arXiv subjects

Amiel Sternberg

Publications and source records attributed to Amiel Sternberg.

88 records · Page 5Linked to original sources

arXiv2006

Bistability in Interstellar Gas-Phase Chemistry

We present an analysis of "bistability" in gas-phase chemical models of dark interstellar clouds. We identify the chemical mechanisms that allow high- and low-ionization solutions to the chemical rate-equations to coexist. We derive simple analytic scaling relations for the gas densities and ionization rates for which the chemistry becomes bistable. We explain why bistability is sensitive to the H3+ dissociative recombination rate coefficient, and why it is damped by gas-grain neutralization.

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arXiv2005

CN and HCN in Dense Interstellar Clouds

We present a theoretical investigation of CN and HCN molecule formation in dense interstellar clouds. We study the gas-phase CN and HCN production efficiencies from the outer photon-dominated regions (PDRs) into the opaque cosmic-ray dominated cores. We calculate the equilibrium densities of CN and HCN, and of the associated species C+, C, and CO, as functions of the far-ultraviolet (FUV) optical depth. We consider isothermal gas at 50 K, with hydrogen particle densities from 10^2 to 10^6 cm^-3. We study clouds that are exposed to FUV fields with intensities 20 to 2*10^5 times the mean interstellar FUV intensity. We assume cosmic-ray H2 ionization rates ranging from 5*10^-17 s^-1, to an enhanced value of 5*10^-16 s^-1. We also examine the sensitivity of the density profiles to the gas-phase sulfur abundance.

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arXiv2004

Photoionized Gas in Dark-Matter Minihalos in the Galactic Halo and Local Group

(Abridged) We present computations of the metals photoionization structures of pressure supported gas clouds in gravitationally dominant DM minihalos in the extended Galactic halo or LG environment. We consider low-metallicity (0.1-0.3 solar) clouds that are photoionized by the metagalactic radiation field, and which are pressure-confined by a hot external medium. We study the properties of LCDM Burkert (and NFW) minihalos with circular velocities 12-30 km/s. We present results for the volume densities and projected column densities of low-ions, CII, NI, OI, SiII and SII, produced in the neutral cores, and high-ions, CIV, NV, OVI, SiIII, SiIV, and SIII produced in the ionized envelopes. We examine the possible relationships between the compact HI high-velocity clouds (CHVCs), dwarf galaxies, and high-velocity ionized CIV absorbers, within the context of photoionized minihalo models. In pressure-confined (P/k ~50 K/cm^3) mini-halo models for the CHVCs the photoionization states are much lower than in the CIV absorbers. However, for lower bounding pressures (<~1 K/cm^3) in more massive dwarf galaxy scale halos, the photoionization states in the outer envelopes resemble those observed in the CIV HVCs. An important exception is OVI for which our photoionization models underpredict the relative abundances by about an order-of-magnitude, suggesting that additional processes are at play as concluded by Sembach et al. For the cloud size-scales expected in median LCDM Burkert halos, a metallicity of 0.3 solar is required to produce absorption-line strengths comparable to those in the CIV HVCs. We argue that fully ionized and starless "dark galaxies" could be detectable in the local universe as UV metal line absorbers with ionization states similar to the CIV absorbers.

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arXiv2003

Ionizing Photon Emission Rates from O- and Early B-type Stars and Clusters

We present new computations of the ionizing spectral energy distributions (SEDs) and Lyman continuum (Lyc) and HeI continuum photon emission rates, for hot O-type and early B-type stars. We consider solar metallicity stars, with effective temperatures ranging from 25,000 to 55,000 K and surface gravities (cm s^-2) logg ranging from 3 to 4, covering the full range of spectral types and luminosity classes for hot stars. We use our updated (WM-basic) code to construct radiation-driven wind atmosphere models for hot stars. Our models include the coupled effects of hydrodynamics and non-LTE radiative transfer in spherically outflowing winds, including the detailed effects of metal line blocking and line blanketing on the radiative transfer and energy balance. We incorporate our hot-star models into our population synthesis code (STARS), and we compute the time-dependent SEDs and resulting Lyc and HeI emission rates for evolving star clusters. We present results for continuous and impulsive star formation for a range of assumed stellar initial mass functions.

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arXiv2003

A mid-infrared spectroscopic survey of starburst galaxies: excitation and abundances

We present spectroscopy of MIR emission lines in twelve starburst regions, located in eleven starburst galaxies, for which a significant number of lines between 2.38 and 45micron were observed with the ISO Short Wavelength Spectrometer, with the intention of providing a reference resource for MIR spectra of starburst galaxies. The observation apertures were centred on actively star forming regions, including those which are inaccessible at optical wavelengths due to high levels of obscuration. We use this data set, which includes fine structure and hydrogen recombination lines, to investigate excitation and to derive gas phase abundances of neon, argon, and sulphur of the starburst galaxies. The derived Ne abundances span approximately an order of magnitude, up to values of ~3 times solar. The excitation ratios measured from the Ne and Ar lines correlate well with each other (positively) and with abundances (negatively). Both in excitation and abundance, a separation of objects with visible Wolf-Rayet features (high excitation, low abundance) is noted from those without (low excitation, high abundance). For a given abundance, the starbursts are of relatively lower excitation than a comparative sample of HII regions, possibly due to ageing stellar populations. By considering the abundance ratios of S with Ne and Ar we find that, in our higher metallicity systems, S is relatively underabundant by a factor of ~3. We discuss the origin of this deficit and favour depletion of S onto dust grains as a likely explanation. This weakness of the MIR fine structure lines of S has ramifications for future infrared missions such as SIRTF and Herschel since it indicates that the S lines are less favourable tracers of star formation than is suggested by nebular models which do not consider this effect.

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arXiv2002

The ISO Galactic Metallicity Gradient Revisited

Two independent groups (Giveon et al. 2002; Martin-Hernandez et al. 2002) have recently investigated the Galactic metallicity gradient as probed by ISO observations of mid-infrared emission lines from HII regions. We show that the different gradients inferred by the two groups are due to differing source selection and differing extinction corrections. We show that both data sets in fact provide consistent results if identical assumptions are made in the analysis. We present a consistent set of gradients in which we account for extinction and variation in electron temperature across the disk.

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arXiv2001

The Excitation and Metallicity of Galactic HII Regions from ISO-SWS Observations of Mid-Infrared Fine-Structure Lines

We present mid-infrared ISO-SWS observations of the fine-structure emissions lines [NeII]12.8, [NeIII]15.6, [NeIII]36, [ArII]6.9, [ArIII]8.9, [SIII]18.7, [SIII]33.5, [SIV]10.5, and the recombination lines Br_alpha and Br_beta, in a sample of 112 Galactic HII regions and 37 nearby extra-galactic HII regions in the LMC, SMC, and M33. The Galactic sources have a wide range in galactocentric distance (0<R<18kpc), which enables us to study excitation and metallicity variations over large Galactic scales. We detect a steep rise in the [NeIII]15.6/[NeII]12.8,[ArIII]8.9/[ArII]6.9, and [SIV]10.5/[SIII]33.5 excitation ratios from the inner Galaxy outwards, and a moderate decrease in metallicity, from twice solar in the inner Galaxy to solar in the outer disk. The extra galactic sources in our sample show low gas density, low metallicity and high excitation. We find a good correlation between the neon and the argon excitation ratios in our sample. The observed correlation is well-reproduced by theoretical nebular models which incorporate new generation wind-driven NLTE model stellar atmospheres for the photoionizing stars. In particular, the NLTE atmospheres can account for the production of [NeIII] emission in the HII regions. We conclude that the increase in nebular excitation with galactocentric radius is due to an increase in stellar effective temperature (as opposed to a hardening of the stellar SEDs due to the metallicity gradient). The good fit between observations and our models support the conclusion of Thornley et al. (2000), that the low neon ratios observed in extra-galactic sources are due to global aging effects.

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arXiv2001

The Super Star Cluster NGC 1569-A Resolved on Sub-Parsec Scales with Hubble Space Telescope Spectroscopy

We present 3000--10000 Ang HST/STIS long-slit spectroscopy of the bright super star cluster A (SSC-A) in the dwarf starburst galaxy NGC 1569. The 0.05" HST angular resolution allows, for the first time, to probe for spatial variations in the stellar population of a ~ 10^6 M_sun SSC. Integrated ground-based spectra of SSC-A have previously revealed young Wolf-Rayet (WR) signatures that coexist with features from supposedly older, red supergiant (RSG), populations. We find that the WR emission complexes come solely from the subcluster A2, identified in previous HST imaging, and are absent from the main cluster A1, thus resolving the question of whether the WR and RSG features arise in a single or distinct clusters. The equivalent widths of the WR features in A2 --- including the CIV 5808 complex which we detect in this object for the first time --- are larger than previously observed in other WR galaxies. Models with sub-solar metallicity, as inferred from the nebular emission lines of this galaxy, predict much lower equivalent widths. On the ``clean'' side of A1, opposite to A2, we find no evidence for radial gradients in the observed stellar population at 0.05"<R<0.40" (~0.5 to 5 pc), neither in broad-band, low-resolution, spectra nor in medium-resolution spectra of the infrared CaII triplet.

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arXiv2001

An Ultraviolet through Infrared Look at Star Formation and Super Star Clusters in Two Circumnuclear Starburst Rings

We present broad-band (U, V, I,and H) and narrow-band(H-alpha+[N II] and Paschen-alpha) images of the circumnuclear starburst rings in two nearby spiral galaxies, NGC 1512 and NGC 5248, obtained with WFPC2 and NICMOS on HST. Combined with HST images at 2300 Ang, these data provide a particularly wide wavelength range with which to study the properties of the stellar populations, the gas, and the dust in the rings. Some large (50-pc scale) line emitting regions have little associated continuum emission, but a Pa equivalent width indicating a few-Myr-old embedded stellar population. The Ha/Pa intensity ratios suggest the gas is mixed with dust, making it effective at obscuring some of the young clusters. We identify about 1000 compact continuum sources (super star clusters and individual stars) and analyze their spectral energy distributions (SEDs) from 0.2 to 1.6 micron by fitting them with a grid of spectral synthesis models with a range of ages and extinctions. Most of the visible clusters are only mildly reddened, with A_V=0 to 1 mag, suggesting that the processes that clear out the gas and dust of the stellar birth clouds are efficient and fast. The patchiness of the dust distribution makes it difficult to estimate reliably the star formation rate, based on UV continuum slope or hydrogen emission-line ratios. The cluster SEDs are consistent with a range in ages, from 1 Myr to 300 Myr, but with only a minority older than a few tens of Myr. However, after accounting for an age bias, the fraction of old clusters is consistent with continuous star formation in the rings over the past ~300 Myr. Some of the brightest young clusters have excess emission in the IR that may be radiation by circumstellar dust. The cluster mass functions follow an m^-2 power-law distribution, distinct from those of old globular clusters.

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arXiv2000

Infrared spectroscopy of NGC 1068: Probing the obscured ionizing AGN continuum

The ISO-SWS 2.5-45 um infrared spectroscopic observations of the nucleus of the Seyfert 2 galaxy NGC 1068 (see companion paper) are combined with a compilation of UV to IR narrow emission line data to determine the spectral energy distribution (SED) of the obscured extreme-UV continuum that photoionizes the narrow line emitting gas in the active galactic nucleus. We search a large grid of gas cloud models and SEDs for the combination that best reproduces the observed line fluxes and NLR geometry. Our best fit model reproduces the observed line fluxes to better than a factor of 2 on average and is in general agreement with the observed NLR geometry. It has two gas components that are consistent with a clumpy distribution of dense outflowing gas in the center and a more extended distribution of less dense and more clumpy gas farther out that has no net outflow. The best fit SED has a deep trough at ~4 Ryd, which is consistent with an intrinsic Big Blue Bump that is partially absorbed by ~6x10^19 cm^-2 of neutral hydrogen interior to the NLR.

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arXiv1999

Near-Infrared Microlensing of Stars by the Super-Massive Black Hole in the Galactic Center

We investigate microlensing amplification of faint stars in the dense stellar cluster in the Galactic Center (GC) by the super-massive black hole (BH). Such events would appear very close to the position of the radio source SgrA*, which is thought to coincide with the BH, and could be observed during the monitoring of stellar motions in the GC. We use the observed K-band (2.2 um) luminosity function (KLF) in the GC and in Baade's Window, as well as stellar population synthesis computations, to construct KLF models for the inner 300 pc of the Galaxy. These, and the observed dynamical properties of this region, are used to compute the rates of microlensing events, which amplify stars above specified detection thresholds. We present computations of the lensing rates and amplifications as functions of the event durations (weeks to years), for a range of detection thresholds. We find that short events dominate the total rate and that long events tend to have large amplifications. For the current detection limit of K=17 mag, the total microlensing rate is 0.003 1/yr, and the rate of events with durations >1 yr is 0.001 1/yr. Recent GC proper motion studies have revealed the possible presence of one or two variable K-band sources very close to SgrA* (Genzel et al 97; Ghez et al 98). These sources may have attained peak brightnesses of K~15 mag, about 1.5-2 mag above the observational detection limits, and appear to have varied on a timescale of ~1 yr. This behavior is consistent with long-duration microlensing of faint stars by the BH. However, we estimate that the probability that such an event could have been detected during the course of the recent proper motion studies is \~0.5%. A ten-fold improvement in the detection limit and 10 yr of monthly monitoring would increase the total detection probability to ~20%. (Abridged)

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arXiv1998

The Ratio of Ortho- to Para-H2 in Photodissociation Regions

We discuss the ratio of ortho- to para-H2 in photodissociation regions (PDRs). We draw attention to an apparent confusion in the literature between the ortho-to-para ratio of molecules in FUV-pumped vibrationally excited states, and the H2 ortho-to-para abundance ratio. These ratios are not the same because the process of FUV-pumping of fluorescent H2 emission in PDRs occurs via optically thick absorption lines. Thus, gas with an equilibrium ratio of ortho- to para-H2 equal to 3 will yield FUV-pumped vibrationally excited ortho-to-para ratios smaller than 3, because the ortho-H2 pumping rates are preferentially reduced by optical depth effects. Indeed, if the ortho and para pumping lines are on the ``square root'' part of the curve-of-growth, then the expected ratio of ortho and para vibrational line strengths is the square root of 3, ~ 1.7, close to the typically observed value. Thus, contrary to what has sometimes been stated in the literature, most previous measurements of the ratio of ortho- to para-H2 in vibrationally excited states are entirely consistent with a total ortho-to-para ratio of 3, the equilibrium value for temperatures greater than 200 K. We present an analysis and several detailed models which illustrate the relationship between the total ratios of ortho- to para-H2 and the vibrationally excited ortho-to-para ratios in PDRs. Recent Infrared Space Observatory (ISO) measurements of pure rotational and vibrational H2 emissions from the PDR in the star-forming region S140 provide strong observational support for our conclusions.

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arXiv1998

Infrared spectroscopy of NGC4151: Probing the obscured ionizing AGN continuum

The ISO-SWS infrared spectroscopic observations of the nucleus of Seyfert galaxy NGC4151, which are described in a companion paper, are used together with a compilation of UV to IR narrow emission line data to determine the spectral shape of the obscured extreme-UV continuum that photoionizes the narrow line emitting gas in the active galactic nucleus. We present a new method to determine the best fitting photoionizing continuum and emission line cloud model from a heterogeneous set of emission line data. For NGC4151, we find a best-fit model that reproduces the observed line fluxes to within a factor of 2 on average, and which is consistent with the observed geometry of the optical Narrow Line Region (NLR). Our model consists of a clumpy, optically thick (ionization bounded) gas distribution, with a hydrogen gas density of ~1000 cm^-3 and a volume filling factor of 6.5*10^-4. Our best fitting spectral energy distribution (SED) falls sharply beyond the Lyman limit and then rises sharply again towards 100 eV. In particular, it does not display a `Big Blue Bump' signature of a hot accretion disk. We find that this SED, which best reproduces the NLR line emission on the 100--500 pc scale, does not have enough UV photons to produce the observed BLR recombination emission from the inner 1 pc. This suggests that the BLR is photoionized by the intrinsic continuum source, which does have a strong UV component (perhaps a Big Blue Bump), but that this UV component is absorbed by material located between the NLR and BLR. Our analysis suggests that the absorber consists of \~5*10^19 cm^-2 of neutral hydrogen. Such an absorber was independently detected by UV absorption lines (Kriss et al. 1992, 1995).

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arXiv1998

The Initial Mass Functions in the Super-Star-Clusters NGC 1569A and NGC 1705-1

I use recent photometric and stellar velocity dispersion measurements of the super-star-clusters (SSCs) NGC 1569A and NGC 1705-1 to determine their present-day luminosity/mass (L_V/M) ratios. I then use the inferred L_V/M ratios, together with population synthesis models of evolving star-clusters, to constrain the initial-mass-functions (IMFs) in these objects. I find that (L_V/M)_solar=28.9 in 1569A, and (L_V/M)_solar=126 in 1705-1. It follows that in 1569A the IMF is steep with alpha~2.5 for m**(-alpha)dm IMFs which extend to 0.1 M_sun. This implies that most of the stellar mass in 1569A is contained in low-mass (< 1 M_sun) stars. However, in 1705-1 the IMF is either flat, with alpha<2$, or it is truncated at a lower mass-limit between 1 and 3 M_sun. I compare the inferred IMFs with the mass functions (MFs) of Galactic globular clusters. It appears that 1569A has a sufficient reservoir of low-mass stars for it to plausibly evolve into an object similar to Galactic globular clusters. However, the apparent deficiency of low-mass stars in 1705-1 may make it difficult for this SSC to become a globular cluster. If low-mass stars do dominate the cluster mass in 1705-1, the large L_V/M ratio in this SSC may be evidence that the most massive stars have formed close to the cluster cores.

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arXiv1998

The Ultraviolet Spectra of LINERs: A Comparative Study

Imaging studies have shown that about 25% of LINER galaxies display a compact nuclear UV source. As part of a program to study the nature of LINERs and their connection to the active galaxy phenomenon, we compare the HST ultraviolet (1150-3200 Ang) spectra of seven such UV-bright LINERs. Data for three of the galaxies (NGC 404, NGC 4569, and NGC 5055) are presented for the first time, while data for four others (M81, NGC 4594, NGC 4579, and NGC 6500) have been recently published. The spectra of NGC 404, NGC 4569, and NGC 5055 show clear absorption-line signatures of massive stars, indicating a stellar origin for the UV continuum. Similar features are probably present in NGC 6500. The same stellar signatures may be present but undetectable in NGC 4594, due to the low signal-to-noise ratio of the spectrum, and in M81 and NGC 4579, due to superposed strong, broad emission lines. The compact central UV continuum source that is observed in these galaxies is a nuclear star cluster rather than a low-luminosity active galactic nucleus (AGN), at least in some cases. Except for the two LINERs with broad emission lines (M81 and NGC 4579), the LINERs have weak or no detectable UV emission lines. At least four of the LINERs suffer from an ionizing photon deficit, in the sense that the ionizing photon flux inferred from the observed far-UV continuum is insufficient to drive the optical H I recombination lines. Examination of the nuclear X-ray flux of each galaxy shows a high X-ray/UV ratio in the four ``UV-photon starved'' LINERs. Future observations can determine whether the UV continuum in LINERs is always dominated by a starburst or, alternatively, that there are two types of UV-bright LINERs: starburst-dominated and AGN-dominated.

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arXiv1996

Hubble Space Telescope Ultraviolet Images of Five Circumnuclear Star-Forming Rings

We present UV (2300 Ang) images, obtained with the HST Faint Object Camera, of the central 20'' of five galaxies containing circumnuclear star-forming rings. The five galaxies are from a well-defined sample of 103 normal, nearby galaxies we have observed with HST. At the HST resolution (0.05''), the rings break up into discrete star-forming clumps, each composed of many luminous and compact ($R \ltorder 5$ pc) star clusters. These objects are similar to those that have been recently reported in colliding and starburst galaxies, and in several other circumnuclear rings. A large fraction, 15%--50%, of the UV emission originates in these compact clusters. Compact clusters therefore may be the preferred mode of star formation in starburst environments. For one galaxy, NGC 2997, we measure the UV-optical colors of the individual clusters using an archival HST WFPC2 image at 6000 Ang . Comparing the colors and luminosities to starburst population synthesis models, we show that the clusters are less than 100 Myr old and have masses of at least a few $10^3 M_{\odot}$, with some as high as $10^5 M_{\odot}$. The UV extinction to those clusters that are detected in the UV is at most a factor of 10. In NGC 2997, the limits on the masses and the ages of the young clusters indicate that these objects will remain bound and evolve into globular clusters. The luminosity function of the clusters in the rings is similar in shape to those measured for super star clusters in other star-forming galaxies, and extends to luminosities lower by several orders of magnitude. All five of the UV-detected circumnuclear rings occur in barred or weakly barred spiral galaxies of type Sc or earlier. None of the five rings has an active nucleus at its center, arguing against a direct correspondence between circumnuclear star formation and nuclear

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