arXiv subjects

Robert C. Kennicutt Jr.

Publications and source records attributed to Robert C. Kennicutt Jr..

27 records · Page 2Linked to original sources

arXiv2006

The Nature of Infrared Emission in the Local Group Dwarf Galaxy NGC 6822 As Revealed by Spitzer

We present Spitzer imaging of the metal-deficient (Z ~30% Z_sun) Local Group dwarf galaxy NGC 6822. On spatial scales of ~130 pc, we study the nature of IR, H alpha, HI, and radio continuum emission. Nebular emission strength correlates with IR surface brightness; however, roughly half of the IR emission is associated with diffuse regions not luminous at H alpha (as found in previous studies). The global ratio of dust to HI gas in the ISM, while uncertain at the factor of ~2 level, is ~25 times lower than the global values derived for spiral galaxies using similar modeling techniques; localized ratios of dust to HI gas are about a factor of five higher than the global value in NGC 6822. There are strong variations (factors of ~10) in the relative ratios of H alpha and IR flux throughout the central disk; the low dust content of NGC 6822 is likely responsible for the different H alpha/IR ratios compared to those found in more metal-rich environments. The H alpha and IR emission is associated with high-column density (> ~1E21 cm^-2) neutral gas. Increases in IR surface brightness appear to be affected by both increased radiation field strength and increased local gas density. Individual regions and the galaxy as a whole fall within the observed scatter of recent high-resolution studies of the radio-far IR correlation in nearby spiral galaxies; this is likely the result of depleted radio and far-IR emission strengths in the ISM of this dwarf galaxy.

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arXiv2006

Ultraviolet Through Far-Infrared Spatially Resolved Analysis of the Recent Star Formation in M81 (NGC3031)

The recent star formation (SF) in the early-type spiral galaxy M81 is characterized using imaging observations from the far-ultraviolet (UV) to the far-infrared (IR). We compare these data with models of the stellar, gas, and dust emission for sub-galactic regions. Our results suggest the existence of a diffuse dust emission not directly linked to the recent SF. We find a radial decrease of the dust temperature and dust mass density, and in the attenuation of the stellar light. The IR emission in M81 can be modeled with three components: 1) cold dust with a temperature =18+-2 K, concentrated near the HII regions but also presenting a diffuse distribution; 2) warm dust with T_w=53+-7 K, directly linked with the HII regions; and 3) aromatic molecules, with diffuse morphology peaking around the HII regions. We derive several relationships to obtain total IR luminosities from IR monochromatic fluxes, and we compare five different star formation rate (SFR) estimators for HII regions in M81 and M51: the UV, Halpha, and three estimators based on Spitzer data. We find that the Halpha luminosity absorbed by dust correlates tightly with the 24 microns emission. The correlation with the total IR luminosity is not as good. Important variations from galaxy to galaxy are found when estimating the total SFR with the 24 microns or the total IR emission alone. The most reliable estimations of the total SFRs are obtained by combining the Halpha emission (or the UV) and an IR luminosity (especially the 24 microns emission), which probe the unobscured and obscured SF, respectively. For the entire M81 galaxy, about 50% of the total SF is obscured by dust. The percentage of obscured SF ranges from 60% in the inner regions of the galaxy to 30% in the outer zones.

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arXiv2004

The Most Exciting Massive Binary Cluster in NGC 5128: Clues to the Formation of Globular Clusters

VLT images in BVI are used to identify the ionizing source centered on Sersic 13, the largest HII region of the giant nearby galaxy NGC 5128 with log L_Halpha=39.6 erg/s. This ionizing source turns out to be a close pair of bright and blue star cluster candidates. Spectroscopy obtained with the Magellan I telescope confirms that these are massive young clusters physically associated with the giant HII region Sersic 13. The spectra of both clusters show prominent Wolf-Rayet type emission features, and prominent lines of HI and HeI, indicative of a very young age (t ~ few x 10^6 yr). Their luminosities make each of them at least as luminous as the massive young cluster R136 in 30 Doradus in the LMC, and their individual masses are estimated to be 1-7.5x10^5 M_sun. In addition, the projected separation of the cluster pair is 42 pc. The measured velocity difference between the clusters is small, ΔV=49+/-21 km/s, and within 2 sigma of the expected orbital velocity V_orb=5-12 km/s if they are bound. Dynamical models predict that binary clusters with these properties would merge in a short timescale of a few orbital periods (P=20-50x10^6 yr). The discovery of this binary cluster suggests that mergers of young massive clusters could lead to the formation of the most massive globular clusters such as omega Cen in our Galaxy and G1 in M31. Alternatively, if they are not gravitationally bound, these objects would individually evolve into two normal globular clusters.

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arXiv2001

The effects of dust in simple environments: Large Magellanic Cloud HII regions

We investigate the effects of dust on Large Magellanic Cloud (LMC) HII region spectral energy distributions using arcminute-resolution far-ultraviolet (FUV), H-alpha, far-infrared (FIR), and radio images. Widely-used indicators of the amount of light lost to dust (attenuation) at H-alpha and in the FUV correlate with each other, although often with substantial scatter. There are two interesting systematic discrepancies. First, H-alpha attenuations estimated from the Balmer decrement are lower than those estimated from the H-alpha-to-thermal radio luminosity ratio. Our data, at this stage, cannot unambiguously identify the source of this discrepancy. Second, the attenuation at 1500 angstroms and UV spectral slope, beta, correlate, although the slope and scatter are substantially different from the correlation first derived for starbursting galaxies by Calzetti et al. Combining our result with those of Meurer et al. for ultra-luminous infrared galaxies and Calzetti et al. for starbursting galaxies, we conclude that no single relation between beta and 1500 angstrom attenuation is applicable to all star-forming systems.

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arXiv2000

A Comparison of UIT Far-Ultraviolet and H alpha Star Formation Rates

We have used archival ultraviolet (UV) imaging of 50 nearby star-forming galaxies obtained with the Ultraviolet Imaging Telescope (UIT) to derive integrated near-UV and far-UV magnitudes, and have combined these data with H alpha, far-infrared, and thermal radio continuum measurements to explore the consistency of UV and H alpha star formation rates (SFRs). In agreement with previous studies, we find that the UV and H alpha SFRs are qualitatively consistent, even before corrections for extinction are applied. The uncorrected UV SFRs are systematically lower by a factor of 1.5 (with a factor of two scatter) among luminous galaxies with SFRs over 1 solar mass per year, indicating a higher effective attenuation of the far-UV radiation. Among less luminous galaxies there is no significant offset between the H alpha and far-UV SFR scales. Far-infrared and thermal radio continuum data available for a subset of our sample allow us to estimate the attenuation in the UV and at H alpha independently. The UV and H alpha attenuations appear to be correlated, and confirm systematically higher attenuations in the UV. Although the galaxies in our sample show modest levels of attenuation (with median values of 0.9 mag at H alpha and 1.4 mag at 1550 Angstroms), the range across the sample is large, around 4 mag for H alpha and around 5 mag in the far-UV (1550 Angstroms). This indicates that the application of a single characteristic extinction correction to H alpha or UV SFRs is only realistic for large, well-defined and well-studied galaxy samples, and that extinction bias may be important for UV or emission-line selected samples of star-forming galaxies.

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arXiv2000

Comparing Ultraviolet and H alpha Star Formation Rates

We have used a sample of 43 star-forming galaxies imaged in the far-ultraviolet (FUV) by the UIT supplemented with 33 galaxies observed by FAUST, to explore the consistency of UV and H alpha derived star formation rates (SFRs). We find, even before correction for dust, that UV and H alpha SFRs are quantitatively consistent for low-luminosity galaxies, and that higher luminosity galaxies have H alpha SFRs a factor of 1.5 higher than their UV SFRs: this reflects the influence of dust. Our results are consistent with a scenario where the UV dust extinction is a factor of less than 2 larger than the H alpha dust extinction in a given galaxy, and that there is over a 4 magnitude range of H alpha dust extinctions which correlate loosely with galaxy luminosity such that low luminosity galaxies tend to have lower extinctions than their higher luminosity counterparts.

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arXiv2000

Lopsided Galaxies, Weak Interactions and Boosting the Star Formation Rate

To investigate the link between weak tidal interactions in disk galaxies and the boosting of their recent star formation, we obtain images and spatially integrated spectra (3615A < lambda < 5315A) for 40 late-type spiral galaxies (Sab-Sbc) with varying degrees of lopsidedness (a dynamical indicator of weak interactions). We quantify lopsidedness as the amplitude of the m=1 Fourier component of the azimuthal surface brightness distribution, averaged over a range of radii. We compare the young stellar content, quantified by EW(Hδ_abs) and the strength of the 4000 Angstrom break (D_4000), with lopsidedness and find a 3-4 sigma correlation between the two. We also find a 3.2 sigma correlation between EW(Hβ_emission) and lopsidedness. Using the evolutionary population synthesis code of Bruzual & Charlot we model the spectra as an ``underlying population'' and a superimposed ``boost population'' with the aim of constraining the fractional boost in the SFR averaged over the past 0.5 Gyr (the characteristic lifetime of lopsidedness). From the difference in both EW(Hδ_abs) and D_4000 between the most and least symmetric thirds of our sample, we infer that ~ 1x10^9 M_solar of stars are formed over the duration of a lopsided event in addition to the ``underlying'' SFH (assuming a final galactic stellar mass of 10^10 M_solar). This corresponds to a factor of 8 increase in the SFR over the past 5x10^8 years. For the nuclear spectra, all of the above correlations except D_4000 vs. are weaker than for the disk, indicating that in lopsided galaxies, the SF boost is not dominated by the nucleus.

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arXiv1998

On Measuring Chemical Abundances in Distant Galaxies Using Global Emission Line Spectra

The advent of 8--10 meter class telescopes enables direct measurement of the chemical properties in the ionized gas of cosmologically--distant galaxies with the same nebular analysis techniques used in local H II regions. We show that spatially unresolved (i.e., global) emission line spectra can reliably indicate the chemical properties of distant star-forming galaxies. However, standard nebular chemical abundance measurement methods (those with a measured electron temperature from [O III] lambda4363) may be subject to small systematic errors when the observed volume includes a mixture of gas with diverse temperatures, ionization parameters, and metallicities. To characterize these systematic effects, we compare physical conditions derived from spectroscopy of individual H II regions with results from global galaxy spectroscopy. We consider both low-mass, metal poor galaxies with uniform abundances and larger galaxies with internal chemical gradients. Well-established empirical calibrations using strong-line ratios can serve as reliable (~0.2 dex) indicators of the overall systemic oxygen abundance even when the signal-to-noise of the Hbeta and [O III] emission lines is as low as 8:1. We present prescriptions, directed toward high-redshift observers, for using global emission line spectra to trace the chemical properties of star-forming galaxies in the distant universe. [abridged]

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arXiv1995

Chemical Abundances in Virgo Spiral Galaxies II: Effects of Cluster Environment

We present new measurements of chemical abundances in H II regions in spiral galaxies of the Virgo cluster and a comparison of Virgo galaxies and field spirals. With these new data there now exist nine Virgo spirals with abundance measurements for at least four H II regions. Our sample of Virgo galaxies ranges from H I deficient objects near the core of the cluster to galaxies with normal H I properties, far from the cluster core. We investigate the relationship between H I disk characteristics and chemical abundances to determine whether dynamical process that remove gas from the disk, such as ram pressure stripping by the intracluster medium, also affect the chemical abundances.

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