Old Stellar Populations. VI. Absorption-Line Spectra of Galaxy Nuclei and Globular Clusters (original) (raw)
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Monthly Notices of the Royal Astronomical Society, 2010
We present a stellar population analysis of the absorption line strength maps for 48 earlytype galaxies from the SAURON sample. Using the line strength index maps of Hβ, Fe5015, and Mg b, measured in the Lick/IDS system and spatially binned to a constant signal-tonoise, together with predictions from up-to-date stellar population models, we estimate the simple stellar population-equivalent (SSP-equivalent) age, metallicity and abundance ratio [α/Fe] over a two-dimensional field extending up to approximately one effective radius. A discussion of calibrations and differences between model predictions is given. Maps of SSPequivalent age, metallicity and abundance ratio [α/Fe] are presented for each galaxy. We find a large range of SSP-equivalent ages in our sample, of which ∼40 per cent of the galaxies show signs of a contribution from a young stellar population. The most extreme cases of post-starburst galaxies, with SSP-equivalent ages of 3 Gyr observed over the full fieldof-view, and sometimes even showing signs of residual star-formation, are restricted to low mass systems (σ e 100 km s −1 or ∼2 ×10 10 M ⊙ ). Spatially restricted cases of young stellar populations in circumnuclear regions can almost exclusively be linked to the presence of star-formation in a thin, dusty disk/ring, also seen in the near-UV or mid-IR on top of an older underlying stellar population.
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.
The Astrophysical Journal, 2002
The narrow absorption lines (NALs) that are seen in the rest-frame ultraviolet near the systemic redshift of active galactic nuclei (AGNs) are not always intrinsic to the near-nuclear region but may originate in the host galaxy or in neighboring galaxies intervening along the line of sight. A variety of criteria have been sought-and several identified-as evidence of an intrinsic origin. We have measured both emission and absorption lines in a Hubble Space Telescope Faint Object Spectrograph sample of objects with both C iv and N v NALs within 0005ע km s of the systemic redshift. We find a strong (199.5% confidence) linear correlation between the Ϫ1 N v/C iv ratio in broad emission lines and that in NALs. A control sample of AGNs with NALs separated by larger velocities shows no such correlation. Our finding thus identifies an additional test for the intrinsic nature of NALs in any given object. The correlation shows that the chemical-enrichment histories and/or ionization parameters of the NAL clouds are closely related to those of clouds that produce the broad emission lines.
Monthly Notices of the Royal Astronomical Society, 2013
A method that is widely used to analyse stellar populations in galaxies is to apply the theoretically derived responses of stellar spectra and line indices to element abundance variations, which are hereafter referred to as response functions. These are applied in a differential way, to base models, in order to generate spectra or indices with different abundance patterns. In this paper, sets of such response functions for three different stellar evolutionary stages are tested with new empirical [Mg/Fe] abundance data for the medium-resolution Isaac Newton Telescope library of empirical spectra (MILES). Recent theoretical models and observations are used to investigate the effects of [Fe/H], [Mg/H] and overall [Z/H] on spectra, via ratios of spectra for similar stars. The global effects of changes in abundance patterns are investigated empirically through direct comparisons of similar stars from MILES, highlighting the impact of abundance effects in the blue part of the spectrum, particularly for lower temperature stars. It is found that the relative behaviour of iron-sensitive line indices are generally well predicted by response functions, whereas Balmer line indices are not. Other indices tend to show large scatter about the predicted mean relations. Implications for element abundance and age studies in stellar populations are discussed and ways forward are suggested to improve the match with the behaviour of spectra and line-strength indices observed in real stars.
Stellar abundance gradients in galactic discs - I. Method and spectral line gradients
Monthly Notices of the Royal Astronomical Society, 2005
We describe the technique of absorption line imaging of galaxy disks using the Taurus Tunable Filter on the Anglo-Australian Telescope and demonstrate its sensitivity to the behaviour of spectral features associated with Mg and Fe. Radial profiles of Mg 2 and Fe5270 line-strengths are presented for a sample of eight face-on spiral galaxies spanning a range of Hubble types. Signatures of phenomena including merger-induced star formation, H rings and galactic bars are also reported. This study demonstrates the capacity of tunable filters to measure Mg and Fe line-strengths across the face of spiral galaxies, which can ultimately reveal clues about the star formation history and chemical evolution.
The Astrophysical Journal, 1999
NGC 3516 displays the strongest intrinsic blueshifted UV absorption lines of any Seyfert 1 galaxy. In this paper, we report on UV spectrophotometric monitoring data obtained over a period of 11 months using the Hubble Space T elescope Faint Object Spectrograph (HST /FOS). The observations were at D2 resolution and covered the wavelength range 1150È3300 During the (HST /FOS) observation A A . period, the UV continuum showed a factor of 5 variation, spanning almost the entire range in continuum Ñux seen in over 15 yr of IUE data. The broad (FWHM D 10,000 km s~1) high-ionization emission lines (HILs) of Lya j1215, C IV j1549, N V j1240, and He II j1640 showed signiÐcant variation (a factor of D2), while the low-ionization lines (LILs) of Mg II j2798 and Fe II showed no signiÐcant variation (less than 7%) within the signal-to-noise ratio of our data. No signiÐcant variability was detected in the narrow (FWHM D 1000 km s~1) emission line strengths over the course of our campaign. The proÐle shapes of the HILs also display signiÐcant variation, with the largest Ñux changes occurring in the core of the line, blueward of line center, a picture generally consistent with a kinematically radial outÑowing broad-line region. Interestingly, the proÐle shapes of Lya and C IV are identical, suggesting a common kinematic origin for these lines. This is supported by an absence of variation in their respective line ratios despite signiÐcant Ñux changes. Perhaps even more surprisingly, at high continuum levels the proÐle shapes of C IV and Mg II are indistinguishable. Moreover, although the Mg II emission-line Ñux varies on longer timescales (Dyears), the proÐle shape of the Mg II emission line appears invariant on timescales of a decade. Our spectra clearly show narrow (FWHM D 200È500 km s~1) intrinsic absorption lines of highly ionized species (e.g., Lya j1215, N V j1240, Si IV j1397, and C IV j1549), but we do not observe the higher velocity C IV absorption-line component seen in the historic (pre-1989) IUE/SWP spectra. The absence of a detectable variation in the narrow absorption line strengths over the course of our observing campaign (11 months) suggests absorption in ions close to their peak in fractional abundance. We Ðnd no evidence for a change in the strength of the narrow absorption lines, suggesting that the absorbing gas remains stable on timescales of at least 20 months. The line widths and ionization state of the intrinsic absorbers are found to be consistent with a narrow-line region origin for the absorbing gas.
Line-of-sight velocity distribution corrections for Lick/IDS indices of early-type galaxies
Astronomy and Astrophysics, 2004
We investigate line-of-sight velocity distribution (LOSVD) corrections for absorption line-strength indices of early-type galaxies in the Lick/IDS system. This system is often used to estimate basic stellar population parameters such as luminosity weighted ages and metallicities. Using single stellar population model spectral energy distributions by Vazdekis (1999) we find that the LOSVD corrections are largely insensitive to changes in the stellar populations for old galaxies (age > 3 Gyr). Only the Lick/IDS Balmer series indices show an appreciable effect, which is on the order of the correction itself. Furthermore, we investigate the sensitivity of the LOSVD corrections to non-Gaussian LOSVDs. In this case the LOSVD can be described by a Gauss-Hermite series and it is shown that typical values of h3 and h4 observed in early-type galaxies can lead to significant modifications of the LOSVD corrections and thus to changes in the derived luminosity weighted ages and metallicities. A new, simple parameterisation for the LOSVD corrections, taking into account the h3 and h4 terms, is proposed and calibrations given for a subset of the Lick/IDS indices and two additional indices applicable to old (>3 Gyr) stellar populations.
Stellar abundance gradients in galactic disks. I. Method and spectral line gradients
Mon Notic Roy Astron Soc, 2005
We describe the technique of absorption line imaging of galaxy disks using the Taurus Tunable Filter on the Anglo-Australian Telescope and demonstrate its sensitivity to the behaviour of spectral features associated with Mg and Fe. Radial profiles of Mg2 and Fe5270 line-strengths are presented for a sample of eight face-on spiral galaxies spanning a range of Hubble types. Signatures of phenomena including merger-induced star formation, HII rings and galactic bars are also reported. This study demonstrates the capacity of tunable filters to measure Mg and Fe line-strengths across the face of spiral galaxies, which can ultimately reveal clues about the star formation history and chemical evolution.
Monthly Notices of the Royal Astronomical Society, 2007
We present absorption line strength maps of a sample of 24 representative early-type spiral galaxies, mostly of type Sa, obtained as part of the SAURON survey of nearby galaxies using our custom-built integral-field spectrograph. Using high-quality spectra, spatially binned to a constant signal-to-noise, we measure several key age, metallicity and abundance ratio sensitive indices from the Lick/IDS system over a contiguous two-dimensional field including bulge and inner disc. We present maps of Hβ, Fe 5015, and Mg b, for each galaxy. We find that Sa galaxies on the average have slightly smaller Mg b and Fe 5015 line strengths than ellipticals and S0s, and higher Hβ values, but with a much larger scatter.