Star Formation Activity In Galaxies Research Articles

Large-scale Structures in COSMOS2020: Evolution of Star Formation Activity in Different Environments at 0.4 < z < 4

To study the role of environment in galaxy evolution, we reconstruct the underlying density field of galaxies based on COSMOS2020 (The Farmer catalog) and provide the density catalog for a magnitude-limited (K s < 24.5) sample of ∼210,000 galaxies at 0.4 < z < 5 within the COSMOS field. The environmental densities are calculated using a weighted kernel density estimation approach with the choice of a von Mises–Fisher kernel, an analog of the Gaussian kernel for periodic data. Additionally, we make corrections for the edge effect and masked regions in the field. We utilize physical properties extracted by LePhare to investigate the connection between star formation activity and the environmental density of galaxies in six mass-complete subsamples at different cosmic epochs within 0.4 < z < 4. Our findings confirm a strong anticorrelation between star formation rate (SFR)/specific SFR (sSFR) and environmental density out to z ∼ 1.1. At 1.1 < z < 2, there is no significant correlation between SFR/sSFR and density. At 2 < z < 4, we observe a reversal of the SFR/sSFR–density relation such that both SFR and sSFR increase by a factor of ∼10 with increasing density contrast, δ, from −0.4 to 5. This observed reversal at higher redshifts supports the scenario where an increased availability of gas supply, along with tidal interactions and a generally higher star formation efficiency in dense environments, could potentially enhance star formation activity in galaxies located in rich environments at z > 2.

Star formation efficiency across large-scale galactic environments

ABSTRACT Environmental effects on the formation and evolution of galaxies have been one of the leading questions in galaxy studies during the past few decades. In this work, we investigate the relationship between the star formation activity of galaxies and their environmental matter density using the cosmological hydrodynamic simulation Simba. The galactic star formation activity indicators that we explore include the star formation efficiency (SFE), specific star formation rate (sSFR), and molecular hydrogen mass fraction ($f^{*}_{\mathrm{H_2}}$), and the environment is considered as the large-scale environmental matter density, calculated based on the stellar mass of nearby galaxies on a 1 h−1 Mpc grid using the cloud in cell method. Our sample includes galaxies with $9\lt \log \frac{M_*}{{\rm M}_{\odot }}$ at 0 &amp;lt; z &amp;lt; 4, divided into three stellar mass bins to disentangle the effects of stellar mass and environment on the star formation activity of galaxies. For low- to intermediate-mass galaxies at low redshifts (z &amp;lt; 1.5), we find that the star formation efficiency of those in high-density regions are ∼0.3 dex lower than those in low-density regions. However, there is no significant environmental dependence of the star formation efficiency for massive galaxies over all our redshift range, and low- to intermediate-mass galaxies at high redshifts (z &amp;gt; 1.5). We present a scaling relation for the depletion time of cold molecular hydrogen (tdepl = 1/SFE) as a function of galaxy parameters including environmental density. Our findings provide a framework for quantifying the environmental effects on the star formation activities of galaxies as a function of stellar mass and redshift. The most significant environmental dependence is seen at later cosmic times (z &amp;lt; 1.5) and towards lower stellar masses ($9\lt \log \frac{M_*}{{\rm M}_{\odot }}\lt 10$). Future large galaxy surveys can use this framework to look for the environmental dependence of the star formation activity and examine our predictions.

Nature versus Nurture: Revisiting the Environmental Impact on Star Formation Activities of Galaxies

We present a systematic study of the environmental impact on star formation activities of galaxies using a mass-complete sample of ∼170k galaxies at z < 4 from the latest COSMOS2020 catalog. At z < 1, we find that the mean star formation rate (SFR) of all galaxies decreases with increasing density of the environment. However, when we only consider star-forming galaxies, the mean SFR becomes independent of the environment at z < 1. At z > 2, we observe a clear positive correlation between the SFR and the density of the environment for all the galaxies. On the other hand, the stellar mass of the galaxies increases significantly with the environment at all redshifts except for star-forming galaxies at z < 1. The fraction of quiescent galaxies increases with increasing density of the environment at z < 2, and the morphology–density relation is confirmed to be present up to z ∼ 1. We also find that environmental quenching is negligible at z > 1, whereas mass quenching is the dominant quenching mechanism for massive galaxies at all redshifts. Based on these results, we argue that stellar mass-regulated physical processes might be the major driving force for star formation activities of galaxies. At low redshift (z < 1) massive galaxies are quenched primarily due to their high mass, resulting in a normal SFR–density relation. At high redshift (z > 2) most of the galaxies are star-forming ones tightly following the star-forming main sequence, and the difference in their stellar mass in different environments naturally leads to a reversal of the SFR–density relation.

J0107a: A Barred Spiral Dusty Star-forming Galaxy at z = 2.467

Dusty star-forming galaxies (DSFGs) are among the most massive and active star-forming galaxies during the cosmic noon. Theoretical studies have proposed various formation mechanisms of DSFGs, including major merger-driven starbursts and secular star-forming disks. Here, we report J0107a, a bright (∼8 mJy at observed-frame 888 μm) DSFG at z = 2.467 that appears to be a gas-rich massive disk and might be an extreme case of the secular disk scenario. J0107a has a stellar mass M ⋆ ∼ 5 × 1011 M ⊙, molecular gas mass M mol ≳ 1011 M ⊙, and a star formation rate of ∼500M ⊙ yr−1. J0107a does not have a gas-rich companion. The rest-frame 1.28 μm JWST NIRCam image of J0107a shows a grand-design spiral with a prominent stellar bar extending ∼15 kpc. The Atacama Large Millimeter/submillimeter Array Band 7 continuum map reveals that the dust emission originates from both the central starburst and the stellar bar. 3D disk modeling of the CO(4–3) emission line indicates a dynamically cold disk with rotation-to-dispersion ratio . The results suggest a bright DSFG may have a nonmerger origin, and its vigorous star formation may be triggered by the bar and/or rapid gas inflow.

Discovery of a Low-redshift Hot Dust-obscured Galaxy

We report the discovery of the hyperluminous, highly obscured active galactic nuclei (AGN) WISE J190445.04+485308.9 (W1904+4853, hereafter, L bol ∼ 1.1 × 1013 L ⊙) at z = 0.415. Its well-sampled spectral energy distribution (SED) is dominated by infrared dust emission, though broad emission lines are detected in the optical spectra. These features suggest that W1904+4853 contains an actively accreting supermassive black hole hidden in its dusty cocoon, resembling the observed properties of hot dust-obscured galaxies (Hot DOGs), a population previously only identified at z > 1.0. Using the broad component of the Mg ii 2798 Å emission line, we estimate a black hole mass of . The corresponding Eddington ratio (η) of implies that the central black hole accretion is at the theoretical limit of isotropic accretion. The rest-frame UV-optical SED also indicates that the host galaxy of W1904+4853 harbors strong star formation activity at the rate of 6–84 M ⊙ yr−1 with an independent estimate of star formation rate up to ∼45 M ⊙ yr−1 using the [O ii] emission line. With an estimated stellar mass of 3 × 1010 M ⊙, the host galaxy appears to be a starburst system with respect to the main sequence of the star-forming galaxies at the same redshift. Although blueshifted and asymmetric [O iii] emission provides evidence of an outflow, we estimate it to be an order of magnitude smaller than the star formation rate, indicating that the current obscured AGN activity at the center has not yet produced significant feedback on the host galaxy star formation activity. W1904+4853 supports the interpretation that Hot DOGs are a rare transitional phase of AGN accretion in galaxy evolution, a phase that can persist into the present-day Universe.

The Way to Quench: Galaxy Evolution in A2142

We show how the star formation activity of galaxies is progressively inhibited from the outer region to the center of the massive cluster A2142. From an extended spectroscopic redshift survey of 2239 galaxies covering a circular area of radius ∼11 Mpc from the cluster center, we extract a sample of 333 galaxies with known stellar mass, star formation rate, and spectral index D n 4000. We use the Blooming Tree algorithm to identify the substructures of the cluster and separate the galaxy sample into substructure galaxies, halo galaxies, and outskirt galaxies. The substructure and halo galaxies are cluster members, whereas the outskirt galaxies are only weakly gravitationally bound to the cluster. For the cluster members, the star formation rate per stellar mass decreases with decreasing distance R from the cluster center. Similarly, the spectral index D n 4000 increases with R, indicating an increasing average age of the stellar population in galaxies closer to the cluster center. In addition, star formation in substructure galaxies is generally more active than in halo galaxies and less active than in outskirt galaxies, proving that substructures tend to slow down the transition between field galaxies and cluster galaxies. We finally show that most actively star-forming galaxies are within the cluster infall region, whereas most galaxies in the central region are quiescent.

Limited impact of jet-induced feedback in the multi-phase nuclear interstellar medium of 4C12.50

Context. 4C12.50 (IRAS 13451+1232) at z = 0.122 is an ultraluminous infared radio galaxy that has often been proposed as a prime candidate for the link between ultraluminous infared galaxies and young radio galaxies. It is also an interesting target to investigate whether and how radio-induced feedback affects the evolution of galaxies in the early phases of radio activity. Aims. We study, in detail for the first time, the hot (≥1500 K) molecular gas in 4C12.50. The potential impact of the radio jet on this gas phase, as well as on the star formation activity, are investigated. We also study the ionised (including coronal) gas as traced by the near-infrared lines. Methods. Using near-infrared long slit spectroscopy obtained with EMIR on GTC and X-shooter on VLT, we analyse the emission line spectrum of the ionised, coronal, and, specially, the hot molecular gas in the western nucleus hosting the compact radio jet. Based on high spatial resolution ALMA CO(2–1) data, we also revise the location of 4C12.50 in the Kennicutt-Schmidt diagram in order to investigate whether star formation is suppressed. Results. 4C12.50 hosts (2.1 ± 0.4) × 104 M⊙ of hot molecular gas. An unusually high rotational temperature Trot = 3020 ± 160 K is inferred. The molecular gas mass obeys a power-law temperature distribution, dMH2/dT ∝ T−5, from T ∼ 300 K and up to ∼3000 K. Both results support the idea that shocks (probably induced by the radio jet) contribute to the heating and excitation of the hot molecular gas. A molecular outflow is not detected. The coupling of the outflowing ionised and neutral outflows with the hot molecular gas is poor. Contrary to other studies, we claim that there is no evidence for star formation suppression in this object. Conclusions. If radio-induced feedback can regulate the star formation activity in galaxies, 4C12.50 is a promising candidate to reveal this phenomenon in action. However, we find no solid evidence for a current or past impact of this mechanism on the evolution of this system, neither by clearing out the dusty central cocoon efficiently, nor by suppressing the star formation activity.

Estimating Molecular Gas Content in Galaxies from Polycyclic Aromatic Hydrocarbon Emission

Emission from polycyclic aromatic hydrocarbons (PAHs), a commonly used indicator of star formation activity in galaxies, also has the potential to serve as an effective empirical tracer of molecular gas. We use a sample of 19 nearby galaxies with spatially resolved mid-infrared Spitzer spectroscopy, multiwavelength optical and mid-infrared imaging, and millimeter interferometric CO(1–0) maps to investigate the feasibility of using PAH emission as an empirical proxy to estimate molecular gas mass. PAH emission correlates strongly with CO emission on subkiloparsec scales over the diverse environments probed by our sample of star-forming galaxies and low-luminosity active galactic nuclei. The tight observed correlation, likely a consequence of photoelectronic heating of the diffuse interstellar gas by the PAHs, permits us to derive an empirical calibration to estimate molecular gas mass from the luminosity of PAH emission that has a total scatter of only ∼0.2–0.25 dex. Mid-infrared bands sensitive to PAH emission (e.g., the Spitzer/IRAC4 and WISE/W3 filters) can also be used as a highly effective substitute for this purpose.

A population of ultraviolet-dim protoclusters detected in absorption.

Galaxy protoclusters, which will eventually grow into the massive clusters we see in the local Universe, are usually traced by locating overdensities of galaxies1. Large spectroscopic surveys of distant galaxies now exist, but their sensitivity depends mainly on a galaxy's star-formation activity and dust content rather than its mass. Tracers of massive protoclusters that do not rely on their galaxy constituents are therefore needed. Here we report observations of Lyman-α absorption in the spectra of a dense grid of background galaxies2,3, which we use to locate a substantial number of candidate protoclusters at redshifts 2.2 to 2.8 through their intergalactic gas. We find that the structures producing the most absorption, most of which were previously unknown, contain surprisingly few galaxies compared with the dark-matter content of their analogues in cosmological simulations4,5. Nearly all of the structures are expected to be protoclusters, and we infer that half of their expected galaxy members are missing from our survey because they are unusually dim at rest-frame ultraviolet wavelengths. We attribute this to an unexpectedly strong and early influence of the protocluster environment6,7 on the evolution of these galaxies that reduced their star formation or increased their dust content.

The LEGO Large Program: Constraining the Physics of Line Emission in Galaxy Observations

The IRAM Large Program LEGO studies molecular clouds in the Milky Way to constrain the physics controlling molecular line emission in galaxies. This is done by imaging two dozen clouds with setups that provide continuous spectral coverage of about 85–115 GHz. To give one example, research in this area permits to “calibrate” extragalactic observations of molecules like HCN and N2H+ to explore how the star formation activity in galaxies depends on their dense gas contents (i.e., Gao &amp; Solomon relation). Interestingly, LEGO and other studies now reveal a substantial cloud-to-cloud variation in line ratios. Once understood properly, this diversity can be used to constrain the properties of extragalactic molecular clouds at great detail. Here we outline the LEGO sample and describe the project status.

Star Formation Activity of Galaxies Undergoing Ram Pressure Stripping in the Virgo Cluster

We study galaxies undergoing ram pressure stripping in the Virgo cluster to examine whether we can identify any discernible trend in their star formation activity. We first use 48 galaxies undergoing different stages of stripping based on HI morphology, HI deficiency, and relative extent to the stellar disk, from the VIVA survey. We then employ a new scheme for galaxy classification which combines HI mass fractions and locations in projected phase space, resulting in a new sample of 365 galaxies. We utilize a variety of star formation tracers, which include g - r, WISE [3.4] - [12] colors, and starburstiness that are defined by stellar mass and star formation rates to compare the star formation activity of galaxies at different stripping stages. We find no clear evidence for enhancement in the integrated star formation activity of galaxies undergoing early to active stripping. We are instead able to capture the overall quenching of star formation activity with increasing degree of ram pressure stripping, in agreement with previous studies. Our results suggest that if there is any ram pressure stripping induced enhancement, it is at best locally modest, and galaxies undergoing enhancement make up a small fraction of the total sample. Our results also indicate that it is possible to trace galaxies at different stages of stripping with the combination of HI gas content and location in projected phase space, which can be extended to other galaxy clusters that lack high-resolution HI imaging.

Modelling the quenching of star formation activity from the evolution of the colour-magnitude relation in VIPERS

We study the evolution of the colour-magnitude relation for galaxies in the VIMOS Public Extragalactic Redshift Survey (VIPERS) by introducing the concept of the bright edge, and use this to derive constraints on the quenching of star formation activity in galaxies over the redshift range 0.5 < z < 1.1. The bright-edge of the colour-magnitude diagram evolves with little dependence on galaxy colour, and therefore on the amount of star formation taking place in bright galaxies. We modelled this evolution with delayed exponential star formation histories (SFHs), to better understand the average time-scale of the turn-off in star formation activity. We show that using SFHs without quenching, the transition from the blue cloud to the red sequence is too slow. This indicates that a scenario purely driven by the consumption of the gas inside each galaxy does not reproduce the observed evolution of the colour-magnitude bright edge. For instantaneous quenching, the best match to the observations assumes that galaxies stop their star formation at a randomly distributed time up to ∼2−2.5 Gyr after observation. We argue that quenching is required over a wide range of stellar masses. Qualitatively similar evolution of the bright edge is found in the predictions of a semi-analytical galaxy formation model, but quantitatively there are marked differences with the observations. This illustrates the utility of the bright edge as a test of galaxy formation models. The evolution changes and no longer matches the observed trend if feedback from heating by active galactic nuclei is turned off.

Influence of velocity dispersions on star-formation activities in galaxies

We investigated the influence of the random velocity of molecular gas on star-formation activities of six nearby galaxies. The physical properties of a molecular cloud, such as temperature and density, influence star-formation activities in the cloud. Additionally, local and turbulent motions of molecules in a cloud may exert substantial pressure on gravitational collapse and thus prevent or reduce star formation in the cloud. However, the influence of gas motion on star-formation activities remains poorly understood. We used data from the Atacama Large Millimeter/submillimeter Array to obtain 12CO(J = 1 − 0) flux and velocity dispersion. We then combined these data with 3.6 and 8 micron midinfrared data from the Spitzer Space Telescope to evaluate the effects of gas motion on star-formation activities in several nearby galaxies. We discovered that relatively high velocity dispersion in molecular clouds corresponds with relatively low star-formation activity. Considering the velocity dispersion as an additional parameter, we derived a modified Kennicutt-Schmidt law with a gas surface density power index of 0.84 and velocity dispersion power index of −0.61.

Quenching as a Contest between Galaxy Halos and Their Central Black Holes

Abstract Existing models of galaxy formation have not yet explained striking correlations between structure and star formation activity in galaxies, notably the sloped and moving boundaries that divide star-forming from quenched galaxies in key structural diagrams. This paper uses these and other relations to “reverse engineer” the quenching process for central galaxies. The basic idea is that star-forming galaxies with larger radii (at a given stellar mass) have lower black hole (BH) masses due to lower central densities. Galaxies cross into the green valley when the cumulative effective energy radiated by their BH equals ∼4× their halo gas-binding energy. Because larger-radii galaxies have smaller BHs, one finds that they must evolve to higher stellar masses in order to meet this halo energy criterion, which explains the sloping boundaries. A possible cause of radii differences among star-forming galaxies is halo concentration. The evolutionary tracks of star-forming galaxies are nearly parallel to the green-valley boundaries, and it is mainly the sideways motions of these boundaries with cosmic time that cause galaxies to quench. BH scaling laws for star-forming, quenched, and green-valley galaxies are different, and most BH mass growth takes place in the green valley. Implications include the radii of star-forming galaxies are an important second parameter in shaping their BHs; BHs are connected to their halos but in different ways for star-forming, quenched, and green-valley galaxies; and the same BH–halo quenching mechanism has been in place since z ∼ 3. We conclude with a discussion of BH–galaxy coevolution and the origin and interpretation of BH scaling laws.

Cosmic evolution of molecular gas mass density from an empirical relationship between L1.4 GHz and L′CO

ABSTRACT Historically, GHz radio emission has been used extensively to characterize the star-formation activity in galaxies. In this work, we look for empirical relationships amongst the radio luminosity, the infrared luminosity, and the CO-based molecular gas mass. We assemble a sample of 278 nearby galaxies with measurements of radio continuum and total infrared emission, and the 12CO J = 1–0 emission line. We find a correlation between the radio continuum and the CO emission line (with a scatter of 0.36 dex), in a large sample of different kinds of galaxies. Making use of this correlation, we explore the evolution of the molecular gas mass function and the cosmological molecular gas mass density in six redshift bins up to z = 1.5. These results agree with previous semi-analytic predictions and direct measurements: the cosmic molecular gas density increases up to z = 1.5. In addition, we find a single plane across five orders of magnitude for the explored luminosities, with a scatter of 0.27 dex. These correlations are sufficiently robust to be used for samples where no CO measurements exist.

Galaxy kinematics across different environments in the RXJ1347−1145 cluster complex

Aims. In order to understand the role of the different processes that drive galaxy evolution in clusters, we need comprehensive studies that simultaneously examine several of the most important physical properties of galaxies. In this work we study the interplay between the kinematic state and star formation activity of galaxies in the RXJ1347−1145 cluster complex at z ∼ 0.45. Methods. We used VLT/VIMOS to obtain slit spectra for 95 galaxies across the 40′ × 40′ area where the RXJ1347−1145 cluster complex resides. We determined the cluster membership of our targets by identifying one or more of the available emission lines within the wavelength range. Our spectroscopy is complemented with archival SUBARU/Suprime-Cam deep photometric observations in five optical bands (B, V, Rc, Ic, z′). We examined the kinematic properties of our sample attending to the degree of distortion of the extracted rotation curves. Regular rotating galaxies were included in our Tully–Fisher analysis while the distorted ones were used to study the role of cluster-specific interactions with respect to star formation and AGN activity. Results. Our analysis confirmed the cluster membership for approximately half of our targets. We report a higher fraction of galaxies with irregular gas kinematics in the cluster environment than in the field. Cluster galaxies with regular rotation display a moderate brightening in the B-band Tully–Fisher relation compatible with the gradual evolution of the stellar populations with lookback time, and no significant evolution in the stellar-mass Tully–Fisher relation, in line with previous studies at similar redshift. Average specific star formation rate values are slightly lower in our cluster sample (−0.15 dex) with respect to the main sequence of star-forming galaxies, confirming the role of the environment in the early quenching of star formation in clusters. Finally, we carried out an exploratory observational study on the stellar-to-halo mass relation finding that cluster galaxies tend to have slightly lower stellar mass values for a fixed halo mass compared to their field counterparts.

The better half – asymmetric star formation due to ram pressure in the EAGLE simulations

ABSTRACT We use the EAGLE simulations to study the effects of the intracluster medium on the spatially resolved star formation activity in galaxies. We study three cases of galaxy asymmetry dividing each galaxy into two halves using the plane (i) perpendicular to the velocity direction, differentiating the galaxy part approaching the cluster centre, hereafter dubbed the ‘leading half’, and the opposite ‘trailing half’; (ii) perpendicular to the radial position of the satellite to the centre of the cluster; and (iii) that maximizes the star formation rate ($\rm SFR$) difference between the two halves. For (i), we find an enhancement of the $\rm SFR$, star formation efficiency, and interstellar medium pressure in the leading half with respect to the trailing one and normal star-forming galaxies in the EAGLE simulation, and a clear overabundance of gas particles in their trailing. These results suggest that ram pressure is boosting the star formation by gas compression in the leading half, and transporting the gas to the trailing half. This effect is more pronounced in satellites of intermediate stellar masses $\rm 10^{9.5}\!-\!10^{10.5}\,M_{\odot }$, with gas masses above $\rm 10^{9} M_{\odot }$, and located within one virial radius or in the most massive clusters. In (iii), we find an alignment between the velocity and the vector perpendicular to the plane that maximizes the $\rm SFR$ difference between the two halves. It suggests that finding this plane in real galaxies can provide an insight into the velocity direction.

Large-scale Structures in the CANDELS Fields: The Role of the Environment in Star Formation Activity

Abstract We present a robust method, weighted von Mises kernel density estimation, along with boundary correction to reconstruct the underlying number density field of galaxies. We apply this method to galaxies brighter than Hubble Space Telescope AB mag in the redshift range 0.4 ≤ z ≤ 5 in the five CANDELS fields (GOODS-N, GOODS-S, EGS, UDS, and COSMOS). We then use these measurements to explore the environmental dependence of the star formation activity of galaxies. We find strong evidence of environmental quenching for massive galaxies (M ≳ 1011 M ⊙) out to z ∼ 3.5 such that an overdense environment hosts ≳20% more massive quiescent galaxies than an underdense region. We also find that environmental quenching efficiency grows with stellar mass and reaches ∼60% for massive galaxies at z ∼ 0.5. The environmental quenching is also more efficient than stellar mass quenching for low-mass galaxies (M ≲ 1010 M ⊙) at low and intermediate redshifts (z ≲ 1.2). Our findings concur thoroughly with the “overconsumption” quenching model where the termination of cool gas accretion (cosmological starvation) happens in an overdense environment and the galaxy starts to consume its remaining gas reservoir in depletion time. The depletion time depends on the stellar mass and could explain the evolution of environmental quenching efficiency with stellar mass.

Understanding star formation and feedback in small galaxies

Context. Low-luminosity, active star-forming blue compact galaxies (BCGs) are excellent laboratories for investigating the process of star formation on galactic scales and to probe the interplay between massive stars and the surrounding interstellar (or intergalactic) medium. Aims. We investigate the morphology, structure, and stellar content of BCG Mrk 900, as well as the excitation, ionization conditions, and kinematics of its H II regions and surrounding ionized gas. Methods. We obtained integral field observations of Mrk 900 working with the Visible Multi-Object Spectrograph at the Very Large Telescope. The observations were taken in the wavelength range 4150−7400 Å covering a field of view of 27″ × 27″ on the sky with a spatial sampling of 0.″67. From the integral field data we built continuum, emission, and diagnostic line ratio maps and produced velocity and velocity dispersion maps. We also generated the integrated spectrum of the major H II regions and the nuclear area to determine reliable physical parameters and oxygen abundances. Integral field spectroscopy was complemented with deep broad-band photometry taken at the 2.5 m NOT telescope; the broad-band data, tracing the galaxy up to radius 4 kpc, allowed us to investigate the properties of the low surface brightness underlying stellar host. Results. We disentangle two different stellar components in Mrk 900: a young population, which resolves into individual stellar clusters with ages ∼5.5−6.6 Myr and extends about 1 kpc along the galaxy minor axis, is placed on top of a rather red and regular shaped underlying stellar host, several Gyr old. We find evidence of a substantial amount of dust and an inhomogeneous extinction pattern, with a dust lane crossing the central starburst. Mrk 900 displays overall rotation, although distorted in the central, starburst regions; the dispersion velocity map is highly inhomogeneous, with values increasing up to 60 km s−1 at the periphery of the SF regions, where we also find hints of the presence of shocks. Our observational results point to an interaction or merger with a low-mass object or infalling gas as plausible trigger mechanisms for the present starburst event.

Active and dust obscured star-forming galaxies at z ∼ 4 probed with UV spectral slope beta

AbstractWe investigate the stellar population of star-forming galaxies at z ∼ 4 by focusing on their slope of rest-frame ultraviolet continuum called UV spectral slope β. We analyze the sample of bright Lyman Break Galaxies (LBGs) with Subaru/i′≤26.0in the Subaru/XMM-Newton Deep Survey field. Our detailed SED fitting analysis indicates that the LBGs with observed UV slope &gt; −1.7, , Av &gt; 1.0, and intrinsic UV slope &lt; −2.5 are the intrinsically active star-forming galaxies with star formation rates larger than a few × 102 M⊙yr−1. A significant fraction of the UV-selected LBGs at z ∼ 4 is on-going active and dust obscured star-forming galaxies.