Bursty stellar populations and obscured active galactic nuclei in galaxy bulges (original) (raw)
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Monthly Notices of the Royal Astronomical Society, 2003
We present the integrated properties of the stellar populations in the Universidad Complutense de Madrid (UCM) Survey galaxies. Applying the techniques described in the first paper of this series, we derive ages, burst masses and metallicities of the newly formed stars in our sample galaxies. The population of young stars is responsible for the Hα emission used to detect the objects in the UCM Survey. We also infer total stellar masses and star formation rates in a consistent way taking into account the evolutionary history of each galaxy. We find that an average UCM galaxy has a total stellar mass of ~1010 Msolar, of which about 5 per cent was formed in an instantaneous burst that occurred about 5 Myr ago, and subsolar metallicity. Less than 10 per cent of the sample shows massive starbursts involving more than half of the total mass of the galaxy. Several correlations are found among the derived properties. The burst strength is correlated with the extinction and with the integrated optical colours for galaxies with low obscuration. The current star formation rate is correlated with the gas content. A stellar mass-metallicity relation is also found. Our analysis indicates that the UCM Survey galaxies span a broad range in properties between those of galaxies completely dominated by current/recent star formation and those of normal quiescent spirals. We also find evidence indicating that star formation in the local Universe is dominated by galaxies considerably less massive than L*.
Star formation history in early-type galaxies - I. The line absorption indices diagnostics
Monthly Notices of the Royal Astronomical Society, 2004
To unravel the formation mechanism and the evolutionary history of Elliptical Galaxies (EGs) is one of the goals of modern astrophysics. In a simplified picture of the issue, the question to be answered is whether they have formed by hierarchical merging of pre-existing sub-structures (maybe disc galaxies) made of stars and gas, each merging event likely accompanied by strong star formation, or conversely, they originated from the early aggregation of lumps of gas turned into stars in the remote past via a burst-like episode ever since followed by quiescence so as to mimic a sort of monolithic process. Even if the two alternatives seem to oppose each other, actually they may concur to shaping the final properties of EG's as seen today. Are there distinct signatures of the underlying dominant process in the observational data? To this aim we have examined the line absorption indices on the Lick system of the normal, field EGs of Trager (1997) and the interacting EGs (pair-and shell-objects) of Longhetti et al. (2000). The data show that both normal, field and interacting galaxies have the same scattered but smooth distribution in the H β vs. [MgFe] plane even if the interacting ones show a more pronounced tail toward high H β values. This may suggest that a common physical cause is at the origin of their distribution. There are two straightforward interpretations of increasing complexity: (1) EGs span true large ranges of ages and metallicities. The age youth is the signature of the aggregation mechanism, each event accompanied by metal enrichment. This simple scheme cannot, however, explain other spectro-photometric properties of EGs and has to be discarded. (2) The bulk population of stars is old but subsequent episodes of star formation scatter the EGs in the diagnostic planes. However, this scheme would predict an outstanding clump at low H β values, contrary to what is observed. The model can be cured by supposing that the primary star formation activity lasted for a significant fraction of the Hubble time (5 Gyr T 13 Gyr) accompanied by global metal enrichment. The "younger" galaxies are more metal-rich. The later burst of star formation should be small otherwise too many high H β objects would be observed. Therefore, the distribution of normal, pairand shell-galaxies in the H β vs. [MgFe] plane is due to the global metal enrichment. Even though the above schemes provide a formal explanation, they seem to be too demanding because of the many ad hoc ingredients that have to be introduced. Furthermore they neglect the observationally grounded hint that the stellar content of EGs is likely enhanced in α-elements with [α/F e] ranging from 0.1 to 0.4 dex. We propose here a new scheme, in which the bulk dispersion of galaxies in the H β vs. [MgFe] plane is caused by a different mean degree of enhancement. In this model, neither large age ranges nor universal enrichment law for the old component are required and the observed distribution along H β is naturally recovered. Furthermore, later bursts of stellar activity are a rare event interesting only those galaxies with very high H β (roughly > 2.5). Finally, simulations of the scatter in broad-band colors of EG's seem to confirm that the bulk stars have formed in the remote past, and that mergers and companion star formation in a recent past are not likely, unless the intensity of the secondary activity is very small.
Star Formation in a Complete Spectroscopic Survey of Galaxies
The Astrophysical Journal, 2001
The 15R-North galaxy redshift survey is a uniform spectroscopic survey (S/N ∼10) covering the range 3650-7400Å for 3149 galaxies with median redshift 0.05. The sample is 90% complete to R = 15.4. The median slit covering fraction is 24% of the galaxy, apparently sufficient to minimize the effects of aperture bias on the EW(Hα). Forty-nine percent of the galaxes in the survey have one or more emission lines detected at ≥ 2σ. In agreement with previous surveys, the fraction of absorption-line galaxies increases steeply with galaxy luminosity. We use Hβ, O[III], Hα, and [N ii] to discriminate between star-forming galaxies and AGNs. At least 20% of the galaxies are star-forming, at least 17% have AGN-like emission, and 12% have unclassifiable emission. The unclassified 12% may include a "hybrid" population of galaxies with both star-formation and AGN activity. The AGN fraction increases steeply with luminosity; the fraction of star-forming galaxies decreases. We use the EW(Hα + [N ii]) to estimate the Scalo birthrate parameter, b, the ratio of the current star formation rate to the time averaged star formation rate. The median birthrate parameter is inversely correlated with luminosity in agreement with the conclusions based on smaller samples (Kennicutt, Tamblyn, & Congdon 1994). Because our survey is large, we identify 33 vigorously starforming galaxies with b > 3. We confirm the conclusion of Jansen, Franx, & Fabricant (2001) that EW(O[II]) must be used with caution as a measure of current star formation. Finally, we examine the way galaxies of different spectroscopic type trace the large-scale galaxy distribution. As expected the absorption line fraction decreases and the star-forming emission-line fraction increases as the galaxy density decreases. The AGN fraction is insensitive to the surrounding galaxy density; the unclassified fraction declines slowly as the density increases. For the starforming galaxies, the EW(Hα) increases very slowly as the galaxy number density decreases. Whether a galaxy forms stars or not is strongly correlated with the surrounding galaxy density averaged over a scale of a few Mpc. This dependence reflects, in large part, the morphology-density relation. However, for galaxies forming stars, the stellar birthrate parameter is remarkably insensitive to the galaxy density. This conclusion suggests that the triggering of star formation occurs on a smaller spatial scale.
Monthly Notices of the Royal Astronomical Society, 2010
We pick out the 35 brightest galaxies from Goto's E+A galaxies catalogue which are selected from the Sloan Digital Sky Survey Data Release 5. As E+As have experienced starburst recently and quenched it abruptly, they have been considered as post-starburst galaxies. The spectra of E+As are prominently characterized by the strong Balmer absorption lines but little [Oii] or Hα emission lines. In this work we study the stellar populations of the sample galaxies by fitting their spectra using ULySS, which is a robust full spectrum fitting method. We fit each of the sample with 1-population (a single stellar population-a SSP) and 3-population (3 SSPs) models, separately. By 1-population fits, we obtain SSP-equivalent ages and metallicities which correspond to the 'luminosity-weighted' averages. By 3-population fits, we divide components into three groups in age (old stellar population-OSP, intermediate-age stellar population-ISP, and young stellar population-YSP), and then get the optimal age, metallicity and population fractions in both mass and light for OSP, ISP and YSP. During the fits, both Pegase.HR/Elodie3.1 and Vazdekis/Miles are used as two independent population models. The two models result in generally consistent conclusions as follows: for all the sample galaxies, YSPs (1Gyr) make important contributions to the light. However, the dominant contributors to mass are OSPs. We also reconstruct the smoothing star formation histories (SFHs) by giving star formation rate (SFR) versus evolutionary age. All the sample galaxies have low SFRs in the intermediate evolutionary stage. Eleven of the thirty-five E+As have high SFRs in the early evolutionary stage. However, another 11 have SFRs that are high during the late evolutionary stage. This might be due to the recently happened but abruptly truncated starburst in such galaxies. In addition, we fit the E+A sample and 34 randomly selected elliptical galaxies with 2-population (2 SSPs) model, which could divide the galaxy components into two groups in age (old component and young component). We obtain the equivalent age of old components for each of the E+A sample and elliptical galaxies. By comparison, the old components of E+As are statistically much younger than those of ellipticals. From the standpoint of the stellar population age, this probably provides an evidence for the proposed evolutionary link from E+As to early-types (E/S0s).
Monthly Notices of the Royal Astronomical Society, 2013
We report on results from the analysis of a stellar mass-selected (log(M * /M ⊙ ) 9.0) sample of 1644 galaxies at 0.65<z<1.1 with ultra-deep (m AB <26.5) optical medium-band (R∼50) photometry from the Survey for High-z Absorption Red and Dead Sources (SHARDS). The spectral resolution of SHARDS allows us to consistently measure the strength of the 4000 Å spectral break [D n (4000), an excellent age indicator for the stellar populations of quiescent galaxies] for all galaxies at z∼0.9 down to log(M * /M ⊙ )∼9. The D n (4000) index cannot be resolved from broad-band photometry, and measurements from optical spectroscopic surveys are typically limited to galaxies at least ×10 more massive. When combined with the restframe U-V colour, (U-V) r , D n (4000) provides a powerful diagnostic of the extinction affecting the stellar population that is relatively insensitive to degeneracies with age, metallicity or star formation history. We use this novel approach to estimate de-reddened colours and lightweighted stellar ages for individual sources. We explore the relationships linking stellar mass, (U-V) r , and D n (4000) for the sources in the sample, and compare them to those found in local galaxies. The main results are: a) both D n (4000) and (U-V) r correlate with M * . The dispersion in D n (4000) values at a given M * increases with M * , while the dispersion for (U-V) r decreases due to the higher average extinction prevalent in massive star-forming galaxies. b) for massive galaxies, we find a smooth transition between the blue cloud and red sequence in the intrinsic U-V colour, in contrast with other recent results. c) at a fixed stellar age, we find a positive correlation between extinction and stellar mass. d) the fraction of sources with declining or halted star formation increases steeply with the stellar mass, from ∼5% at log(M * /M ⊙ )=9.0-9.5 to ∼80% at log(M * /M ⊙ )>11, in agreement with downsizing scenarios.
A comprehensive study of intense star formation bursts in irregular and compact galaxies
Arxiv preprint astro-ph/9812072, 1998
We have analyzed the properties of the star formation episodes taking place in a sample of blue compact and irregular galaxies by comparing their multiwavelength observational properties with the predictions of evolutionary population synthesis models. This method has allowed us to constrain the age, star formation regime (instantaneous or extended) and Initial Mass Function (IMF) slope, as well as the shape and strength of the interstellar extinction in these regions. We find that star formation episodes are essentially short with a mean age of 3.5 Myrs. Some galaxies may be undergoing their first global episode of star formation while for the rest of the sample older stars contribute to at most half the optical emission. The Wolf-Rayet star population (WR) is well reproduced by the models and provides the strongest argument in favor of a short duration of the star formation episode. Supernova rates are relatively large. The accumulation of supernova explosions within few Myr has contributed to a quick metal enrichment of the ISM and to its disruption by the release of huge amounts of mechanical energy. V −K colors agree well with the prediction that red supergiant stars are rare in low metallicity regions. A general agreement is found between the predicted and observed far infrared emissions suggesting that the fraction of hidden stars contributing to the ionisation is minimum, except in some specific objects. A saillant result of this study is that the IMF slope appears to be very universal, on average very close to that of the solar neighborhood and with no dependence on the metallicity, contrary to previous claims. We have also found no dependence whatsoever between the shape of the extinction law and the metallicity. It is likely that the strong radiation associated to the bursts destroys the dust component responsible for the 2175Å bump. Finally we confirm that extinction affecting the stellar continuum is in some cases significantly weaker than that derived from the Balmer emission lines. Such a discrepancy can lead to underestimations in the value of the Hβ equivalent width by a factor as large as 2, leading to an overestimation of Send offprint requests to: J.M. Mas-Hesse Based on observations from the International Ultraviolet Explorer obtained at the ESA VILSPA observatory, on observations taken at the Isaac Newton Telescope at the Spanish Observatorio del Roque de Los Muchachos on La Palma island and on observations with the Nançay radiotelescope. the age of the burst. Similarly, the Wolf-Rayet bump to the Hβ luminosities ratio can also be affected by this differential reddening leading to an overestimation of the WR star population. As bursts get older they appear dustier, possibly as a result of dust ejection during the evolution of their most massive stars. Finally, we have found a serious general discrepancy between the predicted and the measured radio luminosities. While part of this discrepancy might be attributed to aperture mismatching in some cases, it points to the presence of additional radio sources not included in present evolutionary models.
Bursty stellar populations and obscured AGN in galaxy bulges
Arxiv preprint arXiv: …, 2007
We investigate trends between the recent star formation history and black hole growth in galaxy bulges in the Sloan Digital Sky Survey (SDSS). The galaxies lie at 0.01 < z < 0.07 where the fibre aperture covers only the central 0.6-4.0 kpc diameter of the galaxy. We find strong trends between black hole growth, as measured by dust-attenuation-corrected [O III] luminosity, and the recent star formation history of the bulges. 56% of the bulges are quiescent with no signs of recent or ongoing star formation and, while almost half of all AGN lie within these bulges, they contribute only ∼10% to the total black hole growth in the local Universe. At the other extreme, the AGN contained within the ∼4% of galaxy bulges that are undergoing or have recently undergone the strongest starbursts, contribute at least 10-20% of the total black hole growth. Much of this growth occurs in AGN with high amounts of dust extinction and thus the precise numbers remain uncertain. The remainder of the black hole growth (>60%) is contributed by bulges with more moderate recent or ongoing star formation. The strongest accreting black holes reside in bulges with a wide range in recent SFH. We conclude that our results support the popular hypothesis for black hole growth occurring through gas inflow into the central regions of galaxies, followed by a starburst and triggering of the AGN. However, while this is a significant pathway for the growth of black holes, it is not the dominant one in the present-day Universe. More unspectacular processes are apparently responsible for the majority of this growth.
Stellar population in star formation regions of galaxies
Open Astronomy
We developed techniques for searching young unresolved star groupings (clusters, associations, and their complexes) and of estimating their physical parameters. Our study is based on spectroscopic, spectrophotometric, and UBVRI photometric observations of 19 spiral galaxies. In the studied galaxies, we found 1510 objects younger than 10 Myr and present their catalogue. Having combined photometric and spectroscopic data, we derived extinctions, chemical abundances, sizes, ages, and masses of these groupings. We discuss separately the specific cases, when the gas extinction does not agree with the interstellar one. We assume that this is due to spatial offset of Hii clouds with respect to the related stellar population.We developed a method to estimate age of stellar population of the studied complexes using their morphology and the relation with associated H emission region. In result we obtained the estimates of chemical abundances for 80, masses for 63, and ages for 57 young object...
Star formation in distant starburst galaxies
Astronomy and Astrophysics
This paper discusses the stellar population content of distant (5 000 km s −1 ≤ V R ≤ 16 000 km s −1 ) galaxies with enhanced star-formation activity. Distinction is made between isolated galaxies and galaxies morphologically disturbed, with clear signs of interaction such as mergers. In these galaxies the International Ultraviolet Explorer (IUE) large aperture samples most of the galaxy's body. Consequently, the resulting integrated spectra arise primarily from blue stellar populations of different ages together with significant contributions from intermediate and old age components, subject to varying reddening amounts. Instead of analysing individual, usually low Signal-to-Noise ratio (S/N) spectra, our approach is to coadd the spectra of objects with similar spectral properties in the UV, considering as well their properties in the visible/near-infrared ranges. Consequently, the resulting high (S/N) template spectra contain the average properties of a rather uniform class of objects, and information on spectral features can now be analysed with more precision. Three groups have been found for the interacting galaxies, corresponding to a red, blue and very blue continuum. Isolated galaxies have been separated into two groups, one with a flat/red continuum and the other with a blue continuum. For comparison, we also include in the present analysis two groups of nearby disturbed galaxies. Stellar populations are analysed by means of a synthesis algorithm based on star cluster spectral components of different ages which fit the observed spectra both in terms of continuum distribution and spectral features. Flux fractions of the different age groups found in the synthesis have been transformed into mass fractions, allowing inferences on the star formation histories. Young stellar populations (age < 500 Myr) are the main flux contributors, except for the groups with a red spectrum not due to extinction, arising from the intermediate (age ≈ 1 − 2 Gyr) and old age populations. We also study the reddening values and the extinction law: a Small Magellanic Cloud-like extinction law is appropriate for all cases. As compared to nearby galaxies with enhanced star-formation, the distant starburst galaxy spectral groups exhibit larger contributions from the intermediate and old age populations. This effect is mainly accounted for by the larger spatial area sampled Send offprint requests to: C. Bonatto