Far-Infrared Spectroscopy of Normal Galaxies: Physical Conditions in the Interstellar Medium

By S. Malhotra, M. Kaufman, D. Hollenbach, G. Helou, R. Rubin, J. Brauher, D. Dale, N. Lu, S. Lord, G. Stacey and 3 more. 2001

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The most important cooling lines of the neutral interstellar medium (ISM) lie in the far-infrared (FIR). We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans a range in properties such as morphology, FIR colors (indicating dust temperature), and FIR/blue ratios (indicating star formation activity and optical depth). In two-thirds of the galaxies in this sample, the [C II] line flux is proportional to FIR dust continuum. The other one-third show a smooth decline in L[C II]/LFIR with increasing Fν(60 μm)/Fν(100 μm) and LFIR/LB, spanning a range of a factor of more than 50. Two galaxies at the warm and active extreme of the range have L[C II]/LFIR < 2 × 10-4 (3 σ upper limit). This is due to increased positive grain charge in the warmer and more active galaxies, which leads to less efficient heating by photoelectrons from dust grains. The ratio of the two principal photodissociation region (PDR) cooling lines L[O I]/L[C II] shows a tight correlation with Fν(60 μm)/Fν(100 μm), indicating that both gas and dust temperatures increase together. We derive a theoretical scaling between [N II] (122 μm) and [C II] from ionized gas and use it to separate [C II] emission from neutral PDRs and ionized gas. Comparison of PDR models of Kaufman et al. with observed ratios of (1) L[O I]/L[C II] and (L[C II] + L[O I])/LFIR and (2) L[O I]/LFIR and Fν(60 μm)/Fν(100 μm) yields far-UV flux G0 and gas density n. The G0 and n values estimated from the two methods agree to better than a factor of 2 and 1.5, respectively, in more than half the sources. The derived G0 and n correlate with each other, and G0 increases with n as G0 ∝ nα, where α ≈ 1.4 . We interpret this correlation as arising from Strömgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions. The high values of PDR surface temperature (270-900 K) and pressure (6 × 104-1.5 × 107 K cm-3) derived also support the view that a significant part of grain and gas heating in the galaxies occurs very close to star-forming regions. The differences in G0 and n from galaxy to galaxy may be due to differences in the physical properties of the star-forming clouds. Galaxies with higher G0 and n have larger and/or denser star-forming clouds.

Subfields: Astronomy and Astrophysics, Spectroscopy

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Citations in the corpus

Infrared Emission of Normal Galaxies from 2.5 to 12 Micron: Infrared Space Observatory Spectra, Near-Infrared Continuum, and Mid-Infrared Emission Features (2003, background): “…one of the following: ISOCAM images at 7 and 15 lm (see Dale et al. 2000), a sparsely sampled ISO Long Wavelength Spectrometer (ISO-LWS) spectrum between 43 and 200 lm targeting fine-structure lines (Malhotra et al. 2001 and references therein), and a PHT-S 2.5–11.6 lm spectrum as presented here.”
EXPLAINING THE [C ii]157.7 μm DEFICIT IN LUMINOUS INFRARED GALAXIES—FIRST RESULTS FROM A HERSCHEL/PACS STUDY OF THE GOALS SAMPLE (2013, background): “…[C ii] emission arising from warm, AGN-dominated systems such as Mrk 231 (Fischer et al. 2010), this interpretation has been questioned in normal star-forming galaxies due to the requirement of extraordinarly large column densities of gas in the PDRs (Luhman et al. 1998, Malhotra et al. 2001).”
Warm Molecular Hydrogen in the Spitzer SINGS Galaxy Sample (2007, methodology): “For NGC 5713, G0 is 2 times higher than that of Malhotra et al. (2001) and n is at least 6 times higher.”
The properties of the interstellar medium within a star-forming galaxy at z= 2.3 (2010, background): “…conditions are similar to local starburst galaxies and ULIRGs (Stacey et al. 1991; Davies et al. 2003) and at the upper end of both density and G0 for normal starforming galaxies (Malhotra et al. 2001), for example the radiation field is ∼ 1000× more intense than in the disk of the Milky Way.”
A Herschel/PACS Far-infrared Line Emission Survey of Local Luminous Infrared Galaxies (2017, methodology): “Optically thick [O i]63 has been already reported by studies with available information of the [O i]145 emission line (Malhotra et al. 2001; Farrah et al. 2013; Rosenberg et al. 2015).”
Physical Properties of 15 Quasars at z ≳ 6.5 (2017, background): “At low redshift (z < 1) both star-forming galaxies (Malhotra et al. 2001, Sargsyan et al. 2014) and more extreme objects, e.g. LIRGS and ULIRGS (DiazSantos et al. 2013, Farrah et al. 2013), show lower luminosity ratios at higher FIR luminosities: this phenomenon is known as the “CII-deficit”.”
[Si II], [Fe II], [C II], and H2 Emission from Massive Star-forming Regions (2006, methodology): “On previous occasions, we have used our PDR models (e.g., Wolfire et al. 1990; K99; Malhotra et al. 2001; Hunter et al. 2001) combined with observations of [O i], [C ii], and far-infrared dust emission to constrain the properties of PDRs.”
Dusty Star Forming Galaxies at High Redshift (2014, background): “The line is thought to be an important coolant in the ISM, and can make up as much as 1% of the far infrared luminosity of a galaxy (Nikola et al., 1998; Malhotra et al., 2001; Stacey et al., 2010).”
The [C II] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies Revisited (2003, methodology): “A high G0/ n cause for the ULIRG [C ii] deficit would agree with the conclusion of Malhotra et al. (1997, 2001), who suggest that high G0/ n PDRs (specifically, high n and higher G0) account for the [C ii] flux deficiencies seen in the FIR-bright objects among their sample of normal galaxies.”
Big Three Dragons: A z = 7.15 Lyman-break galaxy detected in [O iii] 88 μm, [C ii] 158 μm, and dust continuum with ALMA (2018, background): “In fact, based on Herschel spectroscopy for local dwarf galaxies, Cormier et al. (2015) have demonstrated that [Oiii] 88 µm becomes brighter than [Cii] at low metallic- ity (see also Malhotra et al. 2001).”
A 158 μm [C ii] LINE SURVEY OF GALAXIES AT z ∼ 1–2: AN INDICATOR OF STAR FORMATION IN THE EARLY UNIVERSE (2010, background): “The PDR gas mass fraction in star-forming galaxies ranges from a few percent for quiescent systems like the Milky Way up to more than 50% for starburst galaxies like M82 making PDRs important on galactic scales (Stacey et al. 1985; Crawford et al. 1985; S91; Malhotra et al. 2001).”
An ALMA [C ii] Survey of 27 Quasars at z > 5.94 (2018, background): “…and drops by an order of magnitude toward the bright end (the so-called “[C II] deficit”), in particular in the presence of high-temperature, compact dust emission (Malhotra et al. 2001; Díaz-Santos et al. 2013, 2014, 2017; Farrah et al. 2013; Sargsyan et al. 2014; Herrera-Camus et al. 2015).”
A Compendium of Far-Infrared Line and Continuum Emission for 227 Galaxies Observed by the Infrared Space Observatory (2008, methodology): “Thus, the [N II] 122 µm/[C II] 158 µm ratio, on average, is nearly constant across a broad range of heating environments (Malhotra et al. 2001).”
Cool Gas in High-Redshift Galaxies (2013, background): “The ratio of [C II]/FIR luminosity for the Milky Way is 0.003, and this value holds in nearby disk galaxies, with a relatively large scatter (factor three; e.g. Malhotra et al. 1997, 2001).”
AN INTENSELY STAR-FORMING GALAXY AT z ∼ 7 WITH LOW DUST AND METAL CONTENT REVEALED BY DEEP ALMA AND HST OBSERVATIONS (2013, background): “Since the C+ zones extend over the dust extinction up to Av ≤ 4 (Malhotra et al. 2001), heavy dust extinction in ISM does not allow to make a large C+ zones emitting [Cii].”
First detection of [CII]158 μm at high redshift : vigorous star formation in the early universe (2005, background): “For sources with far-infrared luminosities in excess of 1012 L⊙ or with increasing star formation activity, this ratio drops by an order of magnitude (Malhotra et al., 2001; Luhman et al., 1998, 2003; Negishi et al., 2001).”
The assembly of ‘normal’ galaxies at z ∼ 7 probed by ALMA (2015, background): “…from powerful AGNs, hence resulting in an inappropriate measurement of the star formation rate from the far-IR emission (Sargsyan et al. 2012; Malhotra et al. 2001), or associated to different physical conditions in ULIRGs, which may suppress the emission of [CII] (Kaufman et al. 1999;…”