Star-forming Early-type Galaxies Research Articles

The Origin of Star Formation in Early-type Galaxies Inferred from Spatially Resolved Spectroscopy

We investigate the origin of star formation activity in early-type galaxies with current star formation using spatially resolved spectroscopic data from the Mapping Nearby Galaxies at Apache Point Observatory in the Sloan Digital Sky Survey (SDSS). We first identify star-forming early-type galaxies from the SDSS sample, which are morphologically early-type but show current star formation activity in their optical spectra. We then construct comparison samples with different combinations of star formation activity and morphology, which include star-forming late-type galaxies, quiescent early-type galaxies, and quiescent late-type galaxies. Our analysis of the optical spectra reveals that the star-forming early-type galaxies have two distinctive episodes of star formation, which is similar to late-type galaxies but different from quiescent early-type galaxies with a single star formation episode. Star-forming early-type galaxies have properties in common with star-forming late-type galaxies, which include stellar population, gas and dust content, mass, and environment. However, the physical properties of star-forming early-type galaxies derived from spatially resolved spectroscopy differ from those of star-forming late-type galaxies in the sense that the gas in star-forming early-type galaxies is more concentrated than their stars, and is often kinematically misaligned with stars. The age gradient of star-forming early-type galaxies also differs from those of star-forming late-type galaxies. Our findings suggest that the current star formation in star-forming early-type galaxies has an external origin including galaxy mergers or accretion gas from the cosmic web.

Star-forming early-type galaxies and quiescent late-type galaxies in the local Universe

Aims. The general consensus is that late-type galaxies undergo intense star-formation, activity while early-type galaxies are mostly inactive. We question this general rule and investigate the existence of star-forming early-type and quiescent late-type galaxies in the local Universe. By computing the physical properties of these galaxies and by using information on their structural properties as well as the density of their local environment, we seek to understand the differences from their ‘typical’ counterparts. Methods. We made use of the multi-wavelength photometric data (from the ultraviolet to the sub-millimetre), for 2209 morphologically classified galaxies in the Galaxy And Mass Assembly survey. Furthermore, we separated the galaxies into subsets of star-forming and quiescent based on their dominant ionising process, making use of established criteria based on the WHα width and the [NII/Hα] ratio. Taking advantage of the spectral energy distribution fitting code CIGALE, we derived galaxy properties, such as the stellar mass, dust mass, and star-formation rate, and we also estimated the unattenuated and the dust-absorbed stellar emission, for both the young (≤200 Myr) and old (> 200 Myr) stellar populations. Results. We find that about 47% of E/S0 galaxies in our sample show ongoing star-formation activity and 8% of late-type galaxies are quiescent. The star-forming elliptical galaxies, together with the little blue spheroids, constitute a population that follows the star-forming main sequence of spiral galaxies very well. The fraction of the luminosity originating from young stars in the star-forming early-type galaxies is quite substantial (∼25%) and similar to that of the star-forming late-type galaxies. The stellar luminosity absorbed by the dust (and used to heat the dust grains) is highest in star-forming E/S0 galaxies (an average of 35%) followed by star-forming Sa-Scd galaxies (27%) with this fraction becoming significantly smaller for their quiescent analogues (6% and 16%, for E/S0 and Sa-Scd, respectively). Star-forming and quiescent E/S0 galaxies donate quite different fractions of their young stellar luminosities to heat up the dust grains (74% and 36%, respectively), while these fractions are very similar for star-forming and quiescent Sa-Scd galaxies (59% and 60%, respectively). Investigating possible differences between star-forming and quiescent galaxies, we find that the intrinsic (unattenuated) shape of the SED of the star-forming galaxies is, on average, very similar for all morphological types. Concerning their structural parameters, quiescent galaxies tend to show larger values of the r-band Sérsic index and larger effective radii (compared to star-forming galaxies). Finally, we find that star-forming galaxies preferably reside in lower density environments compared to the quiescent ones, which exhibit a higher percentage of sources being members of groups.

Comparison of Composite and Star-forming Early-type Galaxies

Abstract I assemble 4684 star-forming early-type galaxies (ETGs) and 2011 composite ETGs (located in the composite region on the BPT diagram) from the catalog of the Sloan Digital Sky Survey Data Release 7 MPA-JHU emission-line measurements. I compare the properties of both ETG samples and investigate their compositions, stellar masses, specific star formation rates (sSFRs), and excitation mechanisms. Compared with star-forming ETGs, composite ETGs have higher stellar mass and lower sSFR. In the stellar mass and u − r color diagram, more than 60% of star-forming ETGs and composite ETGs are located in the green valley, showing that the two ETG samples may have experienced star formation and that ∼17% of star-forming ETGs lie in the blue cloud, while ∼30% of composite ETGs lie in the red sequence. In the [N II]/Hα versus EWHα (the Hα equivalent width) diagram, all star-forming ETGs and most of the composite ETGs are located in the star-forming galaxy region, and composite ETGs have lower EWHα than their counterparts. We show the relations between 12+log(O/H) and log(N/O) for both ETG samples, and suggest that nitrogen production of some star-forming ETGs can be explained by the evolution scheme of Coziol et al., while the prodution of composite ETGs may be a consequence of the inflowing of metal-poor gas and these more evolved massive galaxies.

Morphological Transformation and Star Formation Quenching of Massive Galaxies at 0.5 ≤ z ≤ 2.5 in 3D-HST/CANDELS

Abstract To figure out the effect of stellar mass and local environment on morphological transformation and star formation quenching in galaxies, we use the massive (M * ≥ 1010 M ⊙) galaxies at 0.5 ≤ z ≤ 2.5 in five fields of 3D-HST/CANDELS. Based on the UVJ diagnosis and the possibility of possessing a spheroid, our sample of massive galaxies is classified into four populations: quiescent early-type galaxies (qEs), quiescent late-type galaxies (qLs), star-forming early-type galaxies (sEs), and star-forming late-type galaxies (sLs). It is found that the quiescent fraction is significantly elevated at the high ends of mass and local environmental overdensity, which suggests a clear dependence of quenching on both mass and local environment. Over cosmic time, the mass dependence of galaxy quiescence decreases while the local environment dependence increases. The early-type fraction is found to be larger only at the high-mass end, indicating an evident mass dependence of morphological transformation. This mass dependence becomes more significant at lower redshifts. Among the four populations, the fraction of active galactic nuclei (AGNs) in the qLs peaks at 2 < z ≤ 2.5, and rapidly declines with cosmic time. The sEs are found to have higher AGN fractions of 20%–30% at 0.5 ≤ z < 2 . The redshift evolution of AGN fractions in the qLs and sEs suggests that AGN feedback could have played important roles in the formation of the qLs and sEs.

Estimating the metallicity of star-forming early-type galaxies

ABSTRACT We derive data for 4615 star-forming early-type galaxies (ETGs), which come from a cross-match of the Galaxy Zoo and the catalogue of the Max Planck Institute for Astrophysics–Johns Hopkins University (MPA–JHU) emission-line measurements for the Sloan Digital Sky Survey Data Release 7. Our sample is distributed mainly at $-0.7< \log({\rm SFR}/({\rm M}_{\odot }\, {\rm yr}^{-1})) < 1.2$, and the median value of our star formation rates (SFRs) is slightly higher than that shown in Davis & Young. We show that there is a significant trend of lower/higher stellar mass ETGs having lower/higher SFRs, and we obtain our sample best fit of $\log(\rm SFR/(M_{\odot} \rm yr^{-1}))=(0.74\pm0.01)\rm log(M_{*}/M_{\odot})-(7.64\pm0.10)$. We find the same slope as found by Cano-Díaz et al. In our star-forming ETG sample, we demonstrate clearly the correlation of the stellar mass–metallicity (MZ) relation. We find that higher metallicity measurements may be introduced by the diffuse ionized gas, when the D16, Sanch18 and Sander18 indicators are used to calibrate the metallicity of ETGs. We show the relations between the SFR and 12 + log(O/H) with different metallicity estimators, and we suggest that their correlations may be a consequence of the SFR–stellar mass and MZ relations in ETGs.

Far-infrared–radio correlation and magnetic field strength in star-forming early-type galaxies

A tight far-infrared - radio correlation similar to that in star-forming late-type galaxies is also indicated in star-forming blue early-type galaxies, in which the nuclear optical-line emissions are primarily due to star-forming activities without a significant contribution from active galactic nuclei. The average value of far-infrared to 1.4 GHz radio flux-ratio commonly represented as the $'q'$ parameter is estimated to be $2.35\pm0.05$ with a scatter of 0.16 dex. The average star formation rate estimated using 1.4 GHz radio continuum is $\sim6$ M$_{\odot}$ yr$^{-1}$ in good agreement with those estimated using far-infrared and H$\alpha$ luminosities. The radio emission is detected mainly from central region which could be associated with the star-forming activities, most likely triggered by recent tidal interactions. The average thermal contribution to total radio flux is estimated to be $\sim12$ per cent. The average value of the magnetic field strengths in the star-forming early-type galaxies is estimated to be 12$^{+11}_{-4}$ $\mu$G. These magnetic fields are very likely generated via fast amplification in small-scale turbulent dynamos acting in the star-bursting regions.

THE SLOW DEATH (OR REBIRTH?) OF EXTENDED STAR FORMATION INz∼ 0.1 GREEN VALLEY EARLY-TYPE GALAXIES

UV observations in the local universe have uncovered a population of early-type galaxies with UV flux consistent with low-level recent or ongoing star formation. We present resolved UV-optical photometry of a sample of 19 SDSS early-type galaxies at z~0.1 drawn from the sample originally selected by Salim & Rich (2010) to lie in the bluer part of the green valley in the UV-optical color-magnitude diagram as measured by GALEX. Utilizing high-resolution HST far-UV imaging provides unique insight into the distribution of UV light in these galaxies, which we call "extended star-forming early-type galaxies" (ESF-ETGs) because of extended UV emission that is indicative of recent star formation. The UV-optical color profiles of all ESF-ETGs show red centers and blue outer parts. Their outer colors require the existence of a significant underlying population of older stars in the UV-bright regions. Analysis of stacked SDSS spectra reveals weak LINER-like emission in their centers. Using a cross-matched SDSS DR7/GALEX GR6 catalog, we search for other green valley galaxies with similar properties to these ESF-ETGs and estimate that ~13% of dust-corrected green valley galaxies of similar stellar mass and UV-optical color are likely ESF-candidates, i.e., ESF-ETGs are not rare. Our results are consistent with star formation that is gradually declining in existing disks, i.e., the ESF-ETGs are evolving onto the red sequence for the first time, or with rejuvenated star formation due to accreted gas in older disks provided that the gas does not disrupt the structure of the galaxy and the resulting star formation is not too recent and bursty. ESF-ETGs may typify an important subpopulation of galaxies that can linger in the green valley for up to several Gyrs, based on their resemblance to nearby gas-rich green valley galaxies with low-level ongoing star formation. (abridged)

A simple model for AGN feedback in nearby early-type galaxies

Recent work indicates that star-forming early-type galaxies (ETGs) residing in the blue cloud migrate rapidly to the red sequence within around a Gyr, passing through several phases of increasingly strong AGN activity in the process (Schawinski et al. 2007, MNRAS, 382, 1415; S07 hereafter). We show that natural depletion of the gas reservoir through star formation (i.e. in the absence of any feedback from the AGN) induces a blue-to-red reddening rate that is several factors lower than that observed in S07. This is because the gas depletion rate due to star formation alone is too slow, implying that another process needs to be invoked to remove gas from the system and accelerate the reddening rate. We develop a simple phenomenological model, in which a fraction of the AGN's luminosity couples to the gas reservoir over a certain 'feedback timescale' and removes part of the gas mass from the galaxy, while the remaining gas continues to contribute to star formation. We use the model to investigate scenarios which yield migration times consistent with the results of S07. We find that acceptable models have feedback timescales <0.2 Gyrs. The mass fraction in young stars in the remnants is <5% and the residual gas fractions are less than 0.6%, in good agreement with the recent literature. At least half of the initial gas reservoir is removed as the galaxies evolve from the blue cloud to the red sequence. If we restrict ourselves to feedback timescales similar to the typical duty cycles of local AGN (a few hundred Myrs) then a few tenths of a percent of the luminosity of an early-type Seyfert (10^11 LSun) must couple to the gas reservoir in order to produce migration times that are consistent with the observations.

Current star formation in early-type galaxies and the KA phenomenon

We present the results of an effort to identify and study a sample of the likely progenitors of elliptical (E) and lenticular (S0) K+A galaxies. To achieve this, we have searched a sample ∼11 000 nearby (m r < 16) early-type galaxies selected by morphology from the Sloan Digital Sky Survey (SDSS) main spectroscopic sample for actively star-forming E and S0 galaxies. Using emission-line ratios and visual inspection of SDSS g-band images, we have identified 335 galaxies from the SDSS Fourth Data Release (DR4) as actively star-forming E and S0 galaxies. These galaxies make up about 3 per cent of the total early-type sample and less than 1 per cent of all main galaxies with m r < 16. We also identified a sample of ∼400 K+A galaxies from DR4 with m r < 16; more than half of these are E and S0 galaxies. We find that star-forming early-type galaxies and K+A galaxies have similar mass distributions; they are on average less massive than typical early-type galaxies but more massive than the average starforming galaxy. Both of these types of galaxies are found in higher fractions among all galaxies in lower density environments. The fractions of star-forming E and S0 galaxies and E and S0 K+A galaxies depend on environment in nearly the same way. Model spectra fit to the stellar continua of the star-forming E and S0 galaxies showed that their properties are consistent with star formation episodes of � 1 Gyr in duration. The modelling results imply that on average, the star formation episodes will increase the stellar masses by about 4 per cent. The results also imply that only episodes that increase the stellar mass by more than 2‐5 per cent will lead to K+A galaxies as we have defined them and that this is true for roughly 80 per cent of the star-forming E and S0 galaxies in our sample. The estimated typical increase in stellar mass

Starbursts versus Truncated Star Formation in Nearby Clusters of Galaxies

We present long-slit spectroscopy, B and R bandpass imaging, and 21 cm observations of a sample of early-type galaxies in nearby clusters which are known to be either in a star-forming phase or to have had star formation which recently terminated. From the long-slit spectra, obtained with the Blanco 4-m telescope, we find that emission lines in the star-forming cluster galaxies are significantly more centrally concentrated than in a sample of field galaxies. The broadband imaging reveals that two currently star-forming early-type galaxies in the Pegasus I cluster have blue nuclei, again indicating that recent star formation has been concentrated. In contrast, the two galaxies for which star formation has already ended show no central color gradient. The Pegasus I galaxy with the most evident signs of ongoing star formation (NGC7648), exhibits signatures of a tidal encounter. Neutral hydrogen observations of that galaxy with the Arecibo radiotelescope reveal the presence of ~4 x 10^8 solar masses of HI. Arecibo observations of other current or recent star-forming early-type galaxies in Pegasus I indicate smaller amounts of gas in one of them, and only upper limits in others.