Star Formation Indicators Research Articles

A Search for Dense Molecular Gas in High‐Redshift Infrared‐Luminous Galaxies

We present a search for HCN emission from four high redshift far infrared (IR) luminous galaxies. Current data and models suggest that these high $z$ IR luminous galaxies represent a major starburst phase in the formation of spheroidal galaxies, although many of the sources also host luminous active galactic nuclei (AGN), such that a contribution to the dust heating by the AGN cannot be precluded. HCN emission is a star formation indicator, tracing dense molecular hydrogen gas within star-forming molecular clouds (n(H$_2$) $\sim 10^5$ cm$^{-3}$). HCN luminosity is linearly correlated with IR luminosity for low redshift galaxies, unlike CO emission which can also trace gas at much lower density. We report a marginal detection of HCN (1-0) emission from the $z=2.5832$ QSO J1409+5628, with a velocity integrated line luminosity of $L_{\rm HCN}'=6.7\pm2.2 \times10^{9}$ K km s$^{-1}$ pc$^2$, while we obtain 3$\sigma$ upper limits to the HCN luminosity of the $z=3.200$ QSO J0751+2716 of $L_{\rm HCN}'=1.0\times10^{9}$ K km s$^{-1}$ pc$^2$, $L_{\rm HCN}'=1.6\times10^{9}$ K km s$^{-1}$ pc$^2$ for the $z= 2.565$ starburst galaxy J1401+0252, and $L_{\rm HCN}'=1.0\times10^{10}$ K km s$^{-1}$ pc$^2$ for the $z = 6.42$ QSO J1148+5251. We compare the HCN data on these sources, plus three other high-$z$ IR luminous galaxies, to observations of lower redshift star-forming galaxies. The values of the HCN/far-IR luminosity ratios (or limits) for all the high $z$ sources are within the scatter of the relationship between HCN and far-IR emission for low $z$ star-forming galaxies (truncated).

A Very Large Array Search for Water Masers in Six HiiRegions: Tracers of Triggered Low‐Mass Star Formation

We present a search for water maser emission at 22 GHz associated with young low-mass protostars in six H II regions—M16, M20, NGC 2264, NGC 6357, S125, and S140. The survey was conducted with the NRAO Very Large Array from 2000 to 2002. For several of these H II regions, ours are the first high-resolution observations of water masers. We detected 16 water masers: eight in M16, four in M20, three in S140, and one in NGC 2264. All but one of these were previously undetected. No maser emission was detected from NGC 6357 or S125. There are two principle results to our study. (1) The distribution of water masers in M16 and M20 does not appear to be random but instead is concentrated in a layer of compressed gas within a few tenths of a parsec of the ionization front. (2) Significantly fewer masers are seen in the observed fields than expected based on other indications of ongoing star formation, indicating that the maser-exciting lifetime of protostars is much shorter in H II regions than in regions of isolated star formation. Both of these results confirm predictions of a scenario in which star formation is first triggered by shocks driven in advance of ionization fronts and then truncated ~105 yr later when the region is overrun by the ionization front.

History of star formation rate and luminosity density of galaxies

We have computed the time evolution of bolometric, far-infrared, Hα line, ultraviolet (both intrinsic and escaping the star forming region) and the nonthermal radio continuum luminosities for continuous and constant star formation terminating at 95 Myr. The luminosity rises to a plateau value and declines after the termination of starburst, but only gradually. The time evolution profiles are broad and different for different star formation indicators. The broad profiles lead to uncertainties in the star formation rate derived depending on the initial mass function, duration of starburst, its distribution and the observational epoch.

Nonnuclear Hyper/Ultraluminous X-Ray Sources in the Starbursting Cartwheel Ring Galaxy

We report the Chandra/ACIS-S detection of more than 20 ultraluminous X-ray sources (ULXs; L0.5-10 keV ≳ 3 × 1039 ergs s-1) in the Cartwheel collisional ring galaxy system, of which over a dozen are located in the outer active star-forming ring. A remarkable hyperluminous X-ray source (HLX; L0.5-10 keV ≳ 1041 ergs s-1 assuming isotropic radiation), which dominates the X-ray emission from the Cartwheel ring, is located in the same segment of the ring as most ULXs. These powerful H/ULXs appear to be coincident with giant H II region complexes, young star clusters, and radio and mid-infrared hot spots: all strong indicators of recent massive star formation. The X-ray spectra show that H/ULXs have similar properties as those of the most luminous ULXs found in the nearest starbursts and galaxy mergers such as the Antennae galaxies and M82. The close association between the X-ray sources and the starbursting ring strongly suggests that the H/ULXs are intimately associated with the production and rapid evolution of short-lived massive stars. The observations represent the most extreme X-ray luminosities discovered to date associated with star-forming regions—rivaling the X-ray luminosities usually associated with active galactic nuclei.

The First Deep Advanced Camera for Surveys Lyα Images of Local Starburst Galaxies

We report the first results from a deep Lyα imaging program of local starburst galaxies with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The two observed galaxies ESO 350-IG038 and SBS 0335-052 have luminosities similar to those of the Magellanic Clouds but differ in their chemical composition. ESO 350-IG038 has an oxygen abundance of 1/8 solar, whereas SBS 0335-052 is known to have one of the lowest abundances among blue galaxies (~1/30). The ACS imaging reveals a complex Lyα morphology, with sometimes strong offsets between the emission of Lyα and the location of stellar light, ionized gas traced by Hα, and the neutral gas. Overall, more Lyα photons escape from the more metal- and dust-rich galaxy ESO 350-IG038. The absence of clear SBS 0335-052 Lyα emission over all observed knots, whatever their dust content or/and color indices, contradicts model expectations of a lower escape fraction from dust-rich gas because of destruction of Lyα photons by dust grains. Rather, the results are in qualitative agreement with models suggesting the kinematic properties of the gas as the dominant Lyα escape regulator. If the properties of the two observed galaxies are representative of starburst galaxies in general, Lyα will be difficult to interpret as a star formation indicator, in particular if based on Lyα imaging at low spatial resolution.

Star Formation Rates in Interacting Starburst Galaxies

By narrowband imaging in Hα and in the adjacent red stellar continuum we have studied the rate and distribution of star formation in 43 systems of luminous and ultraluminous IR galaxies currently undergoing interaction and merging. These galaxies are amongst the most luminous at 60 μm and range in distance from ~50 up to 100 Mpc. Here we present the Hα and the adjacent red-continuum narrowband images, and we compare the star formation rates derived from Hα with those estimated from the IR luminosity. We find clear evidence for substantial extinction and obscuration of star-forming regions in the optical. Without correction for reddening in the host galaxy or correction for [N II] contamination, the star formation rates derived for Hα are typically 0.5-1.0 dex lower than those estimated from the IR flux, and the scatter in the correlation is very large. However, an unexpected result is that when spectroscopic data are used to eliminate objects dominated by an active nucleus, to determine the galaxian extinction, and to correct the Hα flux for both reddening and for the contamination by the [N II] emission, a remarkably good correlation emerges between the star formation rates estimated from the Hα flux and those derived from the FIR continuum. In addition, a strong correlation is found between the extinction in the line-emitting region, AHα, and the rate of star formation. Our results invalidate the use of Hα imaging as a reliable indicator of star formation in starburst galaxies unless spectroscopic data are also available. This has important implications for the determination of star formation rates in high-redshift galaxies. Finally, we find no correlation between the measured star formation rates, and the interaction class, suggesting that the enhanced star formation rates triggered by the interaction continue throughout the whole of the merging sequence.

Star Formation in the [ITAL]Infrared Space Observatory[/ITAL] Atlas of Bright Spiral Galaxies

We investigate star formation along the Hubble sequence using the ISO Atlas of Spiral Galaxies. Using mid-infrared and far-infrared flux densities normalized by K-band flux densities as indicators of recent star formation, we find several trends. First, star formation activity is stronger in late-type (Sc - Scd) spirals than in early-type (Sa - Sab) spirals. This trend is seen both in nuclear and disk activity. These results confirm several previous optical studies of star formation along the Hubble sequence but conflict with the conclusions of most of the previous studies using IRAS data, and we discuss why this might be so. Second, star formation is significantly more extended in later-type spirals than in early-type spirals. We suggest that these trends in star formation are a result of differences in the gas content and its distribution along the Hubble sequence, and it is these differences that promote star formation in late-type spiral galaxies. We also search for trends in nuclear star formation related to the presence of a bar or nuclear activity. The nuclear star formation activity is not significantly different between barred and unbarred galaxies. We do find that star formation activity appears to be inhibited in LINERs and transition objects compared to HII galaxies. The mean star formation rate in the sample is 1.4 Msun/yr based on global far-infrared fluxes. Combining these data with CO data gives a mean gas consumption time of 6.4 x 10^8 yr, which is ~5 times lower than the values found in other studies. Finally, we find excellent support for the Schmidt Law in the correlation between molecular gas masses and recent star formation in this sample of spiral galaxies.

Far infrared and radio emission in dusty starburst galaxies

We revisit the nature of the far infrared (FIR)/radio correlation by means of the most recent models of star forming galaxies, focusing in particular on the case of obscured starbursts. We model the IR emission with our population synthesis code, GRASIL (Silva et al. [CITE]). For the radio emission, we revisit the simple model of Condon & Yin ([CITE]). We find that a tight FIR/radio correlation is natural when the synchrotron mechanism dominates over the inverse Compton, and the electron cooling time is shorter than the fading time of the supernova (SN) rate. Observations indicate that both these conditions are met in star forming galaxies, from normal spirals to obscured starbursts. However, since the radio non–thermal (NT) emission is delayed, deviations are expected both in the early phases of a starburst, when the radio thermal component dominates, and in the post-starburst phase, when the bulk of the NT component originates from less massive stars. We show that this delay allows the analysis of obscured starbursts with a time resolution of a few tens of Myrs, unreachable with other star formation (SF) indicators. We suggest a strategy to complement the analysis of the deviations from the FIR/radio correlation with the radio slope (q–radio slope diagram) to obtain characteristic parameters of the burst, e.g. its intensity, age and fading time scale. The analysis of a sample of compact ULIRGs shows that they are intense but transient starbursts, to which one should not apply usual SF indicators devised for constant SF rates. We also discuss the possibility of using the q–radio slope diagram to assess the presence of obscured AGN. A firm prediction of the models is an apparent radio excess during the post-starburst phase, which seems to be typical of a class of star forming galaxies in rich cluster cores. Finally we discuss how deviations from the correlation, due to the evolutionary status of the starburst, affect the technique of photometric redshift determination widely used for high-z sources.

The 2dF Galaxy Redshift Survey: galaxy luminosity functions per spectral type

We calculate the optical bj luminosity function of the 2dF Galaxy Redshift Survey (2dFGRS) for different subsets defined by their spectral properties. These spectrally selected subsets are defined using a new parameter, eta, which is a linear combination of the first two projections derived from a Principal Component Analysis. This parameter eta identifies the average emission and absorption line strength in the galaxy rest-frame spectrum and hence is a useful indicator of the present star formation. We use a total of 75,000 galaxies in our calculations, chosen from a sample of high signal-to-noise ratio, low redshift galaxies observed before January 2001. We find that there is a systematic steepening of the faint end slope (alpha) as one moves from passive (alpha = -0.54) to active (alpha = -1.50) star-forming galaxies, and that there is also a corresponding faintening of the rest-frame characteristic magnitude M* - 5\log_10(h) (from -19.6 to -19.2). We also show that the Schechter function provides a poor fit to the quiescent (Type 1) LF for very faint galaxies (M - 5log_10(h) fainter than -16.0), perhaps suggesting the presence of a significant dwarf population. The luminosity functions presented here give a precise confirmation of the trends seen previously in a much smaller preliminary 2dFGRS sample, and in other surveys. We also present a new procedure for determining self-consistent K-corrections and investigate possible fibre-aperture biases.

Radio continuum and CO emission in star-forming galaxies

We combine the radio continuum images from the NRAO VLA Sky Survey with the CO-line observations from the extragalactic CO survey of the Five College Radio Astronomy Observatory to study the relationship between molecular gas and the star formation rate within the disks of 180 spiral galaxies at 45 resolution. We find a tight correlation between these quantities. On average, the ratio between the radio continuum and the CO emission is constant, within a factor of 3, both inside the same galaxy and from galaxy to galaxy. The mean star formation efficiency deduced from the radio continuum corresponds to convert 3.5% of the available molecular gas into stars on a time scale of 10^8 yr and depends weakly on general galaxy properties, such as Hubble type or nuclear activity. A comparison is made with another similar analysis performed using the H_alpha luminosity as star formation indicator. The overall agreement we find between the two studies reinforces the use of the radio luminosity as star formation rate indicator not only on global but also on local scales.

A Large-Scale Study of H13CO+ and C18 (J=1–0) in Orion B

Abstract We have made a new survey for dense cloud cores in the Orion B region using the NANTEN telescope. Two molecular transitions, the $J = 1 - 0$ line of $\mathrm{H}{{13\atop} \mathrm{CO}}^{+}$ and the $J = 1 - 0$ line of $\mathrm{C}{{18\atop} \mathrm{O}}$, were used to study the distribution of dense molecular gas with densities of $10^{4} - 10^{5} \,\mathrm{cm}^{-3}$. We detected 19 $\mathrm{C}{{18\atop} \mathrm{O}}$ clumps, and the mass and the size of the $\mathrm{C}{{18\atop} \mathrm{O}}$ clumps ranged from 13${M}_{\odot}$ to 990${M}_{\odot}$ and from 0.26 pc to 0.69 pc, respectively. Among the physical parameters of these $\mathrm{C}{{18\atop} \mathrm{O}}$ clumps, the molecular column density, $N(\mathrm{H}_2)$, has been found to be the best indicator of star formation; the molecular column density averaged within a $\mathrm{C}{{18\atop} \mathrm{O}}$ clump shows a good correlation with the IRAS luminosity as $L_\mathrm{IRAS} / {{{L}_{\odot}}} \propto N(\mathrm{H}_{2}) \; (\mathrm{cm}^{-2})^{4.8 \pm 0.6}$ (C.C.$= 0.96$). We also detected 11 $\mathrm{H}{{13\atop} \mathrm{CO}}^{+}$ clumps, whose mass ranges from a few ${M}_{\odot}$ to 480${M}_{\odot}$. Five of the $\mathrm{H}{{13\atop} \mathrm{CO}}^{+}$ clumps are of smaller mass, and show no indication of massive star formation; they were newly found in this survey. Five of the eleven $\mathrm{H}{{13\atop} \mathrm{CO}}^{+}$ clumps are particularly massive with an average mass of $\sim 200 \,{{{M}_{\odot}}}$. It is found that these massive $\mathrm{H}{{13\atop} \mathrm{CO}}^{+}$ clumps exhibit active formation of massive stars, as indicated by associated protostellar IRAS point sources, whose luminosities are $10^{2} - 10^{4} \,{{{L}_{\odot}}}$.

Molecular Gas and the Modest Star Formation Efficiency in the “Antennae” Galaxies: Arp 244 = NGC 4038/9

We report here a factor of 5.7 higher total CO flux in Arp 244 (the Antennae galaxies) than that previously accepted in the literature (thus a total molecular gas mass of 1.5 × 1010 M☉), based on our fully sampled CO(1-0) observations at the NRAO 12 m telescope. Currently, much of the understanding and modeling of the star formation in Arp 244 has been derived using a much lower molecular gas mass. It is imperative to reconsider everything, as the high molecular gas mass might provide sufficient fuel for ultraluminous extreme starburst in Arp 244 once the merging advances to late stage. Our observations show that the molecular gas peaks predominately in the disk-disk overlap region between the nuclei, similar to the far-infrared (FIR) and mid-infrared (MIR) emission. The bulk of the molecular gas is forming into stars with a normal star formation efficiency (SFE) LIR/M(H2) ≈ 4.2 L☉/M☉, the same as that of giant molecular clouds in the Galactic disk. Additional supportive evidence is the extremely low fraction of the dense molecular gas in Arp 244, revealed by our detections of the HCN(1-0) emission, which traces the active star-forming gas at density 104 cm-3. Using the high-resolution BIMA + NRAO 12 m telescope, full-synthesis CO(1-0) images and our VLA continuum maps at 20 cm, we estimate the local SFE indicated by the ratio map of the radio continuum to CO(1-0) emission, down to kiloparsec scale. Remarkably, the local SFE stays roughly the same over the bulk of the molecular gas distribution. Only some localized regions show the highest radio-to-CO ratios that we have identified as the sites of the most intense starbursts with SFE 20 L☉/M☉. Here we have assumed that the 20 cm emission is a fairly good indicator of star formation down to kiloparsec scale in Arp 244 because of the well-known correlation between the FIR and the radio continuum emission. These starburst regions are confined exclusively to the dusty patches seen in the Hubble Space Telescope optical images near the CO and FIR peaks where the violent starbursts are presumably heavily obscured. Nevertheless, recent large-scale star formation is going on throughout the system (e.g., concentrations of numerous super-star clusters and MIR hotspots), yet the measured level is more suggestive of a moderate starburst (SFE 10 L☉/M☉) or a weak to normal star formation (SFE ~ 4 L☉/M☉), not necessarily occurring at the high concentrations of the molecular gas reservoir. The overall starburst from the bulk of the molecular gas is yet to be initiated as most of the gas further condenses into a kiloparsec scale in the final coalescence.

A Possible Evolutionary Connection between Active Galactic Nuclei and Starbursts in LINER[CLC]s[/CLC

Our analysis of the two magnitude-limited samples of LINERs suggests a correlation between LFIR/LB, or f(25 μm)/f(60 μm), and Hubble-type index at greater than 99.99% significance level. As LFIR/LB and f(25 μm)/f(60 μm) are considered to be indicators of star formation activity and active galactic nuclei (AGN) activity, respectively, our results suggest that LINERs with higher AGN activity may have a lower star formation contribution. The ones with highest AGN activity and lowest star formation contributions are elliptical galaxies. All well-studied type 1 LINERs belong to this group. On the other hand, LINERs with higher star formation activity present lower AGN contributions. We find all well-studied type 2 LINERs in this parameter space. Most LINERs having inner ring structures belong to this group. Statistics with other indicators of star formation or AGN activity (a nuclear mass-to-light ratio at the H band and the ratio of X-ray to UV power) provide further evidence for such a trend. We have seen that along with the evolution of galaxies from late-type spiral galaxies to early-type ones and up to elliptical galaxies, the intensity of AGN activity increases with decreasing star formation contributions. The above analyses may suggest a possible connection between the host galaxies and nuclear activities, and it might also indicate a possible evolutionary connection between AGNs and starbursts in LINERs.

The Infrared Side of Galaxy Formation. I. The Local Universe in the Semianalytical Framework

We present a new evolutionary model for predicting the far-UV-to-submillimeter properties of the galaxy population. This combines a semianalytic galaxy formation model based on hierarchical clustering (GALFORM) with a spectrophotometric code that includes dust reprocessing (GRASIL). The former provides the star formation and metal enrichment histories, together with the gas mass and various geometrical parameters, for a representative sample of galaxies formed in different density environments. These quantities, together with a few other assumptions concerning the spatial distribution of dust and its optical properties, allow us to model the spectral energy distributions (SEDs) of galaxies, taking into account stellar emission and also dust extinction (absorption plus scattering) and reemission. In the spectrophotometric code, dust is considered only in the disk, but the general radiation field is contributed by both the disk and the bulge components with their own distinct age and metallicity distributions. Two phases are considered for the dust: molecular cloud complexes, where stars are assumed to be born, and the diffuse interstellar medium. The model includes both galaxies forming stars quiescently in disks and starbursts triggered by galaxy mergers. We test our models against the observed spectrophotometric properties of galaxies in the local universe, assuming a cold dark matter cosmology with Ω0 = 0.3 and Λ0 = 0.7. The models reproduce fairly well the SEDs of normal spirals and starbursts from the far-UV to the submillimeter and their internal extinction properties. The starbursts follow the observed relationship between the far-IR-to-UV luminosity ratio and the slope of the UV continuum. They also reproduce the observed starburst attenuation law. This result is remarkable because we use a dust mixture that reproduces the Milky Way extinction law. It suggests that the observed attenuation law is closely related to the geometry of the stars and dust. We compute galaxy luminosity functions over a wide range of wavelengths, which turn out to be in good agreement with observational data in the UV (2000 A), in the B and K bands, and in the IR (12-100 μm). Finally, we investigate the reliability of some star formation indicators that are based on the properties of the continuum SEDs of galaxies. The UV continuum turns out to be a poor star formation indicator for our models, while the infrared luminosity is much more reliable.

A Dark-Cloud Complex in Aquila: Small Molecular Clouds Possibly Associated with the Aquila Rift

Abstract A 12CO(J = 1−0) survey for local molecular clouds was performed toward dark clouds in Aquila (26° < l ≤ 42° and −25° ≤ b < −2°) by using the 4-meter millimeter wave telescope, NANTEN, at Las Campanas Observatory, Chile. A cloud complex consisting of 64 small clouds has been discovered in −25° ≲ b ≲ −12°; at a distance of 220 pc, its height from the galactic plane is ∼ 50–100 pc and the total mass is ∼ 4 × 103M⊙. The spatial and velocity distributions of the complex suggest that it may be connected to the Great Rift in Aquila. This complex, as a whole, has a significantly large virial mass compared with the mass derived from the CO intensities by an order of magnitude, though H I gas of ∼ 104M⊙, possibly associated, may contribute to bind them gravitationally. The individual CO clouds have velocity dispersion and mass similar to those of the high-latitude clouds; also, the clouds are not in gravitational equilibrium. There is no indication of active star formation.

A Near-Infrared Spectroscopic Study of 60 Micron Peakers

We present near-infrared (NIR) spectroscopy of a sample of IRAS-selected galaxies with warm far-infrared colors and spectral energy distributions peaking near 60 ?m. Of the 21 60 ?m peakers classified by optical spectroscopy as Seyfert 2 galaxies, we confirm the existence of dust-obscured broad lines in four (Mkn 3, Mkn 1210, Mkn 463, IRAS 20460). We also propose that two additional 60 ?m peakers may have weak broad lines in the NIR (UGC 02024, IRAS 06488), but we require higher quality data for confirmation. All four of the 60 ?m peakers with confirmed broad NIR lines are known to have broad lines in polarized light, while the two possible candidates have not been observed with spectropolarimetry so far as we are aware. These would be good candidates for future spectropolarimetric observations on large telescopes. Within our sample of 60 ?m peakers, the average ratio of He I/Br? is larger for the starburst galaxies relative to the Seyferts. This is consistent with an evolutionary scenario for 60 ?m peakers, with the starburst class representing an early phase of evolution following an interaction event and the Seyfert class representing a later stage in which the nuclear starburst has faded and has either collapsed to form a central black hole or at least to fuel an existing one. The He I/Br? ratio for the starburst class of 60 ?m peakers is typical of a young starburst with a burst age less than 5 Myr, which is in agreement with their optical spectra. Various near-infrared spectroscopic features ([Fe II]/Pa?, K-band continuum shape, CO absorption, and line widths) are generally consistent with their optical spectroscopic classifications. The exceptions are: IRAS 00160, IRAS 08007A, and IRAS 10567, which are classified as starburst via optical spectroscopy, yet some of their NIR spectral features point toward the existence of a dust-obscured AGN. The stellar absorption CO feature, an indicator of star formation, is detected more strongly in the starburst 60 ?m peakers compared to the Seyfert class, presumably because of dilution by hot dust emission from the active nucleus in the Seyfert 60 ?m peakers.

A Test of the Standard Hypothesis for the Origin of the H [CSC]i[/CSC] Holes in Holmberg II

The nearby irregular galaxy Holmberg II (Ho II, DDO 50) has been extensively mapped in H I using the Very Large Array, revealing intricate structure in its interstellar gas component, as reported by Puche et al. in 1992. An analysis of these structures shows the neutral gas to contain a number of expanding H I holes. The formation of the H I holes has been attributed to multiple supernova (SN) events occurring within wind-blown shells around young, massive star clusters, with as many as 10–200 SNe required to produce many of the holes. From the sizes and expansion velocities of the holes, Puche et al. assigned ages of ~107 to 108 years. If the SN scenario for the formation of the H I holes is correct, it implies the existence of star clusters with a substantial population of late B, A, and F main-sequence stars at the centers of the holes. Many of these clusters should be detectable in deep ground-based CCD images of the galaxy. To test the SN hypothesis for the formation of the H I holes, we have obtained and analyzed deep broadband BVR and narrowband Hα images of Ho II. We compare the optical and H I data and search for evidence of the expected star clusters in and around the H I holes. We also use the H I data to constrain models of the expected remnant stellar population. Assuming that the H I holes are created by multiple SNe, that the number of SNe required can be determined from the observed energetics of the holes, and that the SNe represent the high-mass population of a cluster with a normal initial mass function, we show that in several of the holes the observed upper limits for the remnant cluster brightness are strongly inconsistent with the SN hypothesis described by Puche et al. Moreover, many of the H I holes are located in regions of very low optical surface brightness that show no indication of recent star formation. Here we present our findings, discuss their implications, and explore possible alternative explanations for the existence of the H I holes in Ho II, including the recent suggestion that some of the holes could be produced by gamma-ray burst events.

High-Latitude Radio Emission in a Sample of Edge-on Spiral Galaxies

We have mapped 16 edge-on galaxies at 20 cm using the Very Large Array in its C configuration, and a subset of these galaxies in the D configuration at 6 and/or 20 cm, in a search for extended (1 kpc) radio continuum emission above and below the plane. For five galaxies, we could form spectral index, energy, and magnetic field maps (assuming minimum energy). While the galaxies were partly chosen by radio flux density, they span a variety of star formation rates (SFRs), and only six might be considered starburst galaxies. A range of Hubble type and degree of isolation are also represented. The galaxies largely fall on the FIR–radio continuum correlation. They also display a correlation between IR surface brightness and warmth, extending the previously observed relation of Lehnert & Heckman to galaxies with lower star formation rates. We find that all but one galaxy show evidence for nonthermal high-latitude radio continuum emission, suggesting that cosmic-ray (CR) halos are common in star-forming galaxies. Of these, eight galaxies are new detections. The high-latitude emission is seen over a variety of spatial scales and in discrete and/or smooth features. In some cases, discrete features are seen on large scales, suggesting that smooth radio halos may consist, in part, of discrete features combined with low spatial resolution. In general, the discrete features emanate from the disk, but estimates of CR diffusion lengths suggest that diffusion alone is insufficient to transport the particles to the high latitudes seen (>15 kpc in one case). Thus CRs likely diffuse through low-density regions and/or are assisted by other mechanisms (e.g., winds). We searched for correlations between the prevalence of high-latitude radio emission and a number of other properties, including the global SFR, supernova input rate per unit star-forming area, A, and environment, and do not find clear correlations with any of these properties. A subset of the data allows, at best, for only a weak correlation with A. Our one nondetection (NGC 4517), however, occurs at a threshold level consistent with that found by Dahlem, Lisenfeld, & Golla. The lack of a good correlation with star formation indicators could be the result of the different timescales for star formation processes compared with the duration of the radio emission. Correlations with other properties, such as environment, are more difficult to assess. However, a few isolated galaxies display strong radio halos, indicating that an interaction is not necessary to produce the extraplanar emission.

HCO+in the Starburst Galaxy M82

We report observations and an analysis of the distribution of HCO+(1-0) and HCO+(4-3) emission in the central 1 kpc star forming region of M82. Comparisons are made with other star formation indicators such as the mm continuum, the distribution of radio SNR's and the molecules CO and OH. In a broad sense, the HCO+ is distributed in a way similar to the CO, although there are noticeable differences in detail, including an inward displacement of spiral arm emission relative to CO. A comparison of the position-velocity plots for CO, HCO+(1-0), HCO+(4-3), and ionized gas, with orbits expected in the presence of the nuclear bar suggest an inward transfer of gas associated with star formation toward the nucleus. The HCO+(4-3)/(1-0) line ratios are comparatively uniform in the observed region, and according to an LVG analysis, reflect mean gas densities in the range 10^4 cm^{-3} - 10^5 cm^{-3} for kinetic temperatures in the range 20K - 60K. The comparative uniformity of these conditions and the low filling factor suggest that each sampled point comprises alarge number of clouds occupying a broad range of density, and possibly temperature. We briefly examine fractal type models in the context of the HCO+ data as an alternative way to analyze molecular line emission in M82.

Photometric Observations of Star Formation Activity in Early-Type Spiral Galaxies

We observationally study the current star formation activities of early-type spiral galaxies. We construct a complete sample of 15 early-type spiral galaxies having ratios of far-infrared (FIR) to optical B-band luminosity, log (LFIR/LB), larger than average for this type and present CCD imaging of the R and Hα bands. The equivalent widths of Hα emission increase with increasing LFIR/LB, indicating that log (LFIR/LB) can be an indicator of star formation for such early-type spiral galaxies with star formation activities higher than average. For all of the observed early-type spiral galaxies, the extended H II regions exist at the central regions with some asymmetric features. Hα emission is more concentrated to the galactic center than the R-band light, and the degree of the concentration increases with the star formation activity. We also analyze the relation between the star formation activities and the existence of companion galaxies in the sample galaxies and other bright early-type spiral galaxies. No correlation is found; this suggests that the interaction is not responsible for all of the star formation activities of early-type spiral galaxies.