Instantaneous Star Formation Rate Research Articles

AN EVOLUTIONARY MODEL FOR COLLAPSING MOLECULAR CLOUDS AND THEIR STAR FORMATION ACTIVITY

We present an idealized, semi-empirical model for the evolution of gravitationally contracting molecular clouds (MCs) and their star formation rate (SFR) and efficiency (SFE). The model assumes that the instantaneous SFR is given by the mass above a certain density threshold divided by its free-fall time. The instantaneous number of massive stars is computed assuming a Kroupa IMF. These stars feed back on the cloud through ionizing radiation, eroding it. The main controlling parameter of the evolution turns out to be the maximum cloud mass, $\Mmax$. This allows us to compare various properties of the model clouds against their observational counterparts. A giant molecular cloud (GMC) model ($\Mmax \sim 10^5 \Msun$) adheres very well to the evolutionary scenario recently inferred by Kawamura et al. (2009) for GMCs in the Large Magellanic Cloud. A model cloud with $\Mmax \approx 2000 \Msun$ evolves in the Kennicutt-Schmidt diagram first passing through the locus of typical low- to-intermediate mass star-forming clouds, and then moving towards the locus of high-mass star-forming ones over the course of $\sim 10$ Myr. Also, the stellar age histograms for this cloud a few Myr before its destruction agree very well with those observed in the $\rho$-Oph stellar association, whose parent cloud has a similar mass, and imply that the SFR of the clouds increases with time. Our model thus agrees well with various observed properties of star-forming MCs, suggesting that the scenario of gravitationally collapsing MCs, with their SFR regulated by stellar feedback, is entirely feasible and in agreement with key observed properties of molecular clouds.

The radio-X-ray relation as a star formation indicator: results from the Very Large Array-Extended Chandra Deep Field-South

In order to trace the instantaneous star formation rate (SFR) at high redshift, and thus help in understanding the relation between the different emission mechanisms related to star formation, we combine the recent 4-Ms Chandra X-ray data and the deep Very Large Array radio data in the Extended Chandra Deep Field-South region. We find 268 sources detected both in the X-ray and radio bands. The availability of redshifts for ∼95 per cent of the sources in our sample allows us to derive reliable luminosity estimates and the intrinsic properties from X-ray analysis for the majority of the objects. With the aim of selecting sources powered by star formation in both bands, we adopt classification criteria based on X-ray and radio data, exploiting the X-ray spectral features and time variability, taking advantage of observations scattered across more than 10 years. We identify 43 objects consistent with being powered by star formation. We also add another 111 and 70 star-forming candidates detected only in the radio and X-ray bands, respectively. We find a clear linear correlation between radio and X-ray luminosity in star-forming galaxies over three orders of magnitude and up to z∼ 1.5. We also measure a significant scatter of the order of 0.4 dex, higher than that observed at low redshift, implying an intrinsic scatter component. The correlation is consistent with that measured locally, and no evolution with redshift is observed. Using a locally calibrated relation between the SFR and the radio luminosity, we investigate the LX(2–10 keV)–SFR relation at high redshift. The comparison of the SFR measured in our sample with some theoretical models for the Milky Way and M31, two typical spiral galaxies, indicates that, with current data, we can trace typical spirals only at z≤ 0.2, and strong starburst galaxies with SFRs as high as ∼100 M⊙ yr−1, up to z∼ 1.5.

Large-scale outflows from z≃ 0.7 starburst galaxies identified via ultrastrong Mg ii quasar absorption lines

(Abridged) Star formation-driven outflows are a critical phenomenon in theoretical treatments of galaxy evolution, despite the limited ability of observations to trace them across cosmological timescales. If the strongest MgII absorption-line systems detected in the spectra of background quasars arise in such outflows, "ultra-strong" MgII (USMgII) absorbers would identify significant numbers of galactic winds over a huge baseline in cosmic time, in a manner independent of the luminous properties of the galaxy. To this end, we present the first detailed imaging and spectroscopic study of the fields of two USMgII absorber systems culled from a statistical absorber catalog, with the goal of understanding the physical processes leading to the large velocity spreads that define such systems. Each field contains two bright emission-line galaxies at similar redshift (dv < 300 km/s) to that of the absorption. Lower-limits on their instantaneous star formation rates (SFR) from the observed OII and Hb line fluxes, and stellar masses from spectral template fitting indicate specific SFRs among the highest for their masses at z~0.7. Additionally, their 4000A break and Balmer absorption strengths imply they have undergone recent (~0.01 - 1 Gyr) starbursts. The concomitant presence of two rare phenomena - starbursts and USMgII absorbers - strongly implies a causal connection. We consider these data and USMgII absorbers in general in the context of various popular models, and conclude that galactic outflows are generally necessary to account for the velocity extent of the absorption. We favour starburst driven outflows over tidally-stripped gas from a major interaction which triggered the starburst as the energy source for the majority of systems. Finally, we discuss the implications of these results and speculate on the overall contribution of such systems to the global SFR density at z~0.7.

The H-alpha luminosity function at redshift 2.2

We aim to place new, strengthened constraints on the luminosity function (LF) of H-alpha emitting galaxies at redshift z=2.2, and to further constrain the instantaneous star-formation rate density of the universe (rho*). We have used the new HAWK-I instrument at ESO-VLT to obtain extremely deep narrow-band (line; NB2090) and broad-band (continuum; Ks) imaging observations. The target field is in the GOODS-South, providing us with a rich multi-wavelength auxiliary data set, which we utilise for redshift confirmation and to estimate dust content. We use this new data to measure the faint-end slope (alpha) of LF(H-alpha) with unprecedented precision. The data are well fit by a Schechter function and also a single power-law, yielding alpha=(-1.72 +/- 0.20) and (-1.77 +/- 0.21), respectively. Thus we are able to confirm the steepening of alpha from low- to high-z predicted by a number of authors and observed at other wavelengths. We combine our LF data-points with those from a much shallower but wider survey at z=2.2 (Geach et al. 2008), constructing a LF spanning a factor of 50 in luminosity. Re-fitting the Schechter parameters, we obtain log L*=(43.07+/-0.22)erg s^-1 ; log phi*=(-3.45+/-0.52)Mpc^-3 ; alpha=(-1.60+/-0.15). We integrate over LF(Halpha) and apply a correction for dust attenuation to determine the instantaneous cosmic star-formation rate density at z=2 without assuming alpha or extrapolating it from lower-z. Our measurement of rho* is (0.215+/-0.090) Msun yr^-1 Mpc^-3, integrated over a range of 37 <log(LHhalpha / erg s^-1) < 47.

PASCHEN-α EMISSION IN THE GRAVITATIONALLY LENSED GALAXY SMM J163554.2+661225

We report the detection of the Paschen-alpha emission line in the z=2.515 galaxy SMM J163554.2+661225 using Spitzer spectroscopy. SMM J163554.2+661225 is a sub-millimeter-selected infrared (IR)-luminous galaxy maintaining a high star-formation rate (SFR), with no evidence of an AGN from optical or infrared spectroscopy, nor X-ray emission. This galaxy is lensed gravitationally by the cluster Abell 2218, making it accessible to Spitzer spectroscopy. Correcting for nebular extinction derived from the H-alpha and Pa-alpha lines, the dust-corrected luminosity is L(Pa-alpha) = (2.57+/-0.43) x 10^43 erg s^-1, which corresponds to an ionization rate, Q = (1.6+/-0.3) x 10^55 photons s^-1. The instantaneous SFR is 171+/-28 solar masses per year, assuming a Salpeter-like initial mass function. The total IR luminosity derived using 70, 450, and 850 micron data is L(IR) = (5-10) x 10^11 solar luminosities, corrected for gravitational lensing. This corresponds to a SFR=90-180 solar masses per year, where the upper range is consistent with that derived from the Paschen-alpha luminosity. While the L(8 micron) / L(Pa-alpha) ratio is consistent with the extrapolated relation observed in local galaxies and star-forming regions, the rest-frame 24 micron luminosity is significantly lower with respect to local galaxies of comparable Paschen-alpha luminosity. Thus, SMM J163554.2+661225 arguably lacks a warmer dust component (T ~ 70 K), which is associated with deeply embedded star formation, and which contrasts with local galaxies with comparable SFRs. Rather, the starburst is consistent with star-forming local galaxies with intrinsic luminosities, L(IR) ~ 10^10 solar luminosities, but "scaled-up" by a factor of 10-100.

Testing star formation rate indicators using galaxy merger simulations and radiative transfer

AbstractWe discuss our ongoing project analyzing N-body/smoothed-particle hydrodynamics simulations of isolated and merging galaxies, performed using GADGET-2 (Springel 2005), with the 3-D adaptive grid, polychromatic Monte Carlo radiative transfer code SUNRISE (Jonsson 2006). We apply commonly used UV, optical, and IR star formation rate (SFR) indicators to the integrated spectral energy distributions (SEDs) of the simulated galaxies in order to determine how well the SFR indicators recover the instantaneous SFR in the simulations. The models underlying each SFR indicator must necessarily make assumptions about physical properties of the galaxies, e.g., the star formation history (SFH), whereas all such properties are known in the simulations. This enables us to test and compare SFR indicators in a way that is complementary to observational studies. We present one preliminary result of interest: even after correcting the Hα luminosity for dust using the Calzetti et al. (2000) attenuation law the SFR is significantly underestimated for simulated galaxies with SFR ≳ 10 M⊙ yr−1.

Mid-Infrared Spectra of High-Redshift ( z &gt; 2) Radio Galaxies

We present the first mid-infrared Spitzer/Infrared Spectrograph (IRS) observations of powerful radio galaxies at z > 2. These radio galaxies, 4C +23.56 (z = 2.48) and 6C J1908+7220 (z = 3.53), both show strong mid-infrared continua, but with 6C J1908+7220 also showing strong PAH emission at rest-frame 6.2 and 7.7 μm. In 4C +23.56 we see no obvious PAH features above the continuum. The PAH emission in 6C J1908+7220 is the among the most distant observed to date and implies that there is a large instantaneous star formation rate (SFR). This is consistent with the strong detection of 6C J1908+7220 at far-IR and sub-mm wavelengths, indicative of large amounts of cold dust, ~109 M☉. Powerful radio galaxies at lower redshifts tend to have weak or undetectable PAH features and typically have lower far-IR luminosities. In addition, 4C +23.56 shows moderate silicate absorption as seen in less luminous radio galaxies, indicating τ9.7 μ m = 0.3 ± 0.05. This feature is shifted out of the observed wavelength range for 6C J1908+7220. The correlation of strong PAH features with large amounts of cold dust, despite the presence of a powerful AGN, is in agreement with other recent results and implies that star formation at high redshift is, in some cases at least, associated with powerful, obscured AGN.

Galactic star formation rates gauged by stellar end-products

Young galactic X-ray point sources (XPs) closely trace the ongoing star formation in galaxies. From measured XP number counts we extract the collective 2-10 keV luminosity of young XPs, L_yXP, which we use to gauge the current star-formation rate (SFR) in galaxies. We find that, for a sample of local star-forming galaxies (i.e., normal spirals and mild starbursts), L_yXP correlates linearly with the SFR over three decades in luminosity. A separate, high-SFR sample of starburst ULIRGs can be used to check the calibration of the relation. Using their (presumably SF-related) total 2-10 keV luminosities we find that these sources satisfy the SFR-L_yXP relation, as defined by the weaker sample, and extend it to span about 5 decades in luminosity. The SFR-L_yXP relation is likely to hold also for distant Hubble Deep Field North galaxies, especially so if these high-SFR objects are similar to the (more nearby) ULIRGs. It is argued that the SFR-L_yXP relation provides the most adequate X-ray estimator of instantaneous SFR by the phenomena characterizing massive stars from their birth (FIR emission from placental dust clouds) through their death as compact remnants (emitting X-rays by accreting from a close donor). For local, low/intermediate-SFR galaxies, the simultaneous existence of a correlation of the instantaneous SFR with the total 2-10 keV luminosity, which traces the SFR integrated over (approximately) the last Gyr, suggests that during such epoch the SF in these galaxies has been proceeding at a relatively constant rate.

The Bright Ages Survey. II. Evolution of Luminosity, Dust Extinction, and Star Formation fromz = 0.5 toz = 2.5

The Bright Ages Survey is a K-band-selected redshift survey over six separate fields with UBVRIzJHK imaging covering a total of 75.6 arcmin(2) and reaching K = 20-20.5. Two fields have deep HST imaging, while all are centered on possible overdensities in the z similar to 2 range. Here we report photometric redshifts and spectroscopy for this sample, which has been described in Paper I. We find 18 galaxies with spectroscopic redshifts of z > 1:5. The derived rest-frame R-band luminosity functions show strong evolution out to z = 2. The luminosity function at z = 2 shows more bright galaxies than at any other epoch, even the extrapolated z = 3 luminosity function from Shapley et al. However, the R-band integrated luminosity density remains roughly constant from to z = 0:5 to z = 2. Evolved galaxies (E, S0, Sa) show a decreasing contribution to the total R-band luminosity density with redshift. The dust extinction in our K-selected sample is moderately larger [median z = 2 E(B - V) 0:30] than that found in Lyman break galaxies, although not enough to make a significant impact on the total light or star formation found at high redshift. We measure the extinction-corrected star formation rate density at z 2, finding ρ_(SFR)(z = 1.5-2.5)= 0.093 M_⊙ yr^(-1) Mpc^(-3), consistent with a relatively flat instantaneous star formation rate from z = 1-4.

Star Formation Rates and Extinction Properties of IR‐luminous Galaxies in theSpitzerFirst Look Survey

We investigate the instantaneous star formation rates (SFR) and extinction properties for a large (N=274), near-infrared (2.2 micron) + mid-infrared (24 micron) selected sample of normal to ultra-luminous infrared galaxies (ULIRGs) [10^9<L_{IR}/L_{sun}<10^12.5] with <z>~0.8 in the Spitzer Extragalactic First Look Survey. We combine Spitzer MIPS 24-micron observations with high-resolution, optical Keck Deimos spectroscopy to derive optical emission-line and infrared star formation rates (SFR_{opt} & SFR_{IR}, respectively). Direct comparison of these SFR diagnostics reveals that our sample exhibits a wide range of extinction (1.0<Av<4.0 mag). This is after removing spectroscopic and IRAC color-selected AGN candidates that account for 12% of the sample. Objects with SFRs of a few solar masses per year have Av values consistent with those of normal spirals (Av~1.0 mag). By contrast, LIRGs at z>1, which make up a large fraction of our sample, have SFR~100 M_{sun}/yr and <Av>~2.5 mag. This translates to a 97% mean attenuation of the [OII] forbidden line doublet, with the most extreme sources having as much as 99.7% of their [OII] line flux extinguished by dust. Based on a SFR_{IR}/SFR_{opt} diagnostic, we derive an IR-luminosity-dependent Av^{IR} function [Av^{IR}=0.75*log(L_{IR}/L_{sun})-6.35 mag] that we use to extinction correct our emission line luminosities. Application of this correction results in a correlation between SFR_{IR} and SFR_{opt} that has a dispersion of 0.2 dex. Investigation of the Av dependence on redshift reveals that for a fixed L_{IR}, there is no significant Av evolution. The mean attenuation of our sample is intermediate between that of local optical/UV- and radio-selected samples and has a marginally stronger L_{IR} dependence.

The Rest‐Frame Optical Spectra of SCUBA Galaxies

We present near-infrared spectroscopy and narrowband imaging at the wavelength of redshifted Hα for a sample of 30 high-redshift, far-infrared luminous galaxies. This sample is selected from surveys in the submillimeter, millimeter, and radio wave bands and has complete redshift coverage with a median redshift of z ~ 2.4. We use our data to measure the Hα properties of these systems and to gauge the prevalence of active galactic nuclei (AGNs) in these galaxies through their [N II]/Hα ratios and Hα line widths. Removing obvious AGNs, we find that the predicted Hα star formation rates in this diverse population are suppressed (by a factor of ~10) compared to those derived from their far-infrared luminosities. Using the AGN indicators provided by our near-infrared spectra, we estimate that AGNs are present in at least 40% of the galaxies in our sample. To further investigate this, we construct a composite rest-frame spectrum for both the entire sample and those galaxies that individually show no signs of nuclear activity. We find [N II]/Hα ratios for both composite spectra that suggest that the energy output of the galaxies is star formation rather than AGN dominated. However, we also find that the Hα line in the composite non-AGN spectrum is best fitted with an underlying broad-line component with a narrow/broad flux ratio of 0.45 ± 0.20. The median Hα line width for our sample (removing obvious AGNs) is 400 ± 70 km s-1 (FWHM), and the typical spatial extent of the Hα emission in our narrowband observations is ≲4-8 kpc, which indicates a dynamical mass of (1-2) × 1011 M☉ with corresponding dynamical times of 10-20 Myr. Using both high-resolution imaging and spectroscopically identified velocity offsets, we find that seven of the far-infrared luminous galaxies have companions, suggesting that they are undergoing interactions/mergers, and from their relative velocities we can determine a dynamical mass of (1.5 ± 0.9) × 1011 M☉. These measurements are comparable to millimeter CO estimates for the dynamical masses of these systems on similar scales and larger than recent estimates of the dynamical masses of UV-selected galaxies at similar redshifts derived in an identical manner. Using the [N II]/Hα index to predict abundances, we investigate the luminosity-metallicity relation for these galaxies and find that many have metallicities consistent with UV-selected high-redshift galaxies and slightly lower than local luminous infrared and elliptical galaxies (although we caution that our metallicity estimates have possible systematic uncertainties). We also compared our Hα and far-infrared luminosities with deep Chandra observations of a subset of our survey fields and use these data to further assess their AGN content. We conclude that these high-redshift, far-infrared luminous galaxies represent a population of massive, metal-rich, merging systems with high instantaneous star formation rates, strong dust obscuration, and actively fueled AGNs that are likely to be the progenitors of massive local elliptical galaxies.

Chandra Constraints on the Active Galactic Nucleus Fraction and Star Formation Rate of Red z ≳ 2 Galaxies in the FIRES MS 1054-03 Field

Very deep near-infrared observations in the Faint InfraRed Extragalactic Survey (FIRES) have recently uncovered a significant population of red galaxies at redshifts z > 2. These distant red galaxies (DRGs) are efficiently selected by the criterion Js - Ks > 2.3. We use Chandra data to examine the X-ray emission from DRGs in the 5' × 5' FIRES MS 1054-03 field. Two of 40 DRGs with Ks 1.2 × 1043 ergs s-1. This fraction is smaller than that inferred from optical and near-IR spectroscopy, probably largely owing to strong spectroscopic selection biases. By stacking all undetected DRGs, we find that their average X-ray flux in the 0.5-8 keV band is ≈4.6 × 10-17 ergs s-1 cm-2. The detection is only significant in the soft (0.5-2 keV; 3.4 σ) and full (0.5-8 keV; 3.2 σ) energy bands. The mean detection may result from star formation, the presence of low-luminosity active galactic nuclei (AGNs), or a combination of both. Assuming the detection is due exclusively to star formation, we find an average instantaneous star formation rate of 214 ± 68(random) ± 73(systematic) M☉ yr-1, in excellent agreement with previous results from spectral energy distribution fitting when constant star formation histories are assumed. These results may imply that DRGs contribute significantly to the cosmic star formation rate at z ≈ 2.5. However, the mean X-ray flux strictly provides only an upper limit to the star formation rate owing to the uncertain contribution of low-luminosity, possibly obscured AGNs. Observations at other wavelengths are needed to provide independent estimates of the star formation rate of DRGs.

Star formation rates of distant luminous infrared galaxies derived from Hα and IR luminosities

We present a study of the star formation rate (SFR ) for a sample of 16 distant galaxies detected by ISOCAM at 15 μ m in the CFRS0300+00 and CFRS1400+52 fields. Their high quality and intermediate resolution VLT/FORS spectra have allowed a proper correction of the Balmer emission lines from the underlying absorption. Extinction estimates using the Hβ /Hγ and the Hα /Hβ Balmer decrement are in excellent agreement, providing a robust measurement of the instantaneous SFR based on the extinction-corrected Hα luminosity. Star formation has also been estimated exploiting the correlations between IR luminosity and those at MIR and radio wavelengths. Our study shows that the relationship between the two SFR estimates follow two distinct regimes: (1) for galaxies with SFR IR below ~100 /yr, the SFR deduced from Hα measurements is a good approximation of the global SFR and (2) for galaxies near ULIRG regime. The corrected Hα SFR understimated the SFR by a factor of 1.5 to 2. Our analyses suggest that heavily extincted regions completely hidden in optical bands (such as those found in Arp 220) contribute less than 20% of the global budget of star formation up to .

Star forming rates between z = 0.25 and z = 1.2 from the CADIS emission line survey

The emission line survey within the Calar Alto Deep Imaging Survey (CADIS) detects emission line galaxies by a scan with an imaging Fabry-Perot interferometer. It covers 5 fields of each in three wavelengths windows centered on , 820, and 920 nm, and reaches to a typical limiting line flux of W m-2. This is the deepest emission line survey covering a field of several 100 . Galaxies between and are detected by prominent emission lines (from Hα to [O ii]372.7) falling into the FP scans. Additional observations with a dozen medium band filters allow to establish the line identification and thus the redshift of the galaxies to better than . On the basis of a total of more than 400 emission line galaxies detected in Hα (92 galaxies), [O iii]500.7 (124 galaxies), or [O ii]372.7 (222 galaxies) we measure the instantaneous star formation rate (SFR) in the range . With this purely emission line selected sample we are able to reach much fainter emission line galaxies than previous, continuum-selected samples. Thus completeness corrections are much less important. Although the relative [O iii] emission line strength depends on excitation and metallicity and shows strong variation, the mean line ratios yield SFR[O iii] values consistent with the SFR evolution. Our results substantiates the indications from previous studies (based on small galaxy samples) that the SFR decreases by a factor of ~20 between and today. In fact, for a cosmology, we find an exponential decline Gyr). This decrease of the SFR with time follows an exponential law which is compatible with the decreasing galaxy merger rate as expected from model calculations. The inferred SF density is in perfect agreement with that deduced from the FIR emission of optically selected galaxies which is explained by a large overlap between both populations. We show that self-consistent extinction corrections of both our emission lines and the UV continua lead to consistent results for the SF density.

High‐Redshift Galaxies in Cold Dark Matter Models

We use hydrodynamic cosmological simulations to predict the star formation properties of high-redshift galaxies (z = 2-6) in five variants of the inflationary cold dark matter scenario, paying particular attention to z = 3, the redshift of the largest Lyman break (LBG) samples. Because we link the star formation timescale to the local gas density, the rate at which a galaxy forms stars is governed mainly by the rate at which it accretes cooled gas from the surrounding medium. At z = 3, star formation in most of the simulated galaxies is steady on ~200 Myr timescales, and the instantaneous star formation rate (SFR) is correlated with total stellar mass. However, there is enough scatter in this correlation that a sample selected above a given SFR threshold may contain galaxies with a fairly wide range of masses. The redshift history and global density of star formation in the simulations depend mainly on the amplitude of mass fluctuations in the underlying cosmological model. The three models whose mass fluctuation amplitudes agree with recent analyses of the Lyα forest also reproduce the observed luminosity function of LBGs reasonably well, although the dynamic range of the comparison is small and the theoretical and observational uncertainties are large. The models with higher and lower amplitudes appear to predict too much and too little star formation, respectively, although they are not clearly ruled out. The intermediate amplitude models predict SFR ~ 30-40 M☉ yr-1 for galaxies with a surface density of ~1 arcmin-2 per unit redshift at z = 3. They predict much higher surface densities at lower SFR, and significant numbers of galaxies with SFR > 10 M☉ yr-1 at z ≥ 5.

Hubble Space TelescopeObservations of the Blue Compact Dwarf SBS 0335−052: A Probable Young Galaxy

We present HST WFPC2 V and I images and GHRS UV spectrophotometry of the spectral regions around Lyα and O I λ1302 of the extremely metal-deficient (Z ~ Z☉/41) blue compact dwarf galaxy (BCD) SBS 0335-052. All the star formation in the BCD occurs in six super-star clusters (SSCs) with ages ≤25 Myr, within a region of ~2'' or 520 pc in size. Dust is clearly present and mixed spatially with the SSCs. The SSCs are roughly aligned in the southeast-northwest direction, and there is a systematic increase in reddening of the clusters away from the brightest one. The observed color dependence on position may be the combined effects of differential extinction by dust and color evolution with time due to sequential propagating star formation. There is a supershell of radius ~380 pc, delineating a large supernova cavity. The instantaneous star formation rate is ~0.4 M☉ yr-1. Strong narrow Lyα emission is not observed. Rather there is low-intensity broad (FWZI = 20 A) Lyα emission superposed on even broader Lyα absorption by the H I envelope. This broad low-intensity emission is caused by resonant scattering of Lyα photons. The absence of strong Lyα emission may be due partly to dust absorption, but is due mainly to multiple scattering that removes Lyα photons from the small HST aperture. As the H I cloud is seen nearly edge-on, geometrical effects may also play a role as photons escape more easily in a direction perpendicular to the plane than along it. The BCD appears to be a young galaxy, undergoing one of its very first bursts of star formation. This conclusion is based on the following evidence: (1) The underlying extended low surface brightness component is irregular and filamentary, suggesting that a significant part of the emission comes from ionized gas. Any underlying stellar population must be younger than ~108 yr. (2) The underlying component has very blue colors [-0.34 ≤ (V-I)0 ≤ 0.16], consistent with gaseous emission colors. (3) The O I λ1302 line is not detected in absorption in the Goddard High Resolution Spectrograph spectrum, setting an upper limit for N(O)/N(H) in the H I envelope of the BCD of more than 3000 times smaller than the value in Orion.

Evolution of Galaxy Luminosity in the CDM Model

we analyze the evolution of the luminosity function of galaxies using the CDM model in numerical simulations. There is an observational excess in the number counts of galaxies per square degree in the blur band Nb(i.e. APM, Maddoxet al.1990). Several authors have tried to reproduce it trying to fit the observations. Our model assumes an instantaneous star formation rate (SFR) proportional to a power of the local density. A ‘single star burst’ is produced each time step and we follow the evolution of the luminosity and colour of each ‘stellar group’. The galaxies are identified with a density criterium. We compute U,V,B,K colours and Nband our results agree quite well with observations.