Specific Star Formation Research Articles

A comparative analysis of the active galactic nucleus and star formation characteristics of broad- and narrow-line Seyfert 1 galaxies

We report here our comparative analysis of the active galactic nucleus (AGN) and star formation (SF) characteristics of a sample of narrow-line Seyfert 1 (NLS1) and broad-line Seyfert 1 (BLS1) galaxies. Our sample consisted of 373 BLS1 and 240 NLS1 galaxies and spanned the redshift 0.02 $<$ $z$ $<$ 0.8. The broad-band spectral energy distribution, constructed using data from the ultra-violet to the far-infrared, was modelled using CIGALE to derive the basic properties of our sample. We searched for differences in stellar mass (M$_ ast $), star formation rate (SFR), and AGN luminosity (L$_ AGN $) in the two populations. We also estimated new radiation-pressure-corrected black hole masses for our sample of BLS1 and NLS1 galaxies. While the virial black hole mass (M$_ BH $) of BLS1 galaxies is similar to their radiation-pressure-corrected M$_ BH $ values, the virial M$_ BH $ values of NLS1 galaxies are underestimated. We found that NLS1 galaxies have a lower M$_ BH $ of log (M$_ BH M$_ odot ) = 7.45 pm 0.27 and a higher Eddington ratio of log ($ Edd $) = $-$0.72 pm 0.22 than BLS1 galaxies, which have log (M$_ BH M$_ odot ) and $ Edd $ values of 8.04 pm 0.26 and $-$1.08 pm 0.24, respectively. The distributions of M$_ ast $, SFR, and specific star formation (sSFR = SFR/M$_ ast $) for the two populations are indistinguishable. This analysis is based on an independent approach and contradicts reports in the literature that NLS1 galaxies have a higher SF than BLS1 galaxies. While we found that L$_ AGN $ increases with M$_ ast $, L$_ SF $ flattens at high M$_ ast $ for both BLS1 and NLS1 galaxies. The reason may be that SF is suppressed by AGN feedback at M$_ ast $ higher than sim 1011 M$_ odot $ or that the AGN fuelling mechanism is decoupled from SF. Separating the sample into radio-detected and radio-undetected subsamples, we found no difference in their SF properties suggesting that the effect of AGN jets on SF is negligible.

The evolution of radial gradients of MaNGA quiescent elliptical galaxies: inside-out quenching or outer mass growth?

ABSTRACT Using spatially resolved fossil record analysis on a large sample of ‘red and dead’ elliptical galaxies (classical ellipticals, CLEs) from the MaNGA/SDSS-IV DR15 survey, we reconstruct the archaeological evolution of their radial gradients in mass-to-luminosity ratio (M/L), g − r colour, and specific star formation (SF) rate. We also calculate other metrics that quantify the inside-out SF quenching and external mass growth processes. The M/L gradients, ∇Υ⋆, are approximately flat at high look-back times (tlb), but then they become negative and steeper until an epoch, when this trend reverses. These trends are shifted to later epochs the less massive the galaxies are. Colour gradients follow qualitatively similar trends. We find that these trends are mainly driven by strong inside-out quenching, without significant outer growth or structural changes overall. Our results suggest a scenario where the main progenitors of local CLE galaxies evolved quasi-passively after an early dissipative phase, but underwent radial photometric changes due to the inside-out quenching that led to the systematic decrease of ∇Υ⋆ and to an increase of the light-weighted radius. The late reversing of ∇Υ⋆, tlb≈2 − 4 Gyr, roughly coincides with the global quenching of the CLE galaxies. We have pushed archaeological inferences to the limit, but thanks to the large number of objects and an understanding of how the caveats and assumptions affect our results, we conclude that they offer an average description of evolutionary behaviours of CLE progenitors that is valid at least qualitatively.

Comparing the host galaxy ages of X-ray selected AGN in COSMOS

We explore the properties of the host galaxies of X-ray selected AGN in the COSMOS field using the Chandra Legacy sample and the LEGA-C survey VLT optical spectra. Our main goal is to compare the relative ages of the host galaxies of the obscured and unobscured AGN by means of the calcium break Dn(4000) and the Hδ Balmer line. The host galaxy ages are examined in conjunction with other properties such as the galaxy stellar mass, and star-formation rate as well as the AGN Eddington ratio. Our sample consists of 50 unobscured or mildly obscured (NH < 1023 cm−2) and 23 heavily obscured AGN (NH > 1023 cm−2) in the redshift range z = 0.6 − 1. We take specific caution to create control samples in order to match the exact luminosity and redshift distributions for the obscured and unobscured AGN. The majority of unobscured AGN appear to live in young galaxies in contrast to the obscured AGN which appear to live in galaxies located between the young and old galaxy populations. This finding may be in contrast to those evolutionary AGN unification models which postulate that the AGN begin their life in a heavy obscuration phase. The host galaxies of the obscured AGN have significantly lower levels of specific star-formation. At the same time the obscured AGN have lower Eddington ratios indicating a link between the star-formation and the black hole accretion. We find that the distribution of the stellar masses of the host galaxies of obscured AGN is skewed towards higher stellar masses in agreement with previous findings. Our results on the relative age of obscured AGN are valid when we match our obscured and unobscured AGN samples according to the stellar mass of their host galaxies. All the above results become less conspicuous when a lower column density (log NH(cm−2) = 21.5 or 22) is used to separate the obscured and unobscured AGN populations.

MIRI/JWST observations reveal an extremely obscured starburst in the z = 6.9 system SPT0311-58

Luminous infrared starbursts in the early Universe are thought to be the progenitors of massive quiescent galaxies identified at redshifts 2–4. Using the Mid-IRfrared Instrument (MIRI) on board the James Webb Space Telescope (JWST), we present mid-infrared sub-arcsec imaging and spectroscopy of such a starburst: the slightly lensed hyper-luminous infrared system SPT0311-58 at z = 6.9. The MIRI IMager (MIRIM) and Medium Resolution Spectrometer (MRS) observations target the stellar (rest-frame 1.26 μm emission) structure and ionised (Paα and Hα) medium on kpc scales in the system. The MIRI observations are compared with existing ALMA far-infrared continuum and [C II]158μm imaging at a similar angular resolution. Even though the ALMA observations imply very high star formation rates (SFRs) in the eastern (E) and western (W) galaxies of the system, the Hα line is, strikingly, not detected in our MRS observations. This fact, together with the detection of the ionised gas phase in Paα, implies very high internal nebular extinction with lower limits (AV) of 4.2 (E) and 3.9 mag (W) as well as even larger values (5.6 (E) and 10.0 (W)) by spectral energy distribution (SED) fitting analysis. The extinction-corrected Paα lower limits of the SFRs are 383 and 230 M⊙ yr−1 for the E and W galaxies, respectively. This represents 50% of the SFRs derived from the [C II]158 μm line and infrared light for the E galaxy and as low as 6% for the W galaxy. The MIRIM observations reveal a clumpy stellar structure, with each clump having 3–5×109 M⊙ mass in stars, leading to a total stellar mass of 2.0 and 1.5×1010 M⊙ for the E and W galaxies, respectively. The specific star formation (sSFR) in the stellar clumps ranges from 25 to 59 Gyr−1, assuming a star formation with a 50–100 Myr constant rate. This sSFR is three to ten times larger than the values measured in galaxies of similar stellar mass at redshifts 6–8. Thus, SPT0311-58 clearly stands out as a starburst system when compared with typical massive star-forming galaxies at similar high redshifts. The overall gas mass fraction is Mgas/M* ∼ 3, similar to that of z ∼ 4.5–6 star-forming galaxies, suggesting a flattening of the gas mass fraction in massive starbursts up to redshift 7. The kinematics of the ionised gas in the E galaxy agrees with the known [C II] gas kinematics, indicating a physical association between the ionised gas and the cold ionised or neutral gas clumps. The situation in the W galaxy is more complex, as it appears to be a velocity offset by about +700 km s−1 in the Paα relative to the [C II] emitting gas. The nature of this offset and its reality are not fully established and require further investigation. The observed properties of SPT0311-58, such as the clumpy distribution at sub(kpc) scales and the very high average extinction, are similar to those observed in low- and intermediate-z luminous (E galaxy) and ultra-luminous (W galaxy) infrared galaxies, even though SPT0311-58 is observed only ∼800 Myr after the Big Bang. Such massive, heavily obscured clumpy starburst systems as SPT0311-58 likely represent the early phases in the formation of a massive high-redshift bulge, spheroids and/or luminous quasars. This study demonstrates that MIRI and JWST are, for the first time, able to explore the rest-frame near-infrared stellar and ionised gas structure of these galaxies, even during the Epoch of Reionization.

Emission-line Metallicities from the Faint Infrared Grism Survey and VLT/MUSE

Abstract We derive direct-measurement gas-phase metallicities of for 14 low-mass emission-line galaxies at 0.3 < z < 0.8 identified in the Faint Infrared Grism Survey. We use deep slitless G102 grism spectroscopy of the Hubble Ultra Deep Field, dispersing light from all objects in the field at wavelengths between 0.85 and 1.15 μm. We run an automatic search routine on these spectra to robustly identify 71 emission-line sources, using archival data from Very Large Telescope (VLT)/Multi-Unit Spectroscopic Explorer (MUSE) to measure additional lines and confirm redshifts. We identify 14 objects with 0.3 < z < 0.8 with measurable [O iii]λ4363 Å emission lines in matching VLT/MUSE spectra. For these galaxies, we derive direct electron-temperature gas-phase metallicities with a range of . With matching stellar masses in the range of 107.9 M ⊙ < M ⋆ < 1010.4 M ⊙, we construct a mass–metallicity (MZ) relation and find that the relation is offset to lower metallicities compared to metallicities derived from alternative methods (e.g., R 23, O3N2, N2O2) and continuum selected samples. Using star formation rates derived from the Hα emission line, we calculate our galaxies’ position on the Fundamental Metallicity Relation, where we also find an offset toward lower metallicities. This demonstrates that this emission-line-selected sample probes objects of low stellar masses but even lower metallicities than many comparable surveys. We detect a trend suggesting galaxies with higher Specific Star Formation (SSFR) are more likely to have lower metallicity. This could be due to cold accretion of metal-poor gas that drives star formation, or could be because outflows of metal-rich stellar winds and SNe ejecta are more common in galaxies with higher SSFR.

The dependence of mass and environment on the secular processes of AGNs in terms of morphology, colour, and specific star-formation rate

Context. Galaxy mass and environment play a major role in the evolution of galaxies. In the transition from star-forming to quenched galaxies, active galactic nuclei (AGNs) also have a principal action therein. However, the connections between these three actors are still uncertain. Aims. In this work we investigate the effects of stellar mass and the large-scale structure (LSS) environment on the fraction of optical nuclear activity in a population of isolated galaxies, where AGN would not be triggered by recent galaxy interactions or mergers. Methods. As a continuation of a previous work, we focus on isolated galaxies to study the effect of stellar mass and the LSS in terms of morphology (early- and late-type), colour (red and blue), and specific star-formation rate (quenched and star-forming). To explore where AGN activity is affected by the LSS, we separate galaxies into two groups, of low- and high mass, respectively, and use the tidal strength parameter to quantify the effects. Results. We found that AGN is strongly affected by stellar mass in “active” galaxies (namely late-type, blue, and star-forming), but that mass has no influence on “quiescent” galaxies (namely early-type, red, and quenched), at least for masses down to 1010 M⊙. In relation to the LSS, we found an increase in the fraction of star-forming nuclei galaxies with denser LSS in low-mass star-forming and red isolated galaxies. Regarding AGN, we find a clear increase in the fraction of AGNs with denser environment in quenched and red isolated galaxies, independently of the stellar mass. Conclusions. Active galactic nuclei activity appears to be “mass triggered” in active isolated galaxies. This means that AGN activity is independent of the intrinsic properties of the galaxies, but is dependent on their stellar mass. On the other hand, AGN activity appears to be “environment triggered” in quiescent isolated galaxies, where the fraction of AGNs as a function of specific star formation rate and colour increases from void regions to denser LSS, independently of stellar mass.

Galaxy Formation in Sterile Neutrino Dark Matter Models

Abstract We investigate galaxy formation in models with dark matter (DM) constituted by sterile neutrinos. Given their large parameter space, defined by the combinations of sterile neutrino mass and mixing parameter with active neutrinos, we focus on models with , consistent with the tentative 3.5 keV line detected in several X-ray spectra of clusters and galaxies. We consider (1) two resonant production models with and , to cover the range of mixing parameters consistent with the 3.5 keV line; (2) two scalar-decay models, representative of the two possible cases characterizing such a scenario: a freeze-in and a freeze-out case. We also consider thermal warm DM with particle mass . Using a semianalytic model, we compare the predictions for the different DM scenarios with a wide set of observables. We find that comparing the predicted evolution of the stellar mass function, the abundance of satellites of Milky Way–like galaxies, and the global star formation history of galaxies with observations does not allow us to disentangle the effects of the baryonic physics from those related to the different DM models. On the other hand, the distribution of the stellar-to-halo mass ratios, the abundance of faint galaxies in the UV luminosity function at , and the specific star formation and age distribution of local, low-mass galaxies constitute potential probes for the DM scenarios considered. We discuss how future observations with upcoming facilities will enable us to rule out or to strongly support DM models based on sterile neutrinos.

Extending the Hα Survey for the Local Volume Galaxies

Images in the Ha emission line are presented for 35 nearby objects observed with the 6-m BTA telescope. Three of them, NGC 3377, NGC 3384, and NGC 3390, are bright E and S0 galaxies, one is an edge-on Sd galaxy UGC 7321, two are remote globular clusters associated with M 31, and the rest are dwarf galaxies of morphological types dIr, dTr, dSph, BCD, and Sm. The measured Hα fluxes are used to estimate the integral (SFR) and specific (sSFR) star formation rates for these galaxies. The values of log[sSFR] for all these objects lie below a limit of -0.4(Gyr-1). We note that the emission disk for the nearest super-thin edge-on galaxy UGC 7321 has an extremely large axis ratio of a/b = 38.

Central star formation and metallicity in CALIFA interacting galaxies

We use optical integral-field spectroscopic (IFS) data from 103 nearby galaxies at different stages of the merging event, from close pairs to merger remnants provided by the CALIFA survey, to study the impact of the interaction in the specific star formation and oxygen abundance on different galactic scales. To disentangle the effect of the interaction and merger from internal processes, we compared our results with a control sample of 80 non-interacting galaxies. We confirm the moderate enhancement (2-3 times) of specific star formation for interacting galaxies in central regions as reported by previous studies; however, the specific star formation is comparable when observed in extended regions. We find that control and interacting star-forming galaxies have similar oxygen abundances in their central regions, when normalized to their stellar masses. Oxygen abundances of these interacting galaxies seem to decrease compared to the control objects at the large aperture sizes measured in effective radius. Although the enhancement in central star formation and lower metallicities for interacting galaxies have been attributed to tidally induced inflows, our results suggest that other processes such as stellar feedback can contribute to the metal enrichment in interacting galaxies.

The Second Byurakan Survey Galaxies in Close Pairs

418 pairs containing galaxies from the Second Byurakan Survey (SBS) with dV < 800 km/s and Dp < 100 kpc are selected for study of the dependence of star formation rates and nuclear activity on the kinematics of the pairs and on the morphologies and masses of the paired galaxies. The following basic results are obtained: SBS galaxies with neighbors do not differ spectral type from isolated SBS galaxies. The SBS galaxies are brighter than their neighbors by 0m.5 on the average, but there is no statistically significant difference in their colors, morphologies, and star formation rates. There is a weak, but statistically significant correlation between the morphologies of the paired galaxies. The neighbors of the SBS galaxies have a lower fraction of star forming galaxies and a higher fraction of galaxies in all the other spectral types. The specific (relative) star formation rate (SSFR) is higher for galaxies that are closer to a neighboring galaxy for galaxies of all morphological types. The enhancement is the greatest for galaxies of earlier types (by 1 dex) and less for galaxies of later morphological types. There is an increase of SSFR of galaxies within all ranges of masses. Both major and minor interactions can increase the SSFR by 0.7 dex, but the effect is greater for major interactions, while in the case of minor interactions the SSFR increases only in the massive members of the pairs. The existence of neighbor galaxy of earlier morphological types does not enhance the SSFR, neighbor galaxies of the same morphological types produce a moderate increase in the SSFR, while neighboring galaxies of later morphological types cause a significant increase in the SSFR.

Spot the difference

We present the analysis of photometric, spectroscopic, and morphological properties for differently selected samples of passive galaxies up to z=1 extracted from the zCOSMOS-20k spectroscopic survey. This analysis intends to explore the dependence of galaxy properties on the selection criterion adopted, study the degree of contamination due to star-forming outliers, and provide a comparison between different commonly used selection criteria. We extracted from the zCOSMOS-20k catalog six different samples of passive galaxies, based on morphology, optical colors, specific star-formation rate, a best fit to the observed spectral energy distribution, and a criterion that combines morphological, spectroscopic, and photometric information. The morphological sample has the higher percentage of contamination in colors, specific star formation rate and presence of emission lines, while the red & passive ETGs sample is the purest, with properties mostly compatible with no star formation activity; however, it is also the less economic criterion in terms of information used. The best performing among the other criteria are the red SED and the quiescent ones, providing a percentage of contamination only slightly higher than the red & passive ETGs criterion (on average of a factor of ~2) but with absolute values of the properties of contaminants still compatible with a red, passively evolving population. We also provided two revised definitions of early type galaxies based on restframe color-color and color-mass criteria, that better reproduce the observed bimodalities. The analysis of the number densities shows evidences of mass-assembly downsizing, with galaxies at 10.25<log(M/Msun)<10.75 increasing their number by a factor ~2-4 from z=0.6 to z=0.2, by a factor ~2-3 from z=1 to z=0.2 at 10.75<log(M/Msun)<11, and by only ~10-50% from z=1 to z=0.2 at 11<log(M/Msun)<11.5.

The merger rates and sizes of galaxies across the peak epoch of star formation from the HiZELS survey

We use the HiZELS narrow-band H-alpha survey in combination with CANDELS, UKIDSS and WIRDS near-infrared imaging, to investigate the morphologies, merger rates and sizes of a sample of H-alpha emitting galaxies in the redshift range z=0.40 - 2.23, an epoch encompassing the rise to the peak of the star formation rate density. Merger rates are estimated from space- and ground-based imaging using the M20 coefficient. To account for the increase in the specific star-formation rate (sSFR) of the star forming `main-sequence' with redshift, we normalise the star-formation rate of galaxies at each epoch to the typical value derived from the H-alpha luminosity function. Once this trend in sSFR is removed we see no evidence for an increase in the number density of star-forming galaxies or the merger rate with redshift. We thus conclude that neither is the main driver of the enhanced star-formation rate density at z=1-2, with secular processes such as instabilities within efficiently fuelled, gas-rich discs or multiple minor mergers the most likely alternatives. However, we find that 40-50% of starburst galaxies, those with enhanced specific star formation at their epoch, are major mergers and this fraction is redshift independent. Finally, we find the surprising result that the typical size of a star-forming galaxy of a given mass does not evolve across the redshift range considered, suggesting a universal size-mass relation. Taken in combination, these results indicate a star-forming galaxy population that is statistically similar in physical size, merger rate and mass over the 6 Gyr covered in this study, despite the increase in typical sSFR.

The sizes, masses and specific star formation rates of massive galaxies at 1.3 &lt; z &lt; 1.5: strong evidence in favour of evolution via minor mergers

We report the results of a comprehensive study of the relationship between galaxy size, stellar mass and specific star-formation rate (sSFR) at redshifts 1.3<z<1.5. Based on a mass complete (M_star >= 6x10^10 Msun), spectroscopic sample from the UKIDSS Ultra-deep Survey (UDS), with accurate stellar-mass measurements derived from spectro photometric fitting, we find that at z~1.4 the location of massive galaxies on the size-mass plane is determined primarily by their sSFR. At this epoch we find that massive galaxies which are passive (sSFR <= 0.1 Gyr^-1) follow a tight size-mass relation, with half-light radii a factor f=2.4+/-0.2 smaller than their local counterparts. Moreover, amongst the passive sub-sample we find no evidence that the off-set from the local size-mass relation is a function of stellar population age. Based on a sub-sample with dynamical mass estimates we also derive an independent estimate of f=2.3+/-0.3 for the typical growth in half-light radius between z~1.4 and the present day. Focusing on the passive sub-sample, we conclude that to produce the necessary evolution predominantly via major mergers would require an unfeasible number of merger events and over populate the high-mass end of the local stellar mass function. In contrast, we find that a scenario in which mass accretion is dominated by minor mergers can produce the necessary evolution, whereby an increase in stellar mass by a factor of ~2, accompanied by an increase in size by a factor of ~3.5, is sufficient to reconcile the size-mass relation at z~1.4 with that observed locally. Finally, we note that a significant fraction (44+/-12%) of the passive galaxies in our sample have a disk-like morphology, providing additional evidence that separate physical processes are responsible for the quenching of star-formation and the morphological transformation of massive galaxies (abridged).

On the puzzling plateau in the specific star formation rate at z= 2-7

The observational indications for a constant specific star-formation rate (sSFR) in the redshift range z=2-7 are puzzling in the context of current galaxy-formation models. Despite the tentative nature of the data, their marked conflict with theory motivates a study of the possible implications. The plateau at sSFR ~ 2 Gyr^-1 is hard to reproduce because (a) its level is low compared to the cosmological specific accretion rate at z > 6, (b) it is higher than the latter at z ~ 2, (c) the natural correlation between SFR and stellar mass makes it difficult to manipulate their ratio, and (d) a low SFR at high z makes it hard to produce enough massive galaxies by z ~ 2. Using a flexible semi-analytic model, we explore ad-hoc modifications to the standard physical recipes trying to obey the puzzling observational constraints. Successful models involve non-trivial modifications, such as (a) a suppressed SFR at z > 4 in galaxies of all masses, by enhanced feedback or reduced SFR efficiency, following an initial active phase at z > 7, (b) a delayed gas consumption into stars, allowing the gas that was prohibited from forming stars or ejected at high z to form stars later in more massive galaxies, and (c) enhanced growth of massive galaxies, in terms of either faster assembly or more efficient starbursts in mergers, or by efficient star formation in massive haloes.

SIMULATED VOID GALAXIES IN THE STANDARD COLD DARK MATTER MODEL

We analyze a (120 h−1 Mpc)3 adaptive mesh refinement hydrodynamic simulation that contains a higher resolution 31 × 31 × 35 h−3 Mpc subvolume centered on a ∼30 Mpc diameter void. Our detailed ∼1 kpc resolution allows us to identify 1300 galaxies within this void to a limiting halo mass of ∼1010 M☉. Nearly 1000 galaxies are found to be in underdense regions, with 300 galaxies residing in regions less than half the mean density of the simulation volume. We construct mock observations of the stellar and gas properties of these systems and reproduce the range of colors and luminosities observed in the Sloan Digital Sky Survey for nearby (z < 0.03) galaxies. We find no trends with density for the most luminous (Mr < −18) galaxies, however our dwarf void galaxies (Mr > −16), though they are less reliably resolved, typically appear bluer, with higher rates of star formation and specific star formation and lower mean stellar ages than galaxies in average density environments. We find a significant population of low-luminosity (Mr ∼ −14) dwarf galaxies that is preferentially located in low-density regions and specifically in the void center. This population may help to reduce, but not remove, the discrepancy between the predicted and observed number of void galaxies.

Stellar population gradients from cosmological simulations: dependence on mass and environment in local galaxies

The age and metallicity gradients for a sample of group and cluster galaxies from N-body+hydrodynamical simulation are analyzed in terms of galaxy stellar mass. Dwarf galaxies show null age gradient with a tail of high and positive values for systems in groups and cluster outskirts. Massive systems have generally zero age gradients which turn to positive for the most massive ones. Metallicity gradients are distributed around zero in dwarf galaxies and become more negative with mass; massive galaxies have steeper negative metallicity gradients, but the trend flatten with mass. In particular, fossil groups are characterized by a tighter distribution of both age and metallicity gradients. We find a good agreement with both local observations and independent simulations. The results are also discussed in terms of the central age and metallicity, as well as the total colour, specific star formation and velocity dispersion.

The GALEX Arecibo SDSS Survey - II. The star formation efficiency of massive galaxies

We use measurements of the HI content, stellar mass and star formation rates in ~190 massive galaxies with stellar masses greater than 10^10 Msun, obtained from the Galex Arecibo SDSS Survey (GASS) described in Paper I (Catinella et al. 2010) to explore the global scaling relations associated with the bin-averaged ratio of the star formation rate over the HI mass, which we call the HI-based star formation efficiency (SFE). Unlike the mean specific star formation rate, which decreases with stellar mass and stellar mass surface density, the star formation efficiency remains relatively constant across the sample with a value close to SFE = 10^-9.5 yr^-1 (or an equivalent gas consumption timescale of ~3 Gyr). Specifically, we find little variation in SFE with stellar mass, stellar mass surface density, NUV-r color and concentration. We interpret these results as an indication that external processes or feedback mechanisms that control the gas supply are important for regulating star formation in massive galaxies. An investigation into the detailed distribution of SFEs reveals that approximately 5% of the sample shows high efficiencies with SFE > 10^-9 yr^-1, and we suggest that this is very likely due to a deficiency of cold gas rather than an excess star formation rate. Conversely, we also find a similar fraction of galaxies that appear to be gas-rich for their given specific star-formation rate, although these galaxies show both a higher than average gas fraction and lower than average specific star formation rate. Both of these populations are plausible candidates for "transition" galaxies, showing potential for a change (either decrease or increase) in their specific star formation rate in the near future. We also find that 36+/-5% of the total HI mass density and 47+/-5% of the total SFR density is found in galaxies with stellar mass greater than 10^10 Msun. [abridged]

STAR FORMATION AND FEEDBACK IN SMOOTHED PARTICLE HYDRODYNAMIC SIMULATIONS. II. RESOLUTION EFFECTS

We examine the effect of mass and force resolution on a specific star formation (SF) recipe using a set of N-body/Smooth Particle Hydrodynamic simulations of isolated galaxies. Our simulations span halo masses from 10^9 to 10^13 solar masses, more than four orders of magnitude in mass resolution, and two orders of magnitude in the gravitational softening length, epsilon, representing the force resolution. We examine the total global star formation rate, the star formation history, and the quantity of stellar feedback and compare the disk structure of the galaxies. Based on our analysis, we recommend using at least 10^4 particles each for the dark matter and gas component and a force resolution of epsilon approximately equal to 10^-3 R_vir when studying global SF and feedback. When the spatial distribution of stars is important, the number of gas and dark matter particles must be increased to at least 10^5 of each. Low mass resolution simulations with fixed softening lengths show particularly weak stellar disks due to two-body heating. While decreasing spatial resolution in low mass resolution simulations limits two-body effects, density and potential gradients cannot be sustained. Regardless of the softening, low-mass resolution simulations contain fewer high density regions where SF may occur. Galaxies of approximately 10^10 solar masses display unique sensitivity to both mass and force resolution. This mass of galaxy has a shallow potential and is on the verge of forming a disk. The combination of these factors give this galaxy the potential for strong gas outflows driven by supernova feedback and make it particularly sensitive to any changes to the simulation parameters.

Specific star formation and the relation to stellar mass from 0 &lt;z&lt; 2 as seen in the far-infrared at 70 and 160μm

We use the Spitzer Wide-area InfraRed Extragalactic Legacy Survey (SWIRE) to explore the specific star-formation activity of galaxies and their evolution near the peak of the cosmic far-infrared (FIR) background at 70 and 160um. We use a stacking analysis to determine the mean FIR properties of well defined subsets of galaxies at flux levels well below the FIR catalogue detection limits of SWIRE and other Spitzer surveys. We tabulate the contribution of different subsets of galaxies to the FIR background at 70um and 160um. These long wavelengths provide a good constraint on the bolometric, obscured emission. The large area provides good constraints at low z and in finer redshift bins than previous work. At all redshifts we find that the specific FIR Luminosity (sLFIR) decreases with increasing mass, following a trend L_FIR/M* propto M_* ^beta with beta =-0.38\pm0.14. This is a more continuous change than expected from the {Delucia2007} semi-analytic model suggesting modifications to the feedback prescriptions. We see an increase in the sLFIR by about a factor of ~100 from 0<z<2 and find that the sLFIR evolves as (1+z)^alpha with alpha=4.4\pm0.3 for galaxies with 10.5 < log M*/Msun < 12. This is considerably steeper than the {Delucia2007} semi-analytic model (alpha \sim 2.5). When separating galaxies into early and late types on the basis of the optical/IR spectral energy distributions we find that the decrease in sLFIR with stellar mass is stronger in early type galaxies (beta ~ -0.46), while late type galaxies exhibit a flatter trend (beta \sim -0.15). The evolution is strong for both classes but stronger for the early type galaxies. The early types show a trend of decreasing strength of evolution as we move from lower to higher masses while the evolution of the late type galaxies has little dependence on stellar mass. We suggest that in late-type galaxies we are seeing a consistently declining sSFR..

Can galaxy outflows and re-accretion produce a downsizing in the specific star-formation rate of late-type galaxies?

The observations show that less massive the galaxies are, the higher on average is their specific star formation rate (SSFR = SFR/Ms, Ms is the stellar mass). Such a trend, called the 'SSFR downsizing' (SSFR-DS) phenomenon, is seen for local and high-z (back to z~1-2) galaxy samples. We use observational data related only to disc galaxies and explore the average SSFR change with z for different masses. For Ms in the range ~10^9.5-10^10.5 Msun, the SSFR increases with (1+z) to a power that barely depends on Ms, and at all z's smaller galaxies have ever higher SSFRs. The latter strongly disagree with the LCDM hierarchical mass accretion rates. By means of self-consistent models of disc galaxy evolution inside growing LCDM halos, the effects that disc feedback-driven outflows and gas re-accretion have on the galaxy SSFR histories are explored. The parameters of the outflow and re-accretion schemes are tuned to reproduce the z~0 Mh-Ms relation inferred from observations. In the case of outflows only, the SSFR of individual model galaxies is roughly proportional to (1+z)^2.2 for all the masses with a normalization factor that depends on mass as Ms^0.1, i.e more massive galaxies have slightly larger SSFRs, contrary to the observed strong SSFR-DS trend. For the re-accretion cases, the dependence on z remains approximately the same as without re-infall, but the correlation on Ms even increases for most of the reasonable values of the model parameters. The comparison of models and observations in the SSFR-Ms plane at z~0 shows the divergent trend in SSFR as lower are the masses (upsizing vs downsizing). We explain why the models show the reported trends, and conclude that the SSFR-DS for low-mass galaxies poses a challenge for LCDM-based disc galaxy evolution models. (Abridged)