2).</p

Infrared Space Observatory long-wavelength spectrometer spectroscopy of star-forming regions in M33

We present Infrared Space Observatory (ISO) Long-Wavelength Spectrometer (LWS) far-infrared (FIR) spectra of the nucleus and six giant H II regions in M33 (NGC 595, IC 142, NGC 592, NGC 604, NGC 588, and IC 133). The seven fine-structure lines observed in the FIR are used to model the H II and photodissociation regions (PDRs). There is no observed trend in the FIR properties, observed with the LWS, as a function of galactic radius or metallicity. The cold neutral medium (CNM) is the main reservoir for the atomic gas, containing between 60% and 95% of the gas. The FIRLWS spectral energy distribution can be fitted with a single-temperature graybody spectrum with a temperature in the range 35 K less than or equal to T less than or equal to 49 K. The [C II] 158 mum line flux is 0.2%-0.7% FIRLWS, which is typical of values seen (0.1%-1% FIR) in the nuclei of star-forming galaxies. The [C II]/FIRLWS ratio peaks at the nucleus and is fairly constant across the rest of the sample. Massive star formation is traced by the intensity of the [O III] 88 mum line. The emission from the observed FIR lines that arise solely from H II regions can be modeled as a single component with a given oxygen and nitrogen abundance, effective temperature, density, and ionizing flux. There is no need for an extended low-density component (ELDWIM). Apart from NGC 604 and NGC 595, the fractional [C II] emission that arises from the H II regions and/or PDRs is not well constrained, but typically 5%-50% arises in the H II regions, 10%-35% from the CNM, and the bulk of the emission (40%-90%) in the PDRs. The average PDR in this sample has a gas density [n] similar to 10(3.1) cm(-3), an average incident far-ultraviolet flux (in units of the local interstellar value) [G(0)] = 10(2.4), a gas temperature T similar to 200 K, and an A(V) similar to 10 through the clouds. NGC 604 has 40% of the atomic gas residing in the PDRs, while the rest have a much smaller fraction, similar to5%-15%. The PDRs are similar to those found in other star-forming galaxies such as Centaurus A. [G(0)] is at the lower end of the range observed in samples of spiral and starburst galaxies (2.2 less than or equal to log G(0) less than or equal to 5), and [log(n)] sits comfortably in the middle of the observed range (1.8 less than or equal to log n less than or equal to 4.2)

Infrared Space Observatory long-wavelength spectrometer spectroscopy of star-forming regions in M33

We present Infrared Space Observatory (ISO) Long-Wavelength Spectrometer (LWS) far-infrared (FIR) spectra of the nucleus and six giant H II regions in M33 (NGC 595, IC 142, NGC 592, NGC 604, NGC 588, and IC 133). The seven fine-structure lines observed in the FIR are used to model the H II and photodissociation regions (PDRs). There is no observed trend in the FIR properties, observed with the LWS, as a function of galactic radius or metallicity. The cold neutral medium (CNM) is the main reservoir for the atomic gas, containing between 60% and 95% of the gas. The FIRLWS spectral energy distribution can be fitted with a single-temperature graybody spectrum with a temperature in the range 35 K less than or equal to T less than or equal to 49 K. The [C II] 158 mum line flux is 0.2%-0.7% FIRLWS, which is typical of values seen (0.1%-1% FIR) in the nuclei of star-forming galaxies. The [C II]/FIRLWS ratio peaks at the nucleus and is fairly constant across the rest of the sample. Massive star formation is traced by the intensity of the [O III] 88 mum line. The emission from the observed FIR lines that arise solely from H II regions can be modeled as a single component with a given oxygen and nitrogen abundance, effective temperature, density, and ionizing flux. There is no need for an extended low-density component (ELDWIM). Apart from NGC 604 and NGC 595, the fractional [C II] emission that arises from the H II regions and/or PDRs is not well constrained, but typically 5%-50% arises in the H II regions, 10%-35% from the CNM, and the bulk of the emission (40%-90%) in the PDRs. The average PDR in this sample has a gas density [n] similar to 10(3.1) cm(-3), an average incident far-ultraviolet flux (in units of the local interstellar value) [G(0)] = 10(2.4), a gas temperature T similar to 200 K, and an A(V) similar to 10 through the clouds. NGC 604 has 40% of the atomic gas residing in the PDRs, while the rest have a much smaller fraction, similar to5%-15%. The PDRs are similar to those found in other star-forming galaxies such as Centaurus A. [G(0)] is at the lower end of the range observed in samples of spiral and starburst galaxies (2.2 less than or equal to log G(0) less than or equal to 5), and [log(n)] sits comfortably in the middle of the observed range (1.8 less than or equal to log n less than or equal to 4.2)

Resolving star formation on subkiloparsec scales in the high-redshift galaxy SDP.11 using gravitational lensing

We investigate the properties of the interstellar medium, star formation, and the current-day stellar population in the strongly lensed star-forming galaxy H-ATLAS J091043.1-000321 (SDP.11), at z = 1.7830, using new Herschel and Atacama Large Millimeter/submillimeter Array (ALMA) observations of far-infrared fine-structure lines of carbon, oxygen, and nitrogen. We report detections of the [O iii] 52 μm, [N iii] 57 μm, and [O i] 63 μm lines from Herschel/PACS, and present high-resolution imaging of the [C ii] 158 μm line, and underlying continuum, using ALMA. We resolve the [C ii] line emission into two spatially offset Einstein rings, tracing the red and blue velocity components of the line, in the ALMA/Band 9 observations at 0farcs2 resolution. The values seen in the [C ii]/far-infrared (FIR) ratio map, as low as ~0.02% at the peak of the dust continuum, are similar to those of local ULIRGs, suggesting an intense starburst in this source. This is consistent with the high intrinsic FIR luminosity (~3 × 1012 L ⊙), ~16 Myr gas depletion timescale, and lesssim8 Myr timescale since the last starburst episode, estimated from the hardness of the UV radiation field. By applying gravitational lensing models to the visibilities in the uv-plane, we find that the lensing magnification factor varies by a factor of two across SDP.11, affecting the observed line profiles. After correcting for the effects of differential lensing, a symmetric line profile is recovered, suggesting that the starburst present here may not be the result of a major merger, as is the case for local ULIRGs, but instead could be powered by star formation activity spread across a 3–5 kpc rotating disk