Star Formation Rate Intensity Research Articles

The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER). VI. The High-mass Stellar Initial Mass Function of M33

We measure the high-mass stellar initial mass function (IMF) from resolved stars in M33 young stellar clusters. Leveraging the Hubble Space Telescope’s high resolving power, we fully model the IMF probabilistically. We first model the optical color–magnitude diagram of each cluster to constrain its power-law slope Γ, marginalized over other cluster parameters in the fit (e.g., cluster age, mass, and radius). We then probabilistically model the distribution of mass function (MF) slopes for a highly strict cluster sample of nine clusters more massive than log(Mass/M ⊙) = 3.6; above this mass, all clusters have well-populated main sequences of massive stars and should have accurate recovery of their MF slopes, based on extensive tests with artificial clusters. We find that the ensemble IMF is best described by a mean high-mass slope of Γ¯=1.49±0.18 , with an intrinsic scatter of σΓ2=0.020.00+0.16 , consistent with a universal IMF. We find no dependence of the IMF on environmental impacts such as the local star formation rate (SFR) or galactocentric radius within M33, which serves as a proxy for metallicity. This Γ¯ measurement is consistent with similar measurements in M31, despite M33 having a much higher SFR intensity. While this measurement is formally consistent with the canonical Kroupa (Γ = 1.30) IMF, as well as the Salpeter (Γ = 1.35) value, it is the second Local Group cluster sample to show evidence for a somewhat steeper high-mass IMF slope. We explore the impacts a steeper IMF slope has on a number of astronomical subfields.

Clustering of emission line galaxies with IllustrisTNG – I. Fundamental properties and halo occupation distribution

ABSTRACT Upcoming spectroscopic redshift surveys use emission line galaxies (ELGs) to trace the three-dimensional matter distributions with wider area coverage in the deeper Universe. Since the halos hosting ELGs are young and undergo infall towards more massive halos along filamentary structures, contrary to a widely employed luminous red galaxy sample, the dynamics specific to ELGs should be taken into account to refine the theoretical modelling at non-linear scales. In this paper, we scrutinize the halo occupation distribution (HOD) and clustering properties of ELGs by utilizing IllustrisTNG galaxy formation hydrodynamical simulations. Leveraging stellar population synthesis technique coupled with the photo-ionization model, we compute line intensities of simulated galaxies and construct mock H α and [O ii] ELG catalogues. The line luminosity functions and the relation between the star formation rate and line intensity are well consistent with observational estimates. Next, we measure the HOD and demonstrate that there is a distinct population for the central HOD, which corresponds to low-mass infalling halos. We then perform the statistical inference of HOD parameters from the projected correlation function. Our analysis indicates that the inferred HODs significantly deviate from the HOD measured directly from simulations although the best-fitting model yields a good fit to the projected correlation function. It implies that the information content of the projected correlation function is not adequate to constrain HOD models correctly and thus, it is important to employ mock ELG catalogues to calibrate the functional form of HOD models and add prior information on HOD parameters to robustly determine the HOD.

The Distribution of UV Radiation Field in the Molecular Clouds of Gould Belt

The distribution of ultraviolet (UV) radiation field provides critical constraints on the physical environments of molecular clouds. Within 1 kpc of our solar system and fostering protostars of different masses, the giant molecular clouds in the Gould Belt present an excellent opportunity to resolve the UV field structure in star-forming regions. We performed spectral energy distribution (SED) fitting of the archival data from the Herschel Gould Belt Survey (HGBS). Dust radiative transfer analysis with the DUSTY code was applied to 23 regions in 14 molecular complexes of the Gould Belt, resulting in the spatial distribution of the radiation field in these regions. For 10 of 15 regions with independent measurements of star formation rate, their star formation rate and UV radiation intensity largely conform to a linear correlation found in previous studies.

The spatially resolved star formation history of mergers

This paper presents the spatially resolved star formation history (2D-SFH) of a small sample of four local mergers: the early-stage mergers IC 1623, NGC 6090, and the Mice, and the more advanced merger NGC 2623, by analyzing IFS data from the CALIFA survey and PMAS in LArr mode. Full spectral fitting techniques are applied to the datacubes to obtain the spatially resolved mass growth histories, the time evolution of the star formation rate intensity (ΣSFR), and the local specific star formation rate (sSFR), over three different time scales (30 Myr, 300 Myr, and 1 Gyr). The results are compared with non-interacting Sbc–Sc galaxies, to quantify if there is an enhancement of the star formation and to trace its time scale and spatial extent. Our results for the three LIRGs (IC 1623 W, NGC 6090, and NGC 2623) show that a major phase of star formation is occurring in time scales of 107 yr to few 108 yr, with global SFR enhancements of between approximately two and six with respect to main-sequence star forming (MSSF) galaxies. In the two early-stage mergers IC 1623 W and NGC 6090, which are between first pericentre passage and coalescence, the most remarkable increase of the SFR with respect to non-interacting spirals occurred in the last 30 Myr, and it is spatially extended, with enhancements of factors between two and seven both in the centres (r < 0.5 half light radius, HLR), and in the disks (r > 1 HLR). In the more advanced merger NGC 2623 an extended phase of star formation occurred on a longer time scale of ~1 Gyr, with a SFR enhancement of a factor of approximately two-to-three larger than the one in Sbc–Sc MSSF galaxies over the same period, probably relic of the first pericentre passage epoch. A SFR enhancement in the last 30 Myr is also present, but only in NGC 2623 centre, by a factor of three. In general, the spatially resolved SFHs of the LIRG-mergers are consistent with the predictions from high spatial resolution simulations. In contrast, the star formation in the Mice, specially in Mice B, is not enhanced but inhibited with respect to Sbc–Sc MSSF galaxies. The fact that the gas fraction of Mice B is smaller than in most non-interacting spirals, and that the Mice are close to a prograde orbit, represents a new challenge for the models, which must cover a larger space of parameters in terms of the availability of gas and the orbital characteristics.

STAR FORMATION IN COLLISION DEBRIS: INSIGHTS FROM THE MODELING OF THEIR SPECTRAL ENERGY DISTRIBUTION

During galaxy-galaxy interactions, massive gas clouds can be injected into the intergalactic medium which in turn become gravitationally bound, collapse and form stars, star clusters or even dwarf galaxies. The objects resulting from this process are both "pristine", as they are forming their first generation of stars, and chemically evolved because the metallicity inherited from their parent galaxies is high. Such characteristics make them particularly interesting laboratories to study star formation. After having investigated their star-forming properties, we use photospheric, nebular and dust modeling to analyze here their spectral energy distribution (SED) from the far-ultraviolet to the mid-infrared regime for a sample of 7 star-forming regions. Our analysis confirms that the intergalactic star forming regions in Stephan's Quintet, around Arp 105, and NGC 5291, appear devoid of stellar populations older than 10^9 years. We also find an excess of light in the near-infrared regime (from 2 to 4.5 microns) which cannot be attributed to stellar photospheric or nebular contributions. This excess is correlated with the star formation rate intensity suggesting that it is probably due to emission by very small grains fluctuating in temperature as well as the polycyclic aromatic hydrocarbons (PAH) line at 3.3 micron. Comparing the attenuation via the Balmer decrement to the mid-infrared emission allows us to check the reliability of the attenuation estimate. It suggests the presence of embedded star forming regions in NGC 5291 and NGC 7252. Overall the SED of star-forming regions in collision debris (and Tidal Dwarf Galaxies) resemble more that of dusty star-forming regions in galactic disks than to that of typical star-forming dwarf galaxies.

STAR FORMATION AND DUST OBSCURATION IN THE TIDALLY DISTORTED GALAXY NGC 2442

Abridged: We present a detailed investigation of the morphological distribution and level of star formation and dust obscuration in the nearby tidally distorted galaxy NGC2442. Spitzer images in the IR at 3.6, 4.5, 5.8, 8.0um, and 24um and GALEX images at 1500\AA{} and 2300\AA{} allow us to resolve the galaxy on scales between 240-600pc. We supplement these with archival data in the B, J, H, and K bands. We use the 8um, 24um and FUV (1500\AA) emission to study the star formation rate (SFR). We find that globally, these tracers of star formation give a range of results of ~6-11\msun/yr, with the dust-corrected FUV giving the highest value of SFR. We can reconcile the UV and IR-based estimates by adopting a steeper UV extinction curve that lies in between the starburst (Calzetti) and SMC extinction curves. However, the regions of highest SFR intensity along the spiral arms are consistent with a starburst-like extinction. Overall, the level of star-formation we find is higher than previously published for this galaxy, by about a factor of two, which, contrary to previous conclusions, implies that the interaction that caused the distorted morphology of NGC2442 likely also triggered increased levels of star-formation activity. Outside of the spiral arms, we discover what appears to be a superbubble, ~1.7kpc across in the IRAC images. Significant H{\alpha}, UV and IR emission in the area also suggest vigorous ongoing star-formation. A known, recent supernova (SN1999ga) is located at the edge of this superbubble. Although speculative at this stage, this area suggests a large star-forming region with a morphology shaped by generations of supernovae. Lastly, we discover an 8um (PAH) circumnuclear ring with an ~0.8kpc radius. The H{\alpha} emission is largely concentrated inside that ring and shows a vague spiral structure in the rest of the galaxy.

TOTAL INFRARED LUMINOSITY ESTIMATION OF RESOLVED AND UNRESOLVED GALAXIES

The total infrared (TIR) luminosity from galaxies can be used to examine both star formation and dust physics. We provide here new relations to estimate the TIR luminosity from various Spitzer bands, in particular from the 8 μm and 24 μm bands. To do so, we use data for 45'' subregions within a subsample of nearby face-on spiral galaxies from the Spitzer Infrared Nearby Galaxies Survey (SINGS) that have known oxygen abundances as well as integrated galaxy data from the SINGS, the Local Volume Legacy survey (LVL), and Engelbracht et al. samples. Taking into account the oxygen abundances of the subregions, the star formation rate intensity, and the relative emission of the polycyclic aromatic hydrocarbons at 8 μm, the warm dust at 24 μm, and the cold dust at 70 μm and 160 μm, we derive new relations to estimate the TIR luminosity from just one or two of the Spitzer bands. We also show that the metallicity and the star formation intensity must be taken into account when estimating the TIR luminosity from two wave bands, especially when data longward of 24 μm are not available.

The star formation rate intensity distribution function—: Comparison of observations with hierarchical galaxy formation

Recently, Lanzetta et al. [ApJ (2002) in press] have measured the distribution of star formation rate intensity in galaxies at various redshifts. This data set has a number of advantages relative to galaxy luminosity functions; the effect of surface-brightness dimming on the selection function is simpler to understand, and this data set also probes the size distribution of galactic disks. We predict this function using semi-analytic models of hierarchical galaxy formation in a ΛCDM cosmology. We show that the basic trends found in the data follow naturally from the redshift evolution of dark matter halos. The data are consistent with a constant efficiency of turning gas into stars in galaxies, with a best-fit value of 2%, where dust obscuration is neglected; equivalently, the data are consistent with a cosmic star formation rate which is constant to within a factor of two at all redshifts above two. However, the practical ability to use this kind of distribution to measure the total cosmic star formation rate is limited by the predicted shape of an approximate power law with a smoothly varying power, without a sharp break.

The Star Formation Rate Intensity Distribution Function: Implications for the Cosmic Star Formation Rate History of the Universe

We address the effects of cosmological surface brightness dimming on observations of faint galaxies by examining the distribution of unobscured star formation rate intensities versus redshift. We use the star formation rate intensity distribution function to assess the ultraviolet luminosity density versus redshift, based on our photometry and photometric redshift measurements of faint galaxies in the Hubble Deep Field (HDF) and the Hubble Deep Field-South (HDF-S) Wide Field Planetary Camera 2 and Near-Infrared Camera and Multi-Object Spectrometer fields. We find that (1) previous measurements have missed a dominant fraction of the ultraviolet luminosity density of the universe at high redshifts by neglecting cosmological surface brightness dimming effects, which are important at redshifts larger than z ≈ 2; (2) the incidence of the highest intensity star-forming regions increases monotonically with redshift; and (3) the ultraviolet luminosity density plausibly increases monotonically with redshift through the highest redshifts observed. By measuring the spectrum of the luminosity density versus redshift, we also find that (4) previous measurements of the ultraviolet luminosity density at redshifts z < 2 must be reduced by a factor of ≈2 to allow for the spectrum of the luminosity density between rest-frame wavelengths 1500 and 2800 A. And, by comparing with observations of high-redshift damped Lyα absorption systems detected toward background quasi-stellar objects, we further find that (5) the distribution of star formation rate intensities matches the distribution of neutral hydrogen column densities at redshifts z ≈ 2-5, which establishes a quantitative connection between high-redshift galaxies and high column density gas and suggests that high-redshift damped Lyα absorption systems trace lower star formation rate intensity regions of the same galaxies detected in starlight in the HDF and HDF-S. Because our measurements neglect the effects of obscuration by dust, they represent lower limits to the total star formation rate density.