John M. Cannon

An Ultraviolet-to-Radio Broadband Spectral Atlas of Nearby Galaxies

The ultraviolet-to-radio continuum spectral energy distributions are presented for all 75 galaxies in the Spitzer Infrared Nearby Galaxies Survey (SINGS). A principal component analysis of the sample shows that most of the samples spectral variations stem from two underlying components, one representative of a galaxy with a low infrared-to-ultraviolet ratio and one representative of a galaxy with a high infrared-to-ultraviolet ratio. The influence of several parameters on the infrared-to-ultraviolet ratio is studied (e.g., optical morphology, disk inclination, far-infrared color, ultraviolet spectral slope, and star formation history). Consistent with our understanding of normal star-forming galaxies, the SINGS sample of galaxies in comparison to more actively star-forming galaxies exhibits a larger dispersion in the infrared-to-ultraviolet versus ultraviolet spectral slope correlation. Early-type galaxies, exhibiting low star formation rates and high optical surface brightnesses, have the most discrepant infrared-to-ultraviolet correlation. These results suggest that the star formation history may be the dominant regulator of the broadband spectral variations between galaxies. Finally, a new discovery shows that the 24 μm morphology can be a useful tool for parameterizing the global dust temperature and ultraviolet extinction in nearby galaxies. The dust emission in dwarf/irregular galaxies is clumpy and warm accompanied by low ultraviolet extinction, while in spiral galaxies there is typically a much larger diffuse component of cooler dust and average ultraviolet extinction. For galaxies with nuclear 24 μm emission, the dust temperature and ultraviolet extinction are relatively high compared to disk galaxies.

On extending the mass-metallicity relation of galaxies by 2.5 decades in stellar mass

We report 4.5 � m luminosities for 27 nearby (DP5 Mpc) dwarf irregular galaxies measured with the Spitzer Infrared Array Camera. We have constructed the 4.5 � m luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with secure distance and interstellar medium oxygen abundance measurements. The 4.5 � m L-Z relation is 12þ log (O/H) ¼ (5:78 � 0:21) þ (� 0:122 � 0:012)M½4:5� , where M[4.5] is the absolute magnitude at 4.5 � m. The dispersion in the near-infrared L-Z relation is smaller than the corresponding dispersion in the optical L-Z relation. The subsequently derived stellar mass-metallicity (M� -Z) relation is 12 þ log (O/H) ¼ (5:65 � 0:23) þ (0:298 � 0:030)log M� , and extends the SDSSM� - Zrelation to lower mass by about 2.5 dex. We find that the dispersion in the M� -Z relation is similar over 5 orders of magnitude in stellar mass, and that the relationship between stellar mass and interstellar medium metallicity is similarly tight from high-mass to low-mass systems. We find a larger scatter at low mass in the relation between effective yield and total baryonic mass. In fact, there are a few dwarf galaxies with large yields, which is difficult to explain if galactic winds are ubiquitous in dwarf galaxies. The low scatter in the L‐Z and M� -Z relationships are difficult to understand if galactic superwinds or blowout are responsible for the low metallicities at low mass or luminosity. Naively, one would expect an ever increasing scatter at lower masses, which is not observed. Subject headingg galaxies: dwarf — galaxies: evolution — galaxies: irregular — infrared: galaxies Online material: color figures

THE NATURE OF STARBURSTS. II. THE DURATION OF STARBURSTS IN DWARF GALAXIES

The starburst phenomenon can shape the evolution of the host galaxy and the surrounding intergalactic medium. The extent of the evolutionary impact is partly determined by the duration of the starburst, which has a direct correlation with both the amount of stellar feedback and the development of galactic winds, particularly for smaller mass dwarf systems. We measure the duration of starbursts in twenty nearby, ongoing, and “fossil” starbursts in dwarf galaxies based on the recent star formation histories derived from resolved stellar population data obtained with the Hubble Space Telescope. Contrary to the shorter times of 3–10 Myr often cited, the starburst durations we measure range from 450 to 650 Myr in fifteen of the dwarf galaxies and up to 1.3 Gyr in four galaxies; these longer durations are comparable to or longer than the dynamical timescales for each system. The same feedback from massive stars that may quench the flickering star formation does not disrupt the overall burst event in our sample of galaxies. While five galaxies present fossil bursts, fifteen galaxies show ongoing bursts and thus the final durations may be longer than we report here for these systems. One galaxy shows a burst that has been ongoing for only 20 Myr; we are likely seeing the beginning of a burst event in this system. Using the duration of the starbursts, we calculate that the bursts deposited 10 53.9 –10 57.2 erg of energy into the interstellar medium through stellar winds and supernovae, and produced 3%−26% of the host galaxy’s mass.

The Recent Star Formation Histories of M81 Group Dwarf Irregular Galaxies

We present observations and analysis of nine dwarf irregular galaxies (dIs) in the M81 Group taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. The nine galaxies (the Garland, M81 Dwarf A, DDO 53, Ho IX, Ho I, DDO 165, NGC 2366, Ho II, and IC 2574) span 6 mag in luminosity, a factor of 1000 in current star formation rate, and 0.5 dex in metallicity. We use color-magnitude diagrams of resolved stellar populations to study the star formation histories (SFHs) of these galaxies. Dividing the sample at MB = − 15, we analyze the similarities and differences in the SFHs, birthrate parameters (b), fraction of stars formed per time interval (f), and spatial distribution of stellar components. As function of luminosity, we find only minor differences in SF characteristics among the M81 Group dIs despite a wide range of physical properties. We extend our comparison to select dIs in the Local Group (LG) and, again, find little variation in SF parameters. Our average f parameters are consistent with a constant SFH; however, individual galaxies can show significant departures from this. We find this result underlines the importance of stochastic SF in dIs. We also compare possible formation scenarios of the fainter and candidate tidal dwarfs (TDGs) in the M81 Group. The SFHs and the spatial distribution of associated red stars suggest that the Garland and Ho IX are not dIs and are potentially TDGs. A noteworthy difference between the LG and the M81 Group is the lack of TDGs in the LG.

Extended Mid-Infrared Aromatic Feature Emission in M82

We present new images (ground-based optical and mid-infrared [MIR] from the Spitzer Space Telescope) and spectra (from Spitzer) of the archetypal starburst galaxy M82. The Spitzer data show that the MIR emission extends at least 6 kpc along the minor axis of the galaxy. We use the optical and infrared data to demonstrate that the extended emission is dominated by emission from dust. The colors of the MIR emission and the spectra indicate that there is a strong component of aromatic feature emission (the MIR features commonly attributed to polycyclic aromatic hydrocarbons). The dust continuum and aromatic feature emission are both strong in the well-known superwind region of this galaxy; clearly, the carrier of the aromatic features can survive in close proximity to the wind, far from the plane of the galaxy. We also see significant emission by dust well outside the superwind region, providing the clearest picture to date of the dust distribution in the halo of this galaxy.

Dust and Atomic Gas in Dwarf Irregular Galaxies of the M81 Group: The SINGS and THINGS View

We present observations of the dust and atomic gas phase in seven dwarf irregular galaxies of the M81 group from the Spitzer SINGS and VLA THINGS surveys. The Spitzer observations provide a first glimpse of the nature of the nonatomic ISM in these metal-poor (Z ~ 0.1 Z_☉), quiescent (SFR ~ 0.001-0.1 M_☉ yr^(-1)) dwarf galaxies. Most detected dust emission is restricted to H I column densities >1 × 10^(21) cm^(-2), and almost all regions of high H I column density (>2.5 × 10^(21) cm^(-2)) have associated dust emission. Spitzer spectroscopy of two regions in the brightest galaxies (IC 2574 and Holmberg II) show distinctly different spectral shapes and aromatic features, although the galaxies have comparable gas-phase metallicities. This result emphasizes that the strength of the aromatic features is not a simple linear function of metallicity. We estimate dust masses of ~10^(4)-10^(6) M_☉ for the M81 dwarf galaxies, resulting in an average dust-to-gas ratio (M_(dust)/M_(H I)) of ~3 × 10^(-4) (1.5 × 10^(-3) if only the H I that is associated with dust emission is considered); this is an order of magnitude lower than the typical value derived for the SINGS spirals. The dwarf galaxies are underluminous per unit star formation rate at 70 μm as compared to the more massive galaxies in SINGS by a factor of ~2. However, the average 70/160 μm ratio in the sample dwarf galaxies is higher than what is found in the other galaxies of the SINGS sample. This can be explained by a combination of a lower dust content in conjunction with a higher dust temperature in the dwarfs.

AN INITIAL LOOK AT THE FAR-INFRARED-RADIO CORRELATION WITHIN NEARBY STAR-FORMING GALAXIES USING THE SPITZER SPACE TELESCOPE

We present an initial look at the far-infrared-radio correlation within the star-forming disks of four nearby, nearly face-on galaxies (NGC 2403, NGC 3031, NGC 5194, and NGC 6946). Using Spitzer MIPS imaging, observed as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS), and Westerbork Synthesis Radio Telescope (WSRT) radio continuum data, taken for the WSRT SINGS radio continuum survey, we are able to probe variations in the logarithmic 24 μm/22 cm (q24) and 70 μm/22 cm (q70) surface brightness ratios across each disk at subkiloparsec scales. We find general trends of decreasing q24 and q70 with declining surface brightness and with increasing radius. The residual dispersion around the trend of q24 and q70 versus surface brightness is smaller than the residual dispersion around the trend of q24 and q70 versus radius, on average by ~0.1 dex, indicating that the distribution of star formation sites is more important in determining the infrared/radio disk appearance than the exponential profiles of disks. We have also performed preliminary phenomenological modeling of cosmic-ray electron (CR electron) diffusion using an image-smearing technique and find that smoothing the infrared maps improves their correlation with the radio maps. We find that exponential smoothing kernels work marginally better than Gaussian kernels, independent of projection for these nearly face-on galaxies. This result suggests that additional processes besides simple random walk diffusion in three dimensions must affect the evolution of CR electrons. The best-fit smoothing kernels for the two less active star-forming galaxies (NGC 2403 and NGC 3031) have much larger scale lengths than those of the more active star-forming galaxies (NGC 5194 and NGC 6946). This difference may be due to the relative deficit of recent CR electron injection into the interstellar medium for the galaxies that have largely quiescent disks.

The Lyman Alpha Reference Sample. Ii. Hubble Space Telescope Imaging Results, Integrated Properties, And Trends

We report new results regarding the Ly alpha output of galaxies, derived from the Lyman Alpha Reference Sample, and focused on Hubble Space Telescope imaging. For 14 galaxies we present intensity images in Ly alpha, H alpha, and UV, and maps of H alpha/H beta, Ly alpha equivalent width (EW), and Ly alpha/H alpha. We present Ly alpha and UV radial light profiles and show they are well-fitted by Sersic profiles, but Ly alpha profiles show indices systematically lower than those of the UV (n approximate to 1-2 instead of greater than or similar to 4). This reveals a general lack of the central concentration in Ly alpha that is ubiquitous in the UV. Photometric growth curves increase more slowly for Ly alpha than the far ultraviolet, showing that small apertures may underestimate the EW. For most galaxies, however, flux and EW curves flatten by radii approximate to 10 kpc, suggesting that if placed at high-z only a few of our galaxies would suffer from large flux losses. We compute global properties of the sample in large apertures, and show total Ly alpha luminosities to be independent of all other quantities. Normalized Ly alpha throughput, however, shows significant correlations: escape is found to be higher in galaxies of lower star formation rate, dust content, mass, and nebular quantities that suggest harder ionizing continuum and lower metallicity. Six galaxies would be selected as high-z Ly alpha emitters, based upon their luminosity and EW. We discuss the results in the context of high-z Ly alpha and UV samples. A few galaxies have EWs above 50 angstrom, and one shows f(esc)(Ly alpha) of 80%; such objects have not previously been reported at low-z.

Dust in I Zw 18 from Hubble Space Telescope* Narrowband Imaging

We present new WFPC2 narrowband imaging of the blue compact dwarf galaxy I Zw 18, which is host to the lowest metallicity H II regions known. Images at Ha and Hb are combined with archival broadband images to allow the study of the ionized gas distribution and morphology. Analysis of the Ha/Hb —ux ratio reveals signi—cant enhancements in some areas of both the northwest and southeast regions of the galaxy, with ratios elevated to levels as high as 3.4. The Ha/Hb ratio varies considerably with position throughout the galaxy. Comparing this distribution with the stellar distribution indicates that the regions of enhanced Ha/Hb ratio are not due to the eUects of either collisional excitation or underlying stellar absorption and therefore are most likely interpreted as the presence of dust. This dust has an estimated mass of (2¨5) ] 103 which is consistent with the IRAS far-IR nondetection. Under M _ , the assumption that dust traces the presence of molecular gas, these results suggest that the molecular component of the interstellar medium of I Zw 18, which is needed to fuel its active star formation, is also very clumpy. Such a distribution would be consistent with the recent Far-Ultraviolet Spectroscopic Explorer nondetections of diUuse H 2 . &

The rarity of dust in metal-poor galaxies

Galaxies observed at redshift z > 6, when the Universe was less than a billion years old, thus far very rarely show evidence of the cold dust that accompanies star formation in the local Universe, where the dust-to-gas mass ratio is around one per cent. A prototypical example is the galaxy Himiko (z = 6.6), which—a mere 840 million years after the Big Bang—is forming stars at a rate of 30–100 solar masses per year, yielding a mass assembly time of about 150 × 106 years. Himiko is thought to have a low fraction (2–3 per cent of the Sun’s) of elements heavier than helium (low metallicity), and although its gas mass cannot yet be determined its dust-to-stellar mass ratio is constrained to be less than 0.05 per cent. The local dwarf galaxy I Zwicky 18, which has a metallicity about 4 per cent that of the Sun’s and is forming stars less rapidly (assembly time about 1.6 × 109 years) than Himiko but still vigorously for its mass, is also very dust deficient and is perhaps one of the best analogues of primitive galaxies accessible to detailed study. Here we report observations of dust emission from I Zw 18, from which we determine its dust mass to be 450–1,800 solar masses, yielding a dust-to-stellar mass ratio of about 10−6 to 10−5 and a dust-to-gas mass ratio of 3.2–13 × 10−6. If I Zw 18 is a reasonable analogue of Himiko, then Himiko’s dust mass must be around 50,000 solar masses, a factor of 100 below the current upper limit. These numbers are quite uncertain, but if most high-z galaxies are more like Himiko than like the very-high-dust-mass galaxy SDSS J114816.64 + 525150.3 at z ≈ 6, which hosts a quasar, then our prospects for detecting the gas and dust inside such galaxies are much poorer than hitherto anticipated.