Current Star Formation Activity Research Articles

Star Formation Properties of a Large Sample of Irregular Galaxies

We present the results of Hα imaging of a large sample of irregular galaxies. Our sample includes 94 galaxies with morphological classifications of Im, 26 blue compact dwarfs (BCDs), and 20 Sm systems. The sample spans a large range in galactic parameters, including integrated absolute magnitude (MV of -9 to -19), average surface brightness (20–27 mag arcsec-2), current star formation activity (0–1.3 M⊙ yr-1 kpc-2), and relative gas content (0.02–5 M⊙/LB). The Hα images were used to measure the integrated star formation rates, determine the extents of star formation in the disks, and compare azimuthally averaged radial profiles of current star formation to older starlight. The integrated star formation rates of Im galaxies normalized to the physical size of the galaxy span a range of a factor of 104 with 10% Im galaxies and one Sm system having no measurable star formation at the present time. The BCDs fall, on average, at the high star formation rate end of the range. We find no correlation between star formation activity and proximity to other cataloged galaxies. Two galaxies located in voids are similar in properties to the Sm group in our sample. The H II regions in these galaxies are most often found within the Holmberg radius RH, although in a few systems H II regions are traced as far as 1.7RH. Similarly, most of the star formation is found within three disk scale lengths RD, but in some galaxies H II regions are traced as far as 6RD. A comparison of Hα surface photometry with V-band surface photometry shows that the two approximately follow each other with radius in Sm galaxies, but in most BCDs there is an excess of Hα emission in the centers that drops with radius. In approximately half of the Im galaxies Hα and V correspond well, and in the rest there are small to large differences in the relative rate of falloff with radius. The cases with strong gradients in the LHα/LV ratios and with high central star formation rate densities, which include most of the BCDs, require a significant fraction of their gas to migrate to the center in the last gigayear. We discuss possible torques that could have caused this without leaving an obvious signature, including dark matter bars and past interactions or mergers with small galaxies or H I clouds. There is now a substantial amount of evidence for these processes among many surveys of BCDs. We note that such gas migration will also increase the local pressure and possibly enhance the formation of massive dense clusters but conclude that the star formation process itself does not appear to differ much among BCD, Im, and Sm types. In particular, there is evidence in the distribution function for Hα surface brightness that the turbulent Mach numbers are all about the same in these systems. This follows from the Hα distribution functions corrected for exponential disk gradients, which are log-normal with a nearly constant dispersion. Thus, the influence of shock-triggered star formation is apparently no greater in BCDs than in Im and Sm types.

LX-SFR relation in star-forming galaxies

We compare the results of Grimm, Gilfanov & Sunyaev and Ranalli, Comastri & Seti on the LX–SFR (X-ray luminosity–star formation rate) relation in normal galaxies. Based on the LX–stellar mass dependence for low-mass X-ray binaries (LMXBs), we show that low-SFR (≲1 M⊙ yr−1) galaxies in the Ranalli et al. sample are contaminated by the X-ray emission from LMXBs, unrelated to the current star formation activity. However, the most important conclusion from our comparison is that, after the data are corrected for the ‘LMXB contamination’, the two data sets become consistent with each other, despite differences in their content, variability effects, adopted source distances, X-ray fluxes and SFR determinations, and also in the cosmological parameters used in interpreting the Hubble Deep Field North (HDF-N) data. They also agree well, both in the low- and high-SFR regimes, with the predicted LX–SFR dependence derived from the parameters of the ‘universal’ high-mass X-ray binary (HMXB) luminosity function. This encouraging result emphasizes the potential of the X-ray luminosity as an independent SFR indicator for normal galaxies.

The 2dF Galaxy Redshift Survey: galaxy clustering per spectral type

We have calculated the two-point correlation functions in redshift space, xi(sigma,pi), for galaxies of different spectral types in the 2dF Galaxy Redshift Survey. Using these correlation functions we are able to estimate values of the linear redshift-space distortion parameter, beta = Omega_m^0.6/b, the pairwise velocity dispersion, a, and the real-space correlation function, xi(r), for galaxies with both relatively low star-formation rates (for which the present rate of star formation is less than 10% of its past averaged value) and galaxies with higher current star-formation activity. At small separations, the real-space clustering of passive galaxies is very much stronger than that of the more actively star-forming galaxies; the correlation-function slopes are respectively 1.93 and 1.50, and the relative bias between the two classes is a declining function of radius. On scales larger than 10 h^-1 Mpc there is evidence that the relative bias tends to a constant, b(passive)/b(active) ~ 1. This result is consistent with the similar degrees of redshift-space distortions seen in the correlation functions of the two classes -- the contours of xi(sigma,pi) require beta(active)=0.49+/-0.13, and beta(passive)=0.48+/-0.14. The pairwise velocity dispersion is highly correlated with beta. However, despite this a significant difference is seen between the two classes. Over the range 8-20 h^-1 Mpc, the pairwise velocity dispersion has mean values 416+/-76 km/s and 612+/-92 km/s for the active and passive galaxy samples respectively. This is consistent with the expectation from morphological segregation, in which passively evolving galaxies preferentially inhabit the cores of high-mass virialised regions.

Infrared Emission of Normal Galaxies from 2.5 to 12 Micron:Infrared Space ObservatorySpectra, Near‐Infrared Continuum, and Mid‐Infrared Emission Features

We present ISOPHOT spectra of the regions 2.5-4.9 μm and 5.8-11.6 μm for a sample of 45 disk galaxies from the US Infrared Space Observatory Key Project on Normal Galaxies. The galaxies were selected to span the range in global properties of normal, star-forming disk galaxies in the local universe. The spectra can be decomposed into three spectral components: (1) continuum emission from stellar photospheres, which dominates the near-infrared (NIR; 2.5-4.9 μm) spectral region; (2) a weak NIR excess continuum, which has a color temperature of ~103 K, carries a luminosity of a few percent of the total far-infrared (FIR) dust luminosity LFIR and most likely arises from the interstellar medium (ISM); and (3) the well-known broad emission features at 6.2, 7.7, 8.6, and 11.3 μm, which are generally attributed to aromatic carbon particles. These aromatic features in emission (AFEs) dominate the mid-infrared (MIR; 5.8-11.6 μm) part of the spectrum and resemble the so-called type A spectra observed in many nonstellar sources and the diffuse ISM in our own Galaxy. The few notable exceptions include NGC 4418, where a dust continuum replaces the AFEs in MIR, and NGC 1569, where the AFEs are weak and the strongest emission feature is [S IV] 10.51 μm. The relative strengths of the AFEs vary by 15%-25% among the galaxies. However, little correlation is seen between these variations and either IRAS 60 μm/100 μm flux density ratio R(60/100) or the FIR/blue luminosity ratio LFIR/LB, two widely used indicators of the current star formation activity, suggesting that the observed variations are not a consequence of the radiation field differences among the galaxies. We demonstrate that the NIR excess continuum and AFE emission are correlated, suggesting that they are produced by similar mechanisms and similar (or the same) material. On the other hand, as the current star formation activity increases, the overall strengths of the AFEs and the NIR excess continuum drop significantly with respect to that of the FIR emission from large dust grains. In particular, the summed luminosity of the AFEs falls from ~0.2 LFIR for the most IR-quiescent galaxies to ~0.1 LFIR for the most IR-active galaxies. This is likely a consequence of the preferential destruction in intense radiation fields of the small carriers responsible for the NIR/AFE emission.

Hidden star-formation in the cluster of galaxies Abell 1689

At a redshift of 0.18, Abell 1689 is so far the most distant cluster of galaxies for which substantial mid–infrared (MIR) data have been published. Its mapping with the ISOCAM camera onboard the ISO satellite allowed the detection of 30 cluster members at 6.75 (LW 2 filter) and 16 cluster members at 15 (LW 3 filter) within a clustercentric radius of 0.5 Mpc [CITE][ Paper I]Fadda00b. We present here the follow–up optical photometric and spectroscopic observations which were used to study the individual properties of the galaxy members of A1689. We confirm the high fraction of blue galaxies initially reported in this rich cluster by [CITE], that was challenged by some subsequent studies. We discuss the spectral and morphological properties of all cluster members in our spectroscopic sample, and of the MIR–detected galaxies in particular. Sources with a low [ 15 ] /[ 6.75 ] flux ratio typically consist of luminous passive early–type galaxies while those with a high MIR color index are mainly luminous, blue, emission–line, morphologically disturbed spirals, i.e. the star–forming galaxies usually associated with the “Butcher–Oemler” effect. On the other hand, at least 30% of the 15 sources have optical counterparts showing no evidence for current star–formation activity, while their 15 emission is most likely due to obscured star formation. We argue that the LW 3 luminosity measured in the cluster members is a reliable tracer of the total infrared luminosity which in A1689 galaxies peaks at . We derive from a star–formation rate free of dust extinction, SFR (IR), which we compare with that determined in the optical from the flux of the emission line, SFR (opt). The highest total star formation rates (11 ) and dust extinction are measured in those galaxies exhibiting in their optical spectrum a signature of a dusty starburst. In contrast, none of the galaxies with post-starburst optical spectra has been detected by ISOCAM down to a 15 flux limit corresponding to 1.4 . We find a median SFR (IR) of the LW 3–detected galaxies of 2 that is ten times higher than the median SFR (opt) of the – detected galaxies. The ratio SFR (IR)/SFR (opt) is in fact very high, ranging between 10 and 100 for LW 3–detected galaxies with emission. We conclude that a major part, at least 90%, of the star formation activity taking place in Abell 1689 is hidden. Whether the high extinction measured in the star-forming cluster members results from the cluster environment itself or reflects a comparable extinction in the coeval field is still unclear.

Old Stellar Populations of the Small Magellanic Cloud

We present WFPC2 and ground-based VI photometry of NGC 121 and a nearby field in the outer SMC. For NGC 121, we measure a true distance modulus of (m-M)0 = 18.96 +/- 0.04 (distance of 61.9 +/- 1.1 kpc), age of 10.6 +/- 0.5 Gyr, metallicity of [Fe/H] = -1.03 +/- 0.06, and initial mass of 4.1 +/- 0.4 x 10^5 Msun, assuming a Salpeter IMF with lower cutoff at 0.1 Msun. In the outer SMC field, we find evidence of stars covering a wide range of ages -- from 2 Gyr old to at least 9-12 Gyr old. We have measured the distance, extinction, and star formation history (past star formation rates and enrichment history) using a CMD fitting algorithm. The distance modulus of the SMC is measured to be (m-M)0 = 18.88 +/- 0.08, corresponding to a distance of 59.7 +/- 2.2 kpc. The overall star formation rate appears to have been relatively constant over this period, although there may be small gaps in the star forming activity too small to be resolved. The lack of current star-forming activity is a selection effect, as the field was intentionally chosen to avoid recent activity. The mean metallicity of this field has increased from an average of [Fe/H] = -1.3 +/- 0.3 for stars older than 8 Gyr to [Fe/H] = -0.7 +/- 0.2 in the past 3 Gyr.

H [CSC]i[/CSC] Imaging of Cassiopeia [CSC]i[/CSC

We present H I maps of a recently discovered dwarf galaxy near the IC 342 Maffei 1–2 Group, Cassiopeia 1. Our data show that Cas 1 has a total H I mass of 1.5 × 108 M⊙, assuming a distance of 4.5 Mpc. The highest column density gas forms a ring around a central depression in the H I distribution and the peak of the H I gas is offset from the distribution of starlight. Isovelocity contours indicate that the galaxy is in solid-body rotation; a region of large velocity dispersion is approximately coincident with observed current star formation activity in Cas 1.

High spatial resolution observations of CUDSS14A: a SCUBA-selected ultraluminous galaxy at high redshift

We present a high-resolution millimetre interferometric image of the brightest SCUBA-selected galaxy from the Canada–UK Deep SCUBA survey (CUDSS). We make a very clear detection at 1.3 mm, but fail to resolve any structure in the source. The interferometric position is within 1.5 arcsec of the SCUBA 850-μm centroid, and also within 1.5 arcsec of a 44-μJy radio source and a very faint, extremely red galaxy which we had previously identified as the submillimetre source. We also present new optical and infrared imaging, and infrared spectroscopy of this source. We model the overall spectral energy distribution and conclude that it lies within the redshift range 2<z<4.5. The submillimetre/far-infrared luminosity of CUDSS14A is very weakly dependent on redshift within the constrained range, and is roughly 4×1012 L⊙ (for H0=75 km s−1 Mpc−1 and an assumed Arp 220-like spectrum), which implies a star formation rate ∼1000 M⊙ yr−1. We derive an approximate gas mass of ∼1010 M⊙ which would imply that the current star-forming activity cannot be sustained for longer than about 107 yr. With the present data, however, we are unable to rule out a significant AGN contribution to the total luminosity.

Lopsidedness in dwarf irregular galaxies

We quantify the amplitude of the lopsidedness, the azimuthal angular asymmetry index and the concentration of star-forming regions, as represented by the distribution of the Hα emission, in a sample of 78 late-type irregular galaxies. We bin the observed galaxies into two groups representing blue compact galaxies (BCDs) and low-surface-brightness dwarf galaxies (LSBs). The light distribution is analysed with a novel algorithm, which allows detection of details in the light distribution pattern. We find that while the asymmetry of the underlying continuum light, representing the older stellar generations, is relatively small, the Hα emission is very asymmetric and is correlated in position angle with the continuum light. We show that the concentration of continuum light is correlated with the Hα concentration; this implies that the young star formation has the same spatial properties as the older stellar populations, but that these properties are more strongly expressed by the young stars. We test a model of random star formation over the extent of a galaxy by simulating H ii regions in artificial dwarf galaxies. A galaxy is traced by assuming red star clusters distributed on an underlying exponential disc of radius twice the scalelength. The disc is allowed to change in apparent magnitude, scaleradius, position angle and ellipticity. We compare the asymmetry-concentration distribution predicted by the simulations with the real observed distribution; we find that only LSBs match the distribution predicted by the model. The reason is that, independently of the number of H ii regions, LSBs show no particular location of H ii regions, whereas BCDs show current star formation activity restricted very much to the central parts of the galaxies. A consideration of the properties of the continuum light leads to the conclusion that most of LSBs can be approximated by exponential discs of radius twice their scalelength; BCDs call, however, for much more concentrated underlying systems, with smaller scalelengths than assumed in the simulations. The implication is that random star formation over the full extent of a galaxy may be generated in LSB dwarf irregular galaxies but not in BCD galaxies.

Dust in an Extremely Metal‐Poor Galaxy: Mid‐infrared Observations of SBS 0335−052

The metal-deficient (Z=Z☉/41) blue compact dwarf galaxy SBS 0335-052 was observed with ISOCAM between 5 and 17 μm. With an L12μm/LB ratio of 2.15, the galaxy is unexpectedly bright in the mid-infrared for such a low-metallicity object. The mid-infrared spectrum shows no sign of the unidentified infrared bands, which we interpret as an effect of the destruction of their carriers by the very high UV energy density in SBS 0335-052. The spectral energy distribution (SED) is dominated by a very strong continuum, which makes the ionic lines of [S IV] and [Ne III] very weak. From 5 to 17 μm, the SED can be fitted with a graybody spectrum, modified by an extinction law similar to that observed toward the Galactic center, with an optical depth of AV ~19-21 mag. Such a large optical depth implies that a large fraction (as much as ~75%) of the current star formation activity in SBS 0335-052 is hidden by dust with a mass between 3 × 103 and 5 × 105 M☉. Silicate grains that are present as silicate extinction bands at 9.7 and 18 μm can account for the unusual shape of the MIR spectrum of SBS 0335-052. It is remarkable that such a nearly primordial environment contains as much dust as galaxies that are 10 times more metal-rich. If the hidden star formation in SBS 0335-052 is typical of young galaxies at high redshifts, then the cosmic star formation rate derived from UV/optical fluxes would be underestimated.

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.

Ultraviolet Imaging Telescope Observations of the Magellanic Clouds

We present wide-field far-ultraviolet (FUV; 1300–1800 A) images of the Large and Small Magellanic Clouds (LMC, SMC). These data were obtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-1 (1990 December 1–10) and Astro-2 (1995 March 2–18) missions; the images provide an extensive FUV mosaic of the SMC and contain numerous regions in the LMC, covering a wide range of stellar densities and current star formation activity. A total of 47 LMC/Lucke-Hodge and 37 SMC/Hodge OB associations are completely or partially included in the observed fields. FUV data can identify the hottest OB stars more easily than can optical photometry, and these stars dominate the ionizing flux, which is correlated to the observed Hα flux of the associated H II regions. Of the H II regions in the catalog of Davies, Elliott, & Meaburn (DEM), the UIT fields completely or partially include 102 DEM regions in the LMC and 74 DEM regions in the SMC. We present a catalog of FUV magnitudes derived from point-spread function photometry for 37,333 stars in the LMC (the UIT FUV magnitudes for 11,306 stars in the SMC were presented recently by Cornett et al.), with a completeness limit of mUV ≈ 15 mag and a detection limit of mUV ≈ 17.5. The average uncertainty in the photometry is ~0.1 mag. The full catalog with astrometric positions, photometry, and other information is also available from publicly accessible astronomical data archives. We divide the catalog into field stars and stars that are in DEM regions. We analyze each of these two sets of stars independently, comparing the composite UV luminosity function of our data with UV magnitudes derived from stellar evolution and atmosphere models in order to derive the underlying stellar formation parameters. We find a most probable initial mass function (IMF) slope for the LMC field stars of Γ = -1.80 ± 0.09. The statistical significance of this single slope for the LMC field stars is extremely high, though we also find some evidence for a field star IMF slope of Γ ~ -1.4, roughly equal to the Salpeter slope. However, in the case of the stars in the DEM regions (the stars in all the regions were analyzed together as a single group), we find three IMF slopes of roughly equal likelihood: Γ = -1.0, -1.6, and -2.0. No typical age for the field stars is found in our data for time periods up to a continuous star formation age of 500 Myr, which is the maximum age consistent with the completeness limit magnitude of the catalog's luminosity function. The best age for the collection of cluster stars was found to be t0 = 3.4 ± 1.9 Myr; this is consistent with the age expected for a collection of OB stars from many different clusters.

The Distribution of Mid- and Far-Infrared Emission in 10 Interacting Galaxy Systems

A sample of 10 interacting and merging galaxy systems has been mapped at mid-infrared (10 μm) and far-infrared (100–160 μm) wavelengths with spatial resolutions of 4''and 45'', respectively. The interacting and merging systems are known to be strong IRAS sources; however, the IRAS data contain limited information concerning the distribution of infrared emission between the two galaxies in each pair and no information on the distribution within each galaxy. The observations presented here show that in five of the seven paired systems, one galaxy is the dominant or the only infrared source. There is a good correlation between the Hα and infrared emission levels for these galaxies, indicating that the infrared emission is closely tied to dust heated by young stars. There is also a good spatial correlation between the Hα, infrared, and radio continuum morphologies of these systems, which further strengthens the belief that the infrared, as well as the radio emission, is associated with regions of active star formation. The 10 μm emission is concentrated around the nuclei of most of these galaxies, indicating that the highest levels of current star formation activity are also confined to the near-nuclear regions. Some systems, however, show evidence for significant star formation activity throughout the outer disk regions of the galaxies. All the sample galaxies that are bright in the infrared also have optical indications of high levels of star formation or nuclear activity, suggesting that these conditions are necessary in order for a galaxy to exhibit excess infrared emission.

The Relationship between Gas, Stars, and Star Formation in Irregular Galaxies: A Test of Simple Models

Irregular galaxies are a unique test of models for the physical laws regulating star formation because of their lack of spiral density waves and rotational shear. Here we explore various instability models for the onset of star formation in irregular galaxies. If the gas is unstable, clouds and eventually stars can form, and so these models should predict where star formation occurs. Critical gas densities were calculated for gravitational instabilities in two models, one with a thin, pure-gas disk (Σc) and another with a thick disk composed of gas and a starlike fluid (Σc,2f). We also calculated the stability properties of three-dimensional systems including dark matter, considered the thermal state of the gas, and used a modified threshold column density written in terms of the local rate of shear instead of the epicyclic frequency. The model predictions were compared to the azimuthally averaged present-day star formation activity traced by the Hα surface brightness and to the 1 Gyr integrated star formation activity represented by the stellar surface brightness. We find that the ratio of the observed gas density to the critical gas density, Σg/Σc, is lower by a factor of ~2 in most of the Im galaxies than it is in spiral galaxies, both at the intermediate radii where Σg/Σc is highest and at the outer radii where star formation ends. We also find that although star formation in irregulars usually occurs at intermediate radii where Σg/Σc is highest, this activity often ends before Σg/Σc drops significantly in the outer regions, and it remains high in the inner regions where Σg/Σc is often low. There are also no correlations between the peak, average, or edge values of Σg/Σc and the integrated star formation rates in irregulars. These results suggest that Σg/Σc does not trace star formation with the same detail in irregular galaxies as it appears to trace it in giant spiral galaxies. The low value of α also implies that either the gas in irregulars is more stable than it is in spirals, or Σc is not a good threshold for star-forming instabilities. Dark matter in the disks of irregulars makes the gas more unstable, but stars do the same for the disks of spirals, which leaves the ratio of the two α-values about the same. Moreover, the instability parameter with dark matter still does not follow the star formation activity in irregulars. The thermal model suggests that irregulars have difficulty in sustaining a cool, dense gas phase, and it also fails to predict where star formation occurs. An alternative model in which cloud formation involves a competition between self-gravity and shear, rather than an instability in the usual sense, is more successful in defining the threshold for star formation, but it does not predict where star formation ends either. The failure of these models suggests that processes other than spontaneous instabilities are important for star formation in irregular galaxies. The role of Σg/Σc in spiral galaxies is also questioned. The observed sensitivity of the star formation rate to Σg/Σc may be strongly dependent on instabilities specific to spiral arms and not on general instabilities of the type for which Σg/Σc was originally derived. In that case, large-scale star formation may end in the outer disks of spirals because the stellar density waves end there, at the outer Lindblad resonance. The only azimuthally averaged quantity that correlates with the current star formation activity in irregulars is the stellar surface density. A causal connection is possible if stellar energy input to the interstellar medium acts as a feedback process to star formation. If this process played a key role in initiating star formation in irregulars from the beginning, then it could explain why irregular galaxies began their evolution slowly compared to larger disk systems with spiral arms.

A Large-Scale CO Imaging of the Galactic Center

Molecular gas in the Galactic center region is spatially and kinematically complex, and its physical conditions are distinctively different from those of molecular gas in the Galactic disk (e.g., Morris 1996). Relative paucity of current star formation activity, despite the abundance of dense molecular gas in this region, is one of the problem at issue.

A Comparative Study of Star Formation Thresholds in Gas-Rich Low Surface Brightness Dwarf Galaxies

view Abstract Citations (145) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Comparative Study of Star Formation Thresholds in Gas-Rich Low Surface Brightness Dwarf Galaxies van Zee, Liese ; Haynes, Martha P. ; Salzer, John J. ; Broeils, Adrick H. Abstract We present the results of HI synthesis imaging of six Low Surface Brightness Dwarf Galaxies (LSBDGs) and a comparison sample of four ``normal'' gas-rich dwarf galaxies in order to investigate why the star formation process has been inhibited in the LSBDGs. The LSBDGs are defined by unusually high values of M_H/L_B and the presence of extensive low surface brightness structure. The ``normal'' dwarfs were selected to have similar optical properties as the LSBDGs, but had more typical values of M_H/L_B. Both the LSBDGs and normal dwarfs are rotation dominated systems, with Vrot/sigma > 5. The HI synthesis maps, in conjunction with Hα imaging, were used to investigate the locations of the star formation sites with respect to the HI gas and the gravitational potential. Remarkably, despite differences in the star formation efficiencies, as implied by the mean surface brightness and M_H/L_B of the two samples, no clear distinction is seen in the current star formation activity of LSBDGs and normal dwarfs. The global gas density in all systems is significantly lower than the Toomre instability criterion, suggesting that the star formation process is inefficient in both the normals and LSBDGs. These systems have a moderate amount of HI, but it is spread over a large area, thus reducing the gas surface density. However, local peaks in the HI surface density approach the instability criterion and are correlated with sites of active star formation. Thus, star formation appears to be inhibited globally, but occurs locally in gas-rich dwarf galaxies. Publication: The Astronomical Journal Pub Date: May 1997 DOI: 10.1086/118379 Bibcode: 1997AJ....113.1618V full text sources ADS | data products SIMBAD (12) NED (12)

Comparison of Star Formation in Five Nearby Molecular Clouds

We have calculated bolometric temperature (Tbol) and bolometric luminosity (Lbol) for 383 young stellar objects (YSOs) in five molecular clouds within 200 pc in Corona Australis (CrA), Ophiuchus (Oph), Taurus (Tau), Chamaeleon (Cha), and Lupus (Lup). We used Tbol, Lbol, and bolometric luminosity-temperature (BLT) diagrams to characterize and compare the overall-star-formation activity of the clouds on a self-consistent basis. The main results are the following: (1) the YSO populations in the clouds can be differentiated by the fraction of their low-Tbol sources, which increases systematically from Lup and Cha to Tau and to Oph and CrA. This trend is interpreted as increasing current star-forming activity in the same order; (2) the clouds with higher cold source fractions also seem to have higher bright source fractions; (3) In the BLT diagram, the CrA and Oph sources are more uniformly distributed while the Cha and Lup sources are aggregated near the zero-age mainsequence (ZAMS). Tau sources appear to be an intermediate case. Taurus also seems to contain more cold (Tbol < 1000 K) and low-luminosity (Lbol < 1 L?) sources than the other complexes; (4) the YSOs show a characteristic distribution in the median BLT diagram. This distribution is qualitatively consistent with the early YSO evolution from a protostar to a pre-main-sequence star and provides a unique observational test to star-formation models; (5) for Lup pre-main-sequence stars, the ratio of their Tbol to Teff increases during their approach to the ZAMS. This increase can be explained by the disk and envelope dissipation during the pre-main-sequence evolution; (6) the most active star-forming clouds (Oph and CrA) also have denser molecular cores as measured by C18O J = 1-0 line emission, suggesting that the star formation occurs in the densest parts of the molecular clouds; and (7) we find an anti-correlation between Tbol and C18O emission for the class 0 and I Tau sources (Tbol < 650 K). This shows that Tbol measures the intrinsic redness of YSOs, rather than their disk-envelope orientation. The disk orientation may have a more important effect on Tbol of the pre-main-sequence stars.

Multi‐colour surface photometry of a sample of HI‐detected late type galaxies

AbstractWe perform BV RI surface photometry of 27 late type spiral and dwarf galaxies. All but two galaxies are selected from the HI‐survey of Schneider et al. (1990). We derive integral photometric parameters and the radial light and colour profiles and compare them to those of several LSB and bright field galaxy samples. Our new data confirm the earlier suggestions that the ‘ Schneider's sample’ consists of two different types of galaxies, with probably different star formation histories. First, the nearby (v0 ⩽ 1600 km s‐ 1) galaxies are mostly true dwarfs with the mean absolute magnitude of MB = ‐ 14.9±1.5 and with disk scale lengthes smaller than 1 kpc. These dwarfs are reasonably blue with median B – V = 0.49 and contain a large amount of neutral gas. Second, the distant (v0 ⩾ 4500 km s‐ 1) galaxies are late type LSB galaxies with typical face‐on disk central surface brightnesses of about 23.3 B‐ mag arcsec‐ 2 and ‘ normal’ disk scale lengthes of 3.6±1.5 kpc. Observed LSB galaxies are significantly redder, with median B – V ⋍ 0.72, than are the typical field LSB galaxies. The observed LSB galaxies are rich in HI but the current star formation activity is probably low.

The Peculiar H [CSC]i[/CSC] Kinematics of the Wolf-Rayet Starburst Galaxy NGC 5253

We report the discovery of a peculiar H I velocity field in the nearby amorphous dwarf starburst galaxy NGC 5253 which indicates that the bulk of the neutral atomic gas rotates about the galaxy's optical major axis. There is also weak evidence for a second H I dynamical component rotating about the optical minor axis. We estimate the dynamical mass at 3 × 108/sin2 i M☉. The gasdynamics in NGC 5253 are vaguely reminiscent of what is seen in polar ring galaxies and dust lane ellipticals, but the large gas mass fraction (~0.6 for i = 50°), young starburst population, irregular optical morphology, lack of a stellar ring, and lack of stellar rotation (≤ 7 km s-1) are inconsistent with either type of object. NGC 5253's unique gasdynamics and current star formation activity may be either (1) the result of an interaction with the nearby spiral M83 ~109 yr ago or (2) due to the disruption and accretion of a gas-rich companion on a highly inclined orbit. A combination of wide-field, high-sensitivity H I mapping and new optical spectra measuring the stellar kinematics along the minor axis could discriminate between these two possible interaction scenarios and constrain the original, preinteraction nature of NGC 5253. It is possible that NGC 5253 will later develop into a polar ring galaxy and that we are observing it during its period of ring formation.

Gas content of infrared luminous markarian galaxies

The atomic and molecular hydrogen gas properties of a complete sample of Markarian galaxies with flux density at 60 µm higher than 1.95 Jy are presented. We present the improved far-infrared luminosity function of Markarian galaxies; and its comparison with other samples. We find that 40% of the bright IRAS galaxies of far-infrared luminosity higher than 1010.5L⊙ are Markarian galaxies. There is an absence of correlation between HI content of Markarian galaxies and current star formation activity, implying that star formation in these systems has complex structure and it is not a simple function of the HI content. On the contrary, the H2 content of Markarian galaxies is well correlated with star formation activity. It is argued that tight correlation between HI and H2 contents is a consequence of transformation of atomic hydrogen into molecular.