Ionizing Photon Rate Research Articles

Further observational evidence for a critical ionizing luminosity in active galaxies

We report the results of a survey for HI 21-cm absorption at redshifts of z > 2.6 in a new sample of radio sources with the Green Bank and Giant Metrewave Radio Telescopes. From a total of 25 targets, we report zero detections in the 16 for which optical depth limits could be obtained. Based upon the detection rate for z > 0.1 associated absorption, we would expect approximately four detections. Of the 11 which have previously not been searched, there is sufficient source-frame optical/ultra-violet photometry to determine the ionising photon rate for four. Adding these to the literature, the hypothesis that there is a critical rate of logQ = 56 ionising photons per second is now significant at ~7 sigma. This reaffirms our assertion that searching z > 3 active galaxies for which optical redshifts are available selects sources in which the ultra-violet luminosity is sufficient to ionise all of the neutral gas in the host galaxy.

Atomic and molecular absorption in redshifted radio sources

We report on a survey for associated HI 21-cm and OH 18-cm absorption with the Giant Metrewave Radio Telescope at redshifts z = 0.2-0.4. Although the low redshift selection ensures that our targets are below the critical ultra-violet luminosity, which is hypothesised to ionise all of the neutral gas in the host galaxy, we do not obtain any detections in the six sources searched. Analysing these in context of the previous surveys, in addition to the anti-correlation with the ultra-violet luminosity (ionising photon rate), we find a correlation between the strength of the absorption and the blue -- near-infrared colour, as well as the radio-band turnover frequency. We believe that these are due to the photo-ionisation of the neutral gas, an obscured sight-line being more conducive to the presence of cold gas and the compact radio emission being better intercepted by the absorbing gas, maximising the flux coverage, respectively. Regarding the photo-ionisation, the compilation of the previous surveys increases the significance of the critical ionising photon rate, above which all of the gas in the host galaxy is hypothesised to be ionised, to >5 sigma. This reaffirms that this is an ubiquitous effect, which has profound implications for the detection of neutral gas in these objects with the Square Kilometre Array.

AN ANALYTIC MODEL OF DUSTY, STRATIFIED, SPHERICAL H ii REGIONS

ABSTRACT We study analytically the effect of radiation pressure (associated with photoionization processes and with dust absorption) on spherical, hydrostatic H ii regions. We consider two basic equations, one for the hydrostatic balance between the radiation-pressure components and the gas pressure, and another for the balance among the recombination rate, the dust absorption, and the ionizing photon rate. Based on appropriate mathematical approximations, we find a simple analytic solution for the density stratification of the nebula, which is defined by specifying the radius of the external boundary, the cross section of dust absorption, and the luminosity of the central star. We compare the analytic solution with numerical integrations of the model equations of Draine, and find a wide range of the physical parameters for which the analytic solution is accurate.

THE BRIGHTEST YOUNG STAR CLUSTERS IN NGC 5253

The nearby dwarf starburst galaxy NGC5253 hosts a number of young, massive star clusters, the two youngest of which are centrally concentrated and surrounded by thermal radio emission (the `radio nebula'). To investigate the role of these clusters in the starburst energetics, we combine new and archival Hubble Space Telescope images of NGC5253 with wavelength coverage from 1500 Ang to 1.9 micron in 13 filters. These include H-alpha, P-beta, and P-alpha, and the imaging from the Hubble Treasury Program LEGUS (Legacy Extragalactic UV Survey). The extraordinarily well-sampled spectral energy distributions enable modeling with unprecedented accuracy the ages, masses, and extinctions of the 9 optically brightest clusters (M_V < -8.8) and the two young radio nebula clusters. The clusters have ages ~1-15 Myr and masses ~1x10^4 - 2.5x10^5 M_sun. The clusters' spatial location and ages indicate that star formation has become more concentrated towards the radio nebula over the last ~15 Myr. The most massive cluster is in the radio nebula; with a mass 2.5x10^5 M_sun and an age ~1 Myr, it is 2-4 times less massive and younger than previously estimated. It is within a dust cloud with A_V~50 mag, and shows a clear nearIR excess, likely from hot dust. The second radio nebula cluster is also ~1 Myr old, confirming the extreme youth of the starburst region. These two clusters account for about half of the ionizing photon rate in the radio nebula, and will eventually supply about 2/3 of the mechanical energy in present-day shocks. Additional sources are required to supply the remaining ionizing radiation, and may include very massive stars.

The X-ray luminosity function of active galactic nuclei in the redshift intervalz=3-5

We combine deep X-ray survey data from the Chandra observatory and the wide-area/shallow XMM-XXL field to estimate the AGN X-ray luminosity function in the redshift range z=3-5. The sample consists of nearly 340 sources with either photometric (212) or spectroscopic (128) redshift in the above range. The combination of deep and shallow survey fields provides a luminosity baseline of three orders of magnitude, Lx(2-10keV)~1e43-1e46erg/s at z>3. We follow a Bayesian approach to determine the binned AGN space density and explore their evolution in a model-independent way. Our methodology accounts for Poisson errors in the determination of X-ray fluxes and uncertainties in photometric redshift estimates. We demonstrate that the latter is essential for unbiased measurement of space densities. We find that the AGN X-ray luminosity function evolves strongly between the redshift intervals z=3-4 and z=4-5. There is also suggestive evidence that the amplitude of this evolution is luminosity dependent. The space density of AGN with Lx<1e45erg/s drops by a factor of 5 between the redshift intervals above, while the evolution of brighter AGN appears to be milder. Comparison of our X-ray luminosity function with that of UV/optical selected QSOs at similar redshifts shows broad agreement at bright luminosities, Lx>1e45erg/s. The faint-end slope of UV/optical luminosity functions however, is steeper than for X-ray selected AGN. This implies that the type-I AGN fraction increases with decreasing luminosity at z>3, opposite to trends established at lower redshift. We also assess the significance of AGN in keeping the hydrogen ionised at high redshift. Our X-ray luminosity function yields ionising photon rate densities that are insufficient to keep the Universe ionised at redshift z>4. A source of uncertainty in this calculation is the escape fraction of UV photons for X-ray selected AGN.

BAND-9 ALMA OBSERVATIONS OF THE $[{\rm{N}}\,{\rm{II}}]$ 122μm LINE AND FIR CONTINUUM IN TWO HIGH-zGALAXIES

We present Atacama Large Millimeter Array (ALMA) observations of two high-redshift systems (SMMJ02399-0136 at z_1 ~ 2.8 and the Cloverleaf QSO at z_1 ~ 2.5) in their rest-frame 122 μm continuum (νsky ~ 650 GHz, λsky ~ 450 μm) and [N ii] 122 μm line emission. The continuum observations with a synthesized beam of ~0.25 resolve both sources and recover the expected flux. The Cloverleaf is resolved into a partial Einstein ring, while SMMJ02399-0136 is unambiguously separated into two components: a point source associated with an active galactic nucleus and an extended region at the location of a previously identified dusty starburst. We detect the [N ii] line in both systems, though significantly weaker than our previous detections made with the first generation z (Redshift) and Early Universe Spectrometer. We show that this discrepancy is mostly explained if the line flux is resolved out due to significantly more extended emission and longer ALMA baselines than expected. Based on the ALMA observations we determine that ≥75% of the total [N ii] line flux in each source is produced via star formation. We use the [N ii] line flux that is recovered by ALMA to constrain the N/H abundance, ionized gas mass, hydrogen- ionizing photon rate, and star formation rate. In SMMJ02399-0136 we discover it contains a significant amount (~1000 M_⊙ yr^(−1)) of unobscured star formation in addition to its dusty starburst and argue that SMMJ02399-0136 may be similar to the Antennae Galaxies (Arp 244) locally. In total these observations provide a new look at two well-studied systems while demonstrating the power and challenges of Band-9 ALMA observations of high-z systems.

On the evolution of the cosmic ionizing background

We study the observed cosmic ionizing background as a constraint on the nature of the sources responsible for the reionization of the Universe. In earlier work, we showed that extrapolations of the ultraviolet (UV) luminosity function (LF) of Lyman-break galaxies (LBGs) at fixed Lyman continuum photon escape fraction (fesc) are not able to reproduce the redshift evolution of this background. Here, we employ extrapolations of the high-z LFs to describe the contribution of LBGs to the ionizing photon rate, taking into account the smoothing of the baryonic perturbations, due to the background itself (i.e. the filtering mass), as well as a possible sharp increase of fesc in dwarf galaxies. Under the hypothesis of a dominant contribution of LBGs to cosmic reionization, our results suggest that sources fainter than the current observational limits should be characterized by fesc values of the order of ∼0.1–0.3 (larger than the current estimates for bright galaxies) to account for a z ≳ 6 reionization and the measured evolution of cosmic ionizing background, at the same time. The contribution to the background from quasars turns out to be relevant at z ≲ 3. Overall, our results support the case for dedicated observations of faint galaxies in the rest-frame UV, in order to better determine their physical properties. Observed fesc values outside our proposed range bear relevant consequences on the nature of the astrophysical sources responsible for cosmic reionization and/or its build-up process.

Cluster formation in molecular clouds – I. Stellar populations, star formation rates and ionizing radiation

We present a model for the radiative output of star clusters in the process of star formation suitable for use in hydrodynamical simulations of radiative feedback. Gas in a clump, defined as a region whose density exceeds 10^4 cm^-3, is converted to stars via the random sampling of the Chabrier IMF. A star formation efficiency controls the rate of star formation. We have completed a suite of simulations which follow the evolution of accretion-fed clumps with initial masses ranging from 0 to 10^5 M_sol and accretion rates ranging from 10^-5 to 10^-1 M_sol yr^-1. The stellar content is tracked over time which allows the aggregate luminosity, ionizing photon rate, number of stars, and star formation rate (SFR) to be determined. For a fiducial clump of 10^4 M_sol, the luminosity is ~4x10^6 L_sol with a SFR of roughly 3x10^-3 M_sol yr^-1. We identify two regimes in our model. The accretion-dominated regime obtains the majority of its gas through accretion and is characterized by an increasing SFR while the reservoir-dominated regime has the majority of its mass present in the initial clump with a decreasing SFR. We show that our model can reproduce the expected number of O stars, which dominate the radiative output of the cluster. We find a nearly linear relationship between SFR and mass as seen in observations. We conclude that our model is an accurate and straightforward way to represent the output of clusters in hydrodynamical simulations with radiative feedback.

THE Paα LUMINOSITY FUNCTION OF H II REGIONS IN NEARBY GALAXIES FROMHST/NICMOS

The HII region luminosity function (LF) is an important tool for deriving the birthrates and mass distribution of OB associations, and is an excellent tracer of the newly formed massive stars and associations. To date, extensive work (predominantly in H{\alpha}) has been done from the ground, which is hindered by dust extinction and the severe blending of adjacent (spatially or in projection) HII regions. Reliably measuring the properties of HII regions requires a linear resolution <40 pc, but analyses satisfying this requirement have been done only in a handful of galaxies, so far. As the first space-based work using a galaxy sample, we have selected 12 galaxies from our HST NICMOS Pa{\alpha} survey and studied the luminosity function and size distribution of HII regions both in individual galaxies and cumulatively, using a virtually extinction-free tracer of the ionizing photon rate. The high angular resolution and low sensitivity to diffuse emission of NICMOS also offer an advantage over ground-based imaging by enabling a higher degree of de-blending of the HII regions. We do not confirm the broken power-law LFs found in ground-based studies. Instead, we find that the LFs, both individual and co-added, follow a single power law dN(L)/dlnL ~ L^-1, consistent with the mass function of star clusters in nearby galaxies, and in agreement with the results of the existing analyses with HST data. The individual and co-added size distribution of HII regions are both roughly consistent with dN(D)/dlnD ~ D^-3, but the power-law scaling is probably contaminated by blended regions or complexes.

AN INITIAL MASS FUNCTION STUDY OF THE DWARF STARBURST GALAXY NGC 4214

The production rate of ionizing photons in young < 8 Myr, unresolved stellar clusters in the nearby irregular galaxy NGC 4214 is probed using multi-wavelength Hubble Space Telescope WFC3 data. We normalize the ionizing photon rate by the cluster mass to investigate the upper end of the stellar initial mass function (IMF). We have found that within the uncertainties the upper end of the stellar IMF appears to be universal in this galaxy, and that deviations from a universal IMF can be attributed to stochastic sampling of stars in clusters with masses < 10^3 M_sun. Furthermore, we have found that there does not seem to be a dependence of the maximum stellar mass on the cluster mass. We have also found that for massive clusters, feedback may cause an underrepresentation in Ha luminosities, which needs to be taken into account when conducting this type of analysis.

THE STAR FORMATION IN RADIO SURVEY: GBT 33 GHz OBSERVATIONS OF NEARBY GALAXY NUCLEI AND EXTRANUCLEAR STAR-FORMING REGIONS

We present 33\,GHz photometry of 103 galaxy nuclei and extranuclear star-forming complexes taken with the Green Bank Telescope (GBT) as part of the Star Formation in Radio Survey (SFRS). Among the sources without evidence for an AGN, and also having lower frequency radio data, we find a median thermal fraction at 33GHz of ~76% with a dispersion of ~24%. For all sources resolved on scales <0.5kpc, the thermal fraction is even larger, being >90%. This suggests that the rest-frame 33GHz emission provides a sensitive measure of the ionizing photon rate from young star-forming regions, thus making it a robust star formation rate indicator. Taking the 33GHz star formation rates as a reference, we investigate other empirical calibrations relying on different combinations of warm 24\mu m dust, total infrared (IR; 8-1000\mu m), H\alpha\ line, and far-UV continuum emission. The recipes derived here generally agree with others found in the literature, albeit with a large dispersion that most likely stems from a combination of effects. Comparing the 33GHz to total IR flux ratios as a function of the radio spectral index, measured between 1.7 and 33GHz, we find that the ratio increases as the radio spectral index flattens which does not appear to be a distance effect. Consequently, the ratio of non-thermal to total IR emission appears relatively constant, suggesting only moderate variations in the cosmic-ray electron injection spectrum and ratio of synchrotron to total cooling processes among star-forming complexes. Assuming that this trend solely arises from an increase in the thermal fraction sets a maximum on the scatter of the non-thermal spectral indices among the star-forming regions of \sigma_\alpha^{NT} < 0.13.

COMPLETE IONIZATION OF THE NEUTRAL GAS: WHY THERE ARE SO FEW DETECTIONS OF 21 cm HYDROGEN IN HIGH-REDSHIFT RADIO GALAXIES AND QUASARS

From the first published z > 3 survey of 21-cm absorption within the hosts of radio galaxies and quasars, we found an apparent dearth of cool neutral gas at high redshift. From a detailed analysis of the photometry, each object is found to have a 1216 A continuum luminosity in excess of L ~1e23 W/Hz, a critical value above which 21-cm has never been detected at any redshift. At these wavelengths, and below, hydrogen is excited above the ground state so that it cannot absorb in 21-cm. In order to apply the equation of photoionsation equilibrium, we demonstrate that this critical value also applies to the ionising (< 912 A) radiation. We use this to show, for a variety of gas density distributions, that upon placing a quasar within a galaxy of gas there is always an ultra-violet luminosity above which all of the gas in the galaxy is ionised. While in this state the hydrogen cannot be detected nor cannot engage in star formation. Applying the mean ionising photon rate of all of the sources searched, we find, using canonical values for the gas density and recombination rate coefficient, that the observed critical luminosity gives a scale-length (3 kpc) similar that of the neutral hydrogen (HI) in the Milky Way, a large spiral galaxy. Thus, this simple, yet physically motivated, model can explain the critical luminosity, above which neutral gas is not detected. This indicates that these galaxies are probably devoid of star-forming material, rather the non-detection of 21-cm being due to the sensitivity limits of current radio telescopes.

On the relative contribution of high-redshift galaxies and active galactic nuclei to reionization

In this paper we discuss the contribution of different astrophysical sources to the ionization of neutral hydrogen at different redshifts. We critically revise the arguments in favour/against a substantial contribution of Active Galactic Nuclei (AGNs) and/or Lyman Break Galaxies (LBGs) to the reionization of the Universe at z>5. We consider extrapolations of the high-z QSO and LBG luminosity functions and their redshift evolution as well as indirect constraints on the space density of lower luminosity Active Galactic Nuclei based on the galaxy stellar mass function. Since the hypothesis of a reionization due to LBGs alone requires a significant contribution of faint dwarf galaxies and a LyC photon escape fraction (f_esc) of the order of ~20%, in tension with present observational constraints, we examine under which hypothesis AGNs and LBGs may provide a combined relevant contribution to the reionization. We show that a relatively steep faint-end of the AGN luminosity function, consistent with present constraints, provides a relevant (although sub-dominant) contribution, thus allowing us to recover the required ionizing photon rates with f_esc~5% up to z~7. At higher redshifts, we test the case for a luminosity-dependent f_esc scenario and we conclude that, if the observed LBGs are indeed characterized by very low f_esc, values of the order of f_esc~70% are needed for objects below our detection threshold, for this galaxy population to provide a substantial contribution to reionization. Clearly, the study of the properties of faint sources (both AGNs and LBGs) is crucial.

Prospects of observing a quasar H ii region during the epoch of reionization with the redshifted 21-cm signal

We present a study of the impact of a bright quasar on the redshifted 21cm signal during the Epoch of Reionization (EoR). Using three different cosmological radiative transfer simulations, we investigate if quasars are capable of substantially changing the size and morphology of the H II regions they are born in. We choose stellar and quasar luminosities in a way that is favourable to seeing such an effect. We find that even the most luminous of our quasar models is not able to increase the size of its native H II region substantially beyond those of large H II regions produced by clustered stellar sources alone. However, the quasar H II region is found to be more spherical. We next investigate the prospects of detecting such H II regions in the redshifted 21cm data from the Low Frequency Array (LOFAR) by means of a matched filter technique. We find that H II regions with radii ~ 25 comoving Mpc or larger should have a sufficiently high detection probability for 1200 hours of integration time. Although the matched filter can in principle distinguish between more and less spherical regions, we find that when including realistic system noise this distinction can no longer be made. The strong foregrounds are found not to pose a problem for the matched filter technique. We also demonstrate that when the quasar position is known, the redshifted 21cm data can still be used to set upper limits on the ionizing photon rate of the quasar. If both the quasar position and its luminosity are known, the redshifted 21 cm data can set new constrains on quasar lifetimes.

What produces the diffuse X-ray emission from the Orion nebula? I. Simple spherical models

The expansion of stellar wind bubbles inside evolving H ii regions is studied in spherical symmetry, with stellar wind parameters and ionizing photon rate appropriate to θ1Ori C, the main exciting source of the Orion nebula. The effects of mass loading due to embedded proplyds and thermal conduction at the edge of the hot, shocked stellar wind bubble on the expansion are investigated. The predicted X-ray luminosities and spectra of these models are calculated and compared to published observations. The models all have excess emission at higher energies compared to the observations. This emission comes from hot gas behind the stellar wind shock and its temperature depends solely on the stellar wind velocity, which for θ1Ori C is well established. Possible mechanisms for lowering the temperature of this gas are discussed.

Molecular outflows towards O-type young stellar objects

We have searched for massive molecular outflows in a sample of high-mass star forming regions, and we have characterised both the outflow properties and those of their associated molecular clumps. With a sample composed largely of more luminous objects than previous ones, this work complements analogous surveys performed by other authors by adding the missing highest luminosity sources. The sample under study has been selected so as to favour the earliest evolutionary phases of star formation, and is composed of very luminous objects (L_bol > 2x10^4 L_sun and up to ~10^6 L_sun), possibly containing O-type stars. Each source has been mapped in 13CO(2-1) and C18O(2-1) with the IRAM-30m telescope on Pico Veleta (Spain). The whole sample shows high-velocity wings in the 13CO(2-1) spectra, indicative of outflowing motions. In addition, we have obtained outflow maps in 9 of our 11 sources, which display well-defined blue and/or red lobes. For these sources, the outflow parameters have been derived from the line wing 13CO(2-1) emission. An estimate of the clump masses from the C18O(2-1) emission is also provided and found to be comparable to the virial masses. From a comparison between our results and those found by other authors at lower masses, it is clear that the outflow mechanical force increases with the bolometric luminosity of the clump and with the ionising photon rate of the associated HII regions, indicating that high-mass stars drive more powerful outflows. A tight correlation between outflow mass and clump mass is also found. Molecular outflows are found to be as common in massive star forming regions as in low-mass star forming regions. This, added to the detection of a few tentative large-scale rotating structures suggests that high-mass stars may generally form via accretion, as low-mass stars.

High-Resolution Mid-Infrared Imaging of Radio Ultracompact HiiRegions

We present data from mid-infrared Keck Telescope imaging of 18 radio-selected ultracompact H II region candidates at diffraction-limited resolution. The goal of these observations is to determine the sizes, luminosities, and morphologies of the mid-infrared-emitting dust surrounding the stellar sources. All 18 sources were imaged at 11.7 and 17.65 μm, and 10 of them were also imaged at 24.5 μm. All the sources were resolved. We have generated dust temperature and optical depth maps and combined them with radial velocity measurements and radio data (1.4 and 5 GHz) to constrain the properties of these star-forming regions. Half of our objects are excited by B-type stars, and all our objects have derived types that are later than an O6 star. We find a significant correlation between infrared and radio flux densities and a weaker one between infrared diameters and the central source ionizing photon rates. This latter correlation suggests that the more compact sources result from later spectral types rather than young age. Our new data may suggest a revision to the infrared color selection criteria of ultracompact H II regions at resolutions 1''. These 18 sources are part of a sample of 687 sources dominated by ultracompact H II regions selected by matching radio and infrared maps of the first Galactic quadrant by Giveon and coworkers. The new mid-infrared images constitute a significant improvement in resolving substructure at these wavelengths. If applied to all of this sample, our analysis will improve our understanding of embedded star formation in the Galaxy.

Evolving Spectra of Population III Stars: Consequences for Cosmological Reionization

We examine the significance of the first metal-free stars (Population III) for the cosmological reionization of H i and He ii. These stars have unusually hard spectra, with the integrated ionizing photon rates from a Population III stellar cluster for H i and He ii being 1.6 and 10 5 times stronger, respectively, than those from a Population II cluster. For the currently favored cosmology, we find that Population III stars alone can reionize H i at redshifts of z ’ 9 and 4.7 and He ii at z ’ 5:1 and 0.7 for continuous and instantaneous modes of star formation, respectively. More realistic scenarios involving combinations of Population III and Population II stellar spectra yield similar results for hydrogen. Helium never reionizes completely in these cases; the ionization fraction of He iii reaches a maximum of about 60% at z � 5:6 if Population III star formation lasts for 10 9 yr. Future data on H i reionization can test the amount of small-scale power available to the formation of the first objects and provide a constraint on values of � 8d0:7. Since current UV observations indicate an epoch of reionization for He ii at z � 3, He ii may reionize more than once. Measurements of the He ii Gunn-Peterson effect in the intergalactic medium at redshifts ze3 may reveal the significance of Population III stars for He ii reionization, particularly in ‘‘ void ’’ regions that may contain ‘‘ relic ’’ ionization from early Population III stellar activity. Subject headings: cosmology: theory — intergalactic medium

On the Relative Importance of Photoevaporative and Hydrodynamic Effects in the Ablation of Self‐gravitating Globules in Compact HiiRegions

We investigate in detail the process of hydrodynamic ablation, both for the case of subsonic and for supersonic flows, of isothermal self-gravitating globules. The results are then compared with those for photoevaporative mass loss to estimate which of the two processes is the dominant mechanism for the mass loss of self-gravitating globules embedded in compact H II regions. This material then goes on to mass-load the stellar wind, thereby altering its dynamical properties. Using our results, we perform numerical simulations of the evolution of such H II regions, taking into account both possible mass-loading processes, together with the effect of the finite lifetime of the globules. We find that for compact H II regions with central stars possessing high ionizing photon rates the photoevaporation process dominates.

Radio Recombination Lines from the Nuclear Regions of Starburst Galaxies

Using the Very Large Array (VLA) with an angular resolution of 3'', we have detected the hydrogen recombination line H92α from the galaxies Arp 220, M83, and NGC 2146. The line emission arises from the nuclear regions with a line-to-continuum ratio of 1% or less. In order to fit both the observed H92α line and continuum data in the nuclear regions, we have considered two types of models. First, we utilize a model with a collection of H ii regions. A large number of compact H ii regions are required in this model. With electron temperatures in the range 5 × 103-1 × 104 K and a range of electron densities, this model can account for both the line intensity and the continuum spectrum. In most cases, the H92α line is dominated by internal stimulated emission due to free-free continuum arising within the H ii regions. In a low-density case (ne = 50 cm−3) for Arp 220, about half the line emission comes from external stimulated emission due to the background nonthermal source. Typical rates of ionizing photons predicted from these models are ~5 × 1052 s−1 for M83, ~4 × 10−53 s−1 for NGC 2146, and 5 × 1054 s−1 for Arp 220. We infer that 105 O5 stars are required in Arp 220, which is an order of magnitude greater than in NGC 2146 and 2 orders of magnitude greater than in M83. Alternatively, several uniform slab models with Te ≥ 5 × 103 and ne in the range of 50 − 1 × 104 cm−3 appear to fit both the H92α line and continuum data of Arp 220 and M83. In the low-density models, stimulated emission by the background nonthermal radiation appears to be dominant at low frequencies, and the lines at higher frequencies arise primarily from spontaneous emission. The uniform slab model requires a higher ionizing photon rate than the H ii region model. No slab models with reasonable Te can fit the data observed in M83 and NGC 2146. Combining previous published data with these new observations, a sample of 13 galaxies has been observed for radio recombination lines (RRLs) with the VLA. Nine out of the 13 galaxies have been detected in the H92α line. While the H92α line luminosity appears to be correlated with the Bra line luminosity, we find that nearly all the RRL galaxies show a significant excess in H92α line compared to the expected LTE value. The excess in the H92α line flux suggests that non-LTE effects are important for the H92α line in these starburst nuclei. A strong correlation between H92α and the molecular lines of HCN/HCO+ is also found, indicating that the RRL emitters may be spatially associated with the dense molecular cores. The inferred high electron density also suggests an intimate relation between the RRLs and the dense molecular medium in these galaxies.