Kevin V. Croxall

CALIBRATING EXTINCTION-FREE STAR FORMATION RATE DIAGNOSTICS WITH 33 GHz FREE-FREE EMISSION IN NGC 6946

Using free-free emission measured in the Ka band (26-40 GHz) for 10 star-forming regions in the nearby galaxy NGC 6946, including its starbursting nucleus, we compare a number of star formation rate (SFR) diagnostics that are typically considered to be unaffected by interstellar extinction. These diagnostics include non-thermal radio (i.e., 1.4 GHz), total infrared (IR; 8-1000 μm), and warm dust (i.e., 24 μm) emission, along with hybrid indicators that attempt to account for obscured and unobscured emission from star-forming regions including Hα + 24 μm and UV + IR measurements. The assumption is made that the 33 GHz free-free emission provides the most accurate measure of the current SFR. Among the extranuclear star-forming regions, the 24 μm, Hα + 24 μm, and UV + IR SFR calibrations are in good agreement with the 33 GHz free-free SFRs. However, each of the SFR calibrations relying on some form of dust emission overestimates the nuclear SFR by a factor of ~2 relative to the 33 GHz free-free SFR. This is more likely the result of excess dust heating through an accumulation of non-ionizing stars associated with an extended episode of star formation in the nucleus rather than increased competition for ionizing photons by dust. SFR calibrations using the non-thermal radio continuum yield values which only agree with the 33 GHz free-free SFRs for the nucleus and underestimate the SFRs from the extranuclear star-forming regions by an average factor of ~2 and ~4-5 before and after subtracting local background emission, respectively. This result likely arises from the cosmic-ray (CR) electrons decaying within the starburst region with negligible escape, whereas the transient nature of star formation in the young extranuclear star-forming complexes allows for CR electrons to diffuse significantly further than dust-heating photons, resulting in an underestimate of the true SFR. Finally, we find that the SFRs estimated using the total 33 GHz flux density appear to agree well with those estimated using free-free emission due to the large thermal fractions present at these frequencies even when local diffuse backgrounds are not removed. Thus, rest-frame 33 GHz observations may act as a reliable method to measure the SFRs of galaxies at increasingly high redshift without the need of ancillary radio data to account for the non-thermal emission.

THE EMISSION BY DUST AND STARS OF NEARBY GALAXIES IN THE HERSCHEL KINGFISH SURVEY

Using new far-infrared imaging from the Herschel Space Observatory with ancillary data from ultraviolet (UV) to submillimeter wavelengths, we estimate the total emission from dust and stars of 62 nearby galaxies in the KINGFISH survey in a way that is as empirical and model independent as possible. We collect and exploit these data in order to measure from the spectral energy distributions (SEDs) precisely how much stellar radiation is intercepted and re-radiated by dust, and how this quantity varies with galaxy properties. By including SPIRE data, we are more sensitive to emission from cold dust grains than previous analyses at shorter wavelengths, allowing for more accurate estimates of dust temperatures and masses. The dust/stellar flux ratio, which we measure by integrating the SEDs, has a range of nearly three decades (from 10^(−2.2) to 10^(0.5)). The inclusion of SPIRE data shows that estimates based on data not reaching these far-IR wavelengths are biased low by 17% on average. We find that the dust/stellar flux ratio varies with morphology and total infrared (IR) luminosity, with dwarf galaxies having faint luminosities, spirals having relatively high dust/stellar ratios and IR luminosities, and some early types having low dust/stellar ratios. We also find that dust/stellar flux ratios are related to gas-phase metallicity (log(f_(dust)/f_∗) = −0.66 ± 0.08 and −0.22 ± 0.12 for metal-poor and intermediate-metallicity galaxies, respectively), while the dust/stellar mass ratios are less so (differing by ≈0.2 dex); the more metal-rich galaxies span a much wider range of the flux ratios. In addition, the substantial scatter between dust/stellar flux and dust/stellar mass indicates that the former is a poor proxy of the latter. Comparing the dust/stellar flux ratios and dust temperatures, we also show that early types tend to have slightly warmer temperatures (by up to 5 K) than spiral galaxies, which may be due to more intense interstellar radiation fields, or possibly to different dust grain compositions. Finally, we show that early types and early-type spirals have a strong correlation between the dust/stellar flux ratio and specific star formation rate, which suggests that the relatively bright far-IR emission of some of these galaxies is due to ongoing (if limited) star formation as well as to the radiation field from older stars, which is heating the dust grains.

Mapping the cold dust temperatures and masses of nearby KINGFISH galaxies with Herschel

Taking advantage of the unprecedented combination of sensitivity and angular resolution afforded by the Herschel Space Observatory at far-infrared and submillimetre wavelengths, we aim to characterize the physical properties of cold dust within nearby galaxies, as well as the associated uncertainties, namely the robustness of the parameters we derive using different modified blackbody models. For a pilot subsample of the KINGFISH (Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel) key programme, we perform two-temperature fits of the Spitzer and Herschel photometric data (from 24 to 500 μm), with a warm and a cold component, both globally and in each resolution element. We compare the results obtained from different analysis strategies. At global scale, we observe a range of values of the modified blackbody fit parameters β_c (0.8–2.5) and T_c (19.1–25.1 K). We compute maps of our modelling parameters with β_c fixed or treated as a free parameter to test the robustness of the temperature and dust surface density maps we deduce. When the emissivity is fixed, we observe steeper temperature gradients as a function of radius than when it is allowed to vary. When the emissivity is fitted as a free parameter, barred galaxies tend to have uniform fitted emissivities. Gathering the parameters obtained in each resolution element in a T_c–β_c diagram underlines an anticorrelation between the two parameters. It remains difficult to assess whether the dominant effect is the physics of dust grains, noise, or mixing along the line of sight and in the beam. We finally observe in both cases that the dust column density peaks in central regions of galaxies and bar-ends (coinciding with molecular gas density enhancements usually found in these locations). We also quantify how the total dust mass varies with our assumptions about the emissivity index as well as the influence of the wavelength coverage used in the fits. We show that modified blackbody fits using a shallow emissivity (β < 2.0) lead to significantly lower dust masses compared to the β < 2.0 case, with dust masses lower by up to 50 per cent if β_c = 1.5, for instance. The working resolution affects our total dust mass estimates: masses increase from global fits to spatially resolved fits.

Resolving the Far-IR Line Deficit: Photoelectric Heating and Far-IR Line Cooling in NGC 1097 and NGC 4559

The physical state of interstellar gas and dust is dependent on the processes which heat and cool this medium. To probe heating and cooling of the interstellar medium over a large range of infrared surface brightness, on sub-kiloparsec scales, we employ line maps of [C II] 158 μm, [O I] 63 μm, and [N II] 122 μm in NGC 1097 and NGC 4559, obtained with the Photodetector Array Camera & Spectrometer on board Herschel. We matched new observations to existing Spitzer Infrared Spectrograph data that trace the total emission of polycyclic aromatic hydrocarbons (PAHs). We confirm at small scales in these galaxies that the canonical measure of photoelectric heating efficiency, ([C II] + [O I])/TIR, decreases as the far-infrared (far-IR) color, ν_f ν(70 μm) νf_ν(100 μm), increases. In contrast, the ratio of far-IR cooling to total PAH emission, ([C II] + [O I])/PAH, is a near constant ~6% over a wide range of far-IR color, 0.5 < νf_ν(70 μm) νf_ν(100 μm) ≾ 0.95. In the warmest regions, where νf_ν(70 μm) νf_ν(100 μm) ≳ 0.95, the ratio ([C II] + [OI])/PAH drops rapidly to 4%. We derived representative values of the local ultraviolet radiation density, G_0, and the gas density, n_H, by comparing our observations to models of photodissociation regions. The ratio G_0/n_H, derived from fine-structure lines, is found to correlate with the mean dust-weighted starlight intensity, langUrang, derived from models of the IR spectral energy distribution. Emission from regions that exhibit a line deficit is characterized by an intense radiation field, indicating that small grains are susceptible to ionization effects. We note that there is a shift in the 7.7/11.3 μm PAH ratio in regions that exhibit a deficit in ([C II] + [O I])/PAH, suggesting that small grains are ionized in these environments.

LITHIUM IN THE INTERMEDIATE-AGE OPEN CLUSTER, NGC 3680

High-dispersion spectra centered on the Li 6708 A line have been obtained for 70 potential members of the intermediate-age open cluster NGC 3680, with an emphasis on stars in the turnoff region of the cluster color-magnitude diagram (CMD). A measurable Li abundance has been derived for 53 stars, 39 of which have radial velocities and proper motions consistent with cluster membership. After being transferred to common temperature and abundance scales, previous Li estimates have been combined to generate a sample of 49 members, 40 of which bracket the cluster Li-dip. Spectroscopic elemental analysis of eight giants and five turnoff stars produces [Fe/H] = –0.17 ± 0.07 (sd) and –0.07 ± 0.02 (sd), respectively. We also report measurements of Ca, Si, and Ni which are consistent with scaled-solar ratios within the errors. Adopting [Fe/H] = –0.08 (Section 3.6), Y 2 isochrone comparisons lead to an age of 1.75 ± 0.1 Gyr and an apparent modulus of (m – M) = 10.30 ± 0.15 for the cluster, placing the center of the Li-dip at 1.35 ± 0.03 M ☉. Among the giants, five of the nine cluster members are now known to have measurable Li with A(Li) near 1.0. A combined sample of dwarfs in the Hyades and Praesepe is used to delineate the Li-dip profile at 0.7 Gyr and [Fe/H] = +0.15, establishing its center at 1.42 ± 0.02 M ☉ and noting the possible existence of a secondary dip on its red boundary. When evolved to the typical age of the clusters NGC 752 (age = 1.45 Gyr, (m – M) = 8.4), IC 4651 (age = 1.5 Gyr, (m – M) = 10.4), and NGC 3680, the Hyades/Praesepe Li-dip profile reproduces the observed morphology of the combined Li-dip within the CMDs of the intermediate-age clusters while implying a metallicity dependence for the central mass of the Li-dip given by M/M ☉ = 1.38 ± 0.04 + 0.4 ± 0.2 [Fe/H]. The implications of the similarity of the Li-dichotomy among giants in NGC 752 and IC 4651 and the disagreement with the pattern among NGC 3680 giants are discussed.

A STUDY OF HEATING AND COOLING OF THE ISM IN NGC 1097 WITH HERSCHEL-PACS AND SPITZER-IRS

NGC 1097 is a nearby Seyfert 1 galaxy with a bright circumnuclear starburst ring, a strong large-scale bar, and an active nucleus. We present a detailed study of the spatial variation of the far-infrared (FIR) [C II]158 μm and [O I]63 μm lines and mid-infrared H_2 emission lines as tracers of gas cooling, and of the polycyclic aromatic hydrocarbon (PAH) bands as tracers of the photoelectric heating, using Herschel-PACS and Spitzer-IRS infrared spectral maps. We focus on the nucleus and the ring, and two star-forming regions (Enuc N and Enuc S). We estimated a photoelectric gas heating efficiency ([C II]158 μm+[O I]63 μm)/PAH in the ring about 50% lower than in Enuc N and S. The average 11.3/7.7 μm PAH ratio is also lower in the ring, which may suggest a larger fraction of ionized PAHs, but no clear correlation with [C II]158 μm/PAH(5.5-14 μm) is found. PAHs in the ring are responsible for a factor of two more [C II]158 μm and [O I]63 μm emission per unit mass than PAHs in the Enuc S. spectral energy distribution (SED) modeling indicates that at most 25% of the FIR power in the ring and Enuc S can come from high-intensity photodissociation regions (PDRs), in which case G_0 ~ 10^(2.3) and n_H ~ 10^(3.5) cm^(–3) in the ring. For these values of G_0 and n_H, PDR models cannot reproduce the observed H2 emission. Much of the H2 emission in the starburst ring could come from warm regions in the diffuse interstellar medium that are heated by turbulent dissipation or shocks.

THE DISPLACED DUSTY INTERSTELLAR MEDIUM OF NGC 3077: TIDAL STRIPPING IN THE M 81 TRIPLET

We present the detection of extended (~30?kpc2) dust emission in the tidal H I arm near NGC?3077 (member of the M?81 triplet) using SPIRE on board Herschel. Dust emission in the tidal arm is typically detected where the H I column densities are >1021?cm?2. The SPIRE band ratios show that the dust in the tidal arm is significantly colder (~13?K) than in NGC?3077 itself (~31?K), consistent with the lower radiation field in the tidal arm. The total dust mass in the tidal arm is ~1.8 ? 106?M ? (assuming ? = 2), i.e., substantially larger than the dust mass associated with NGC?3077 (~2 ? 105?M ?). Where dust is detected, the dust-to-gas ratio is (6 ? 3) ? 10?3, consistent within the uncertainties with what is found in NGC?3077 and nearby spiral galaxies with Galactic metallicities. The faint H II regions in the tidal arm cannot be responsible for the detected enriched material and are not the main source of the dust heating in the tidal arm. We conclude that the interstellar medium (atomic H I, molecules, and dust) in this tidal feature was pre-enriched and stripped off NGC?3077 during its recent interaction (~3 ? 108?yr ago) with M?82 and M?81. This implies that interaction can efficiently remove heavy elements and enriched material (dust and molecular gas) from galaxies. As interactions were more frequent at large look-back times, it is conceivable that they could substantially contribute (along with galactic outflows) to the enrichment of the intergalactic medium.

THE 2001-2003 LOW STATE OF NOVA LACERTAE 1950 (DK LAC)

We report on extensive photometry of DK Lac obtained during the interval 1990–2009, which includes a 2 mag low state during 2001–2003. Much of the photometry consists of exposures obtained with a typical spacing of several days, but also includes 26 sequences of continuous photometry each lasting 2–7 hr. We find no evidence for periodicities in our data. We do find that the random variations in the low state are approximately twice those in the high state, when expressed in magnitudes. The lack of orbital-timescale variations is attributed to the nearly face-on presentation of the disk. There is a 0.2 mag decline in the high-state brightness of the system over 19 years, which is consistent with the behavior of other old novae in the decades following outburst. High-state spectra are also presented and discussed. We find that the equivalent width of Hα falls by about double from 1991 to 2008. The photometric properties are discussed in the context of the hibernation scenario for the behavior of novae between outbursts, in which we conclude that low states in old novae are probably unrelated to their possible entrance into hibernation.

The Survey of Lines in M31 (SLIM): The Drivers of the [C ii]/TIR Variation

The ratio of the [CII] 158

CHEMICAL ABUNDANCES OF SEVEN IRREGULAR AND THREE TIDAL DWARF GALAXIES IN THE M81 GROUP

\,\mu