Nancy Ann Silbermann

The Infrared spectral energy distribution of normal star-forming galaxies

We have developed a new phenomenological model for the spectral energy distribution of normal star-forming galaxies between 3 and 1100 μm. These model spectra allow us to determine the infrared energy budget for normal galaxies, and in particular to translate far-infrared fluxes into total (bolometric) infrared fluxes. The 20 to 42 m range appears to show the most significant growth in relative terms as the activity level increases, suggesting that the 20–42 m continuum may be the best dust emission tracer of current star formation in galaxies.

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XXVIII. Combining the Constraints on the Hubble Constant

Since the launch of the Hubble Space Telescope nine years ago, Cepheid distances to 25 galaxies have been determined for the purpose of calibrating secondary distance indicators. A variety of these can now be calibrated, and the accompanying papers by Sakai, Kelson, Ferrarese, and Gibson employ the full set of 25 galaxies to consider the Tully-Fisher relation, the fundamental plane of elliptical galaxies, Type Ia supernovae, and surface brightness fluctuations. When calibrated with Cepheid distances, each of these methods yields a measurement of the Hubble constant and a corresponding measurement uncertainty. We combine these measurements in this paper, together with a model of the velocity field, to yield the best available estimate of the value of H_0 within the range of these secondary distance indicators and its uncertainty. The result is H_0 = 71 +/- 6 km/sec/Mpc. The largest contributor to the uncertainty of this 67% confidence level result is the distance of the Large Magellanic Cloud, which has been assumed to be 50 +/- 3 kpc.

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XIII. The Metallicity Dependence of the Cepheid Distance Scale

Uncertainty in the metal-abundance dependence of the Cepheid variable period-luminosity (PL) relation remains one of the outstanding sources of systematic error in the extragalactic distance scale and in the Hubble constant. To test for such a metallicity dependence, we have used the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST) to observe Cepheids that span a range in oxygen abundance of 0.7 ± 0.15 dex in two fields in the nearby spiral galaxy M101. A differential analysis of the PL relations in V and I in the two fields yields a marginally significant change in the inferred distance modulus on metal abundance, with δ(m-M)0/δ[O/H] = -0.24 ± 0.16 mag dex-1. The trend is in the theoretically predicted sense that metal-rich Cepheids appear brighter and closer than metal-poor stars. External comparisons of Cepheid distances with those derived from three other distance indicators, in particular from the tip of the red giant branch method, further constrain the magnitude of any Z-dependence of the PL relation at V and I. The overall effects of any metallicity dependence on the distance scale derived with HST will be of the order of a few percent or less for most applications, though distances to individual galaxies at the extremes of the metal abundance range may be affected at the 10% level.

The Hubble Space Telescope key project on the extragalactic distance scale XXIV: the calibration of Tully-Fisher relations and the value of the Hubble constant

This paper presents the calibration of BVRIH-0.5 Tully-Fisher relations based on Cepheid distances to 21 galaxies within 25 Mpc and 23 clusters within 10,000 km s-1. These relations have been applied to several distant cluster surveys in order to derive a value for the Hubble constant, H0, mainly concentrating on an I-band all-sky survey by Giovanelli and collaborators, consisting of total I magnitudes and 50% line width data for ~550 galaxies in 16 clusters. For comparison, we also derive the values of H0 using surveys in the B and V bands by Bothun and collaborators, and in H band by Aaronson and collaborators. Careful comparisons with various other databases from the literature suggest that the H-band data, which have isophotal magnitudes extrapolated from aperture magnitudes rather than total magnitudes, are subject to systematic uncertainties. Taking a weighted average of the estimates of Hubble constants from four surveys, we obtain H0 = 71 ± 4 (random) ± 7 (systematic). We have also investigated how the value of H0 is affected by various systematic uncertainties, such as the internal extinction correction method used, Tully-Fisher slopes and shapes, a possible metallicity dependence of the Cepheid period-luminosity relation, and cluster population incompleteness bias.

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XXVI. The Calibration of Population II Secondary Distance Indicators and the Value of the Hubble Constant

A Cepheid-based calibration is derived for four distance indicators that utilize stars in old stellar populations: the tip of the red giant branch (TRGB), the planetary nebula luminosity function (PNLF), the globular cluster luminosity function (GCLF), and the surface brightness fluctuation method (SBF). The calibration is largely based on the Cepheid distances to 18 spiral galaxies within cz = 1500 km s-1 obtained as part of the Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale, but relies also on Cepheid distances from separate HST and ground-based efforts. The newly derived calibration of the SBF method is applied to obtain distances to four Abell clusters in the velocity range 3800-5000 km s-1. Combined with cluster velocities corrected for a cosmological flow model, these distances imply a value of the Hubble constant of H0 = 69 ? 4 (random) ? 6 (systematic) km s-1 Mpc-1. This result assumes that the Cepheid PL relation is independent of the metallicity of the variable stars; adopting a metallicity correction as in Kennicutt et al. would produce a 5% ? 3% decrease in H0. Finally, the newly derived calibration allows us to investigate systematics in the Cepheid, PNLF, SBF, GCLF, and TRGB distance scales.

The Extragalactic Distance Scale Key Project. IV. The Discovery of Cepheids and a New Distance to M100 Using the Hubble Space Telescope

This paper presents initial observations, including the discovery of 30 Cepheids in the nearby galaxy M81, made using the Wide Field Camera (WFC).

Infrared Space Observatory Measurements of [C II] Line Variations in Galaxies

We report measurements of the [C II] fine-structure line at 157.714 ?m in 30 normal star-forming galaxies with the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). The ratio of the line to total far-infrared (FIR) luminosity, LC II/LFIR, measures the ratio of the cooling of gas to that of dust, and thus the efficiency of the grain photoelectric heating process. This ratio varies by more than a factor of 40 in the current sample. About two-thirds of the galaxies have LC II/LFIR ratios in the narrow range of (2-7) ? 10 -->?3. The other one-third show trends of decreasing LC II/LFIR with increasing dust temperature, as measured by the flux ratio of infrared emission at 60 and 100 ?m, F?(60 ?m)/F?(100 ?m), and with increasing star formation activity, measured by the ratio of FIR and blue-band luminosity, LFIR/L -->B. We also find three FIR-bright galaxies that are deficient in the [C II] line, which is undetected with 3 ? upper limits of LC II/LFIR ?4. The trend in the LC II/LFIR ratio with the temperature of dust and with star formation activity may be due to decreased efficiency of photoelectric heating of gas at high UV radiation intensity as dust grains become positively charged, decreasing the yield and the energy of the photoelectrons. The three galaxies with no observed photodissociation region lines have among the highest LFIR/L -->B and F?(60 ?m)/F?(100 ?m) ratios. Their lack of [C II] lines may be due to a continuing trend of decreasing LC II/LFIR with increasing star formation activity and dust temperature seen in one-third of the sample with warm IRAS colors. In that case, the upper limits on LC II/LFIR imply a ratio of UV flux to gas density of G -->0/n>10 cm -->3 (where G -->0 is in units of the local average interstellar field). The low LC II/LFIR ratio could also be due to either weak [C II], owing to self-absorption, or a strong FIR continuum from regions weak in [C II], such as dense H II regions or plasma ionized by hard radiation of active galactic nuclei. The mid-infrared and radio images of these galaxies show that most of the emission comes from a compact nucleus. CO and H I are detected in these galaxies, with H I seen in absorption toward the nucleus.

The Extragalactic Distance Scale Key Project. XVI. Cepheid Variables in an Inner Field of M101

We report on the identification of 255 candidate variable stars in a field located some 17 from the center of the late-type spiral galaxy M101 = NGC 5457, based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. Photometric measurements in the F555W and F814W filters—analyzed independently with the DAOPHOT/ALLFRAME and DoPHOT software suites—have been transformed to the Johnson V and Kron-Cousins I standard magnitude systems. Periods and intensity-averaged mean magnitudes for 61 carefully selected candidate Cepheid variables with periods in the range 10-48 days indicate a reddening-corrected mean distance modulus (m - M)0 = 29.05 ± 0.14 (if the true modulus of the Large Magellanic Cloud is 18.50 ± 0.10, and if there is no dependence of the period-luminosity relation on metal abundance); results consistent with this are obtained whether or not the sample is expanded to include a larger fraction of the candidates. Applying a metallicity-dependent correction of +0.16 ± 0.10 mag would increase this estimate to (m - M)0 = 29.21 ± 0.17 mag.

The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XV. A Cepheid Distance to the Fornax Cluster and Its Implications

Using the Hubble Space Telescope, 37 long-period Cepheid variables have been discovered in the Fornax Cluster spiral galaxy NGC 1365. The resulting V and I period-luminosity relations yield a true distance modulus of ?0=31.35?0.07 mag, which corresponds to a distance of 18.6?0.6 Mpc. This measurement provides several routes for estimating the Hubble constant. (1) Assuming this distance for the Fornax Cluster as a whole yields a local Hubble constant of 70?18 (random) ?7 (systematic) km s?1 Mpc?1. (2) Nine Cepheid-based distances to groups of galaxies out to and including the Fornax and Virgo Clusters yield H0=73?16 (random) ?7 (systematic) km s?1 Mpc?1. (3) Recalibrating the I-band Tully-Fisher relation using NGC 1365 and six nearby spiral galaxies, and applying it to 15 galaxy clusters out to 100 Mpc, give H0=76?3 (random) ?8 (systematic) km s?1 Mpc?1. (4) Using a broad-based set of differential cluster distance moduli ranging from Fornax to Abell 2147 gives H0=72?3 (random) ?6 (systematic) km s?1 Mpc?1. Finally, (5) assuming the NGC 1365 distance for the two additional Type Ia supernovae in Fornax and adding them to the SN Ia calibration (correcting for light-curve shape) gives H0=67?6 (random) ?7 (systematic) km s?1 Mpc?1 out to a distance in excess of 500 Mpc. All five of these H0 determinations agree to within their statistical errors. The resulting estimate of the Hubble constant, combining all of these determinations, is H0=72?5 (random) ?7 (systematic) km s?1 Mpc?1. An extensive tabulation of identified systematic and statistical errors, and their propagation, is given.

The Mid-Infrared Spectra of Normal Galaxies

The mid-infrared spectra (2.5-5 and 5.7-11.6 µm) obtained by ISOPHOT reveal the interstellar medium emission from galaxies powered by star formation to be strongly dominated by the aromatic features at 6.2, 7.7, 8.6, and 11.3 µm. Additional emission appears in between the features, and an underlying continuum is clearly evident at 3-5 µm. This continuum would contribute about a third of the luminosity in the 3-13 µm range. The features together carry 5%-30% of the 40-120 µm far-infrared (FIR) luminosity. The relative fluxes in individual features depend very weakly on galaxy parameters such as the far-infrared colors, direct evidence that the emitting particles are not in thermal equilibrium. The dip at 10 µm is unlikely to result from silicate absorption since its shape is invariant among galaxies. The continuum component has a fnu~nu+0.65 shape between 3 and 5 µm and carries 1%-4% of the FIR luminosity; its extrapolation to longer wavelengths falls well below the spectrum in the 6-12 µm range. This continuum component is almost certainly of nonstellar origin and is probably due to fluctuating grains without aromatic features. The spectra reported here typify the integrated emission from the interstellar medium of the majority of star-forming galaxies and could thus be used to obtain redshifts of highly extincted galaxies up to z=3 with SIRTF.