The star formation histories of elliptical galaxies across the Fundamental Plane (original) (raw)
The Astrophysical Journal, 1995
We present an analysis of the far-ultraviolet upturn phenomenon (UVX) observed in elliptical galaxies and spiral galaxy bulges. Our premise is that the UV radiation from these systems emanates primarily from extreme horizontal branch (EHB) stars and their progeny. Such objects have Zero Age Horizontal Branch envelope masses M 0 env < 0:05M. Local examples of EHB stars exist in some globular clusters and in the Galactic disk field and serve both as a guide and constraint. We re-derive the broad-band UV colors 1500 V and 2500 V for globular clusters and elliptical galaxies from the available satellite data and investigate color-color and color-line strength correlations. There are several important distinctions between clusters and galaxies. They do not occupy a single Mg 2-color sequence. Clusters can be bluer than any galaxy in 15 V and 25 V , implying larger hot star populations, but galaxies are significantly bluer than clusters in 15 25 at a given 15 V. We attribute this primarily to the effect of metal abundance on the mid-UV (2500 A) light. It also implies that the UVX in galaxies is not produced by metal poor subpopulations similar to the clusters. We develop a simple spectral synthesis formulation for all phases of single star evolution from the ZAMS to the white dwarf cooling track that requires only one or two parameters for each choice of age and abundance. We provide the ingredients necessary for constructing models with arbitrary HB morphologies in the age range 2 < t < 20 Gyr and for 6 metallicities in the range 2:26 < [Fe=H] < 0:58; we also consider the effect of enhanced Y in metal rich models. UV properties of the models are predicted using the Kurucz (1991) atmospheres. The maximum lifetime UV output is produced by EHB stars with M 0 env 0:02M , and can be up to 30 times higher than for post-asymptotic-giant-branch (P-AGB) stars. The ultraviolet output of old populations is governed primarily by the distribution of M 0 env , P(M 0 env), on the ZAHB. The UV output is not very sensitive to [Fe=H] or to Y , but it can change very rapidly with M 0 env. Thus, it is extremely sensitive to the precise nature of giant branch mass loss. Because this process is not well understood physically, we choose to leave mass loss as an implicit free parameter. Our models use simple descriptions of P(M 0 env) to bracket the colors produced from any real distribution of stars. Our models accurately predict the range of UV colors observed for the globular clusters, given known constraints on their age, abundances, and HB morphologies. Clusters with "blue HB" morphologies do not require the hotter EHB stars to explain their UV colors, although a small EHB population is consistent with our models. The largest known population of these stars in a cluster, as a fraction of the total HB, is 20% in ! Cen. For [Fe=H] > 0:5, however, blue HB stars will be rare. As a consequence, we find that models with [Fe=H] > 0 which do not contain EHB stars cannot reproduce the colors of most of the galaxies. However, only small EHB fractions are required: < 5% for the bulk of the E galaxies and 20% for those with the strongest UVX. These results are independent of the assumed [Fe/H]. The EHB fraction required for most galaxies is comparable to the fraction of hot subdwarfs in the Galactic disk. Most of these are EHB stars, and their existence considerably strengthens the case for EHB populations as the source of elliptical galaxy UV light. The models also predict that the fraction of the far-UV light from P-AGB stars, which are spatially resolvable in nearby galaxies, is 70% and 20% for moderate UVX and strong UVX systems, respectively. We find that 25 V , but not 15 V , is sensitive to the age and abundance, though these cannot always be cleanly distinguished. The galaxy colors place strong limits of h[Fe=H]i > 0:5 and < 15% on the contribution of globular cluster-type populations to the V light. Galaxy colors are consistent with solar-abundance models with ages in the range 6-14 Gyr. However, the 25 V colors of the galaxies other than the strong UVX systems are too blue to be consistent with [Fe=H] > 0:2 for any age. This may be additional evidence that [Mg/Fe] > 0 in elliptical galaxies. UV colors for M32 are consistent with the solar abundance, intermediate age (4-6 Gyr) population inferred from optical/IR observations.
An Ultraviolet/Optical Atlas of Bright Galaxies
The Astrophysical Journal Supplement Series, 2001
We present wide-Ðeld imagery and photometry of 43 selected nearby galaxies of all morphological types at ultraviolet and optical wavelengths. The ultraviolet (UV) images, in two broad bands at 1500 and 2500
The Astrophysical Journal, 2007
We present the results from the analysis of optical spectra of 31 Hα-selected regions in the extended UV (XUV) disks of M 83 (NGC 5236) and NGC 4625 recently discovered by GALEX. The spectra were obtained using IMACS at Las Campanas Observatory 6.5m Magellan I telescope and COSMIC at the Palomar 200-inch telescope, respectively for M 83 and NGC 4625. The line ratios measured indicate nebular oxygen abundances (derived from the R23 parameter) of the order of Z ⊙ /5-Z ⊙ /10 1 . For most emission-line regions analyzed the line fluxes and ratios measured are best reproduced by models of photoionization by single stars with masses in the range 20-40 M ⊙ and oxygen abundances comparable to those derived from the R23 parameter. We find indications for a relatively high N/O abundance ratio in the XUV disk of M 83. Although the metallicities derived imply that these are not the first stars formed in the XUV disks, such a level of enrichment could be reached in young spiral disks only 1 Gyr after these first stars would have formed. The amount of gas in the XUV disks allow maintaining the current level of star formation for at least a few Gyr.
A Photometric Study of Giant Ellipticals and Their Stellar Halos With VST
Galaxies, 2017
Observations of diffuse starlight in the outskirts of galaxies are thought to be a fundamental source of constraints on the cosmological context of galaxy assembly in the ΛCDM model. Such observations are not trivial because of the extreme faintness of such regions. In this work, we investigated the photometric properties of six massive early-type galaxies (ETGs) in the VST Elliptical GAlaxies Survey (VEGAS) sample (NGC 1399, NGC 3923, NGC 4365, NGC 4472, NGC 5044, and NGC 5846) out to extremely low surface brightness levels with the goal of characterizing the global structure of their light profiles for comparison to state-of-the-art galaxy formation models. We carried out deep and detailed photometric mapping of our ETG sample taking advantage of deep imaging with VST/OmegaCAM in the g and i bands. By fitting the light profiles, and comparing the results to simulations of elliptical galaxy assembly, we have identified signatures of a transition between relaxed and unrelaxed accreted components and can constrain the balance between in situ and accreted stars. The very good agreement of our results with predictions from theoretical simulations demonstrates that the full VEGAS sample of ∼ 100 ETGs will allow us to use the distribution of diffuse light as a robust statistical probe of the hierarchical assembly of massive galaxies.
2013
Aims. We revisit the scaling relations and star-forming histories of local elliptical galaxies using a novel selection method applied to the Sloan Digital Sky Survey DR7. Methods. We combine two probability-based automated spectroscopic and morphological classifications of ∼ 600000 galaxies with z < 0.25 to isolate true elliptical galaxies. Our sample selection method does not introduce artificial cuts in the parameters describing the galaxy but instead it associates to every object a weight measuring the probability of being in a given spectro-morphological class. Thus the sample minimizes the selection biases. Results. We show that morphologically defined ellipticals are basically distributed in 3 spectral classes, which dominate at different stellar masses. The bulk of the population (∼ 50%) is formed by a well defined class of galaxies with old stellar populations that formed their stars at very early epochs in a short episode of star formation. They dominate the scaling relations of elliptical galaxies known from previous works and represent the canonical elliptical class. At the low mass end, we find a population of slightly larger ellipticals, with smaller velocity dispersions at fixed stellar mass, which seem to have experienced a more recent episode of star formation probably triggered by gas-rich minor mergers. The high mass end tends to be dominated by a third spectral class, slightly more metal rich and with more efficient stellar formation than the reference class. This third class contributes to the curvature of the mass-size relation at high masses reported in previous works. Our method is therefore able to isolate typical spectra of elliptical galaxies following different evolutive pathways.
The near-infrared Fundamental Plane of elliptical galaxies
Monthly Notices of the Royal Astronomical Society, 1999
We present results from a near-infrared K-band imaging survey of 59 elliptical galaxies in ve nearby clusters. We measure photometric parameters for each galaxy using surface photometry and draw velocity dispersions from the literature. Three observables de ne a near-infrared Fundamental Plane (FP) of elliptical galaxies with R e / 1:44 0:04 0:79 0:04 e . The scatter in the near-infrared relation is small at 16:5% in distance, which is equivalent to, or less than, the scatter of the optical FP. We suggest that the small deviation of the near-infrared FP relation from the optical FP is due to the reduction of metallicity e ects in the near-infrared bandpass. While the small scatter of the optical FP could be consistent with compensating e ects of age and metallicity, the similarly small scatter of the near-infrared FP is nearly independent of metallicity and hence places a strong constraint on possible age spreads among elliptical galaxies at every point along the FP. We suggest that the departure of the near-infrared FP from the pure virial form R e / 2 1 e , and the corresponding observed relation (M=L) / M 0:16 0:01 , may be explained by slight systematic departures of the structure and dynamics of elliptical galaxies from a homology.
The Ages of Elliptical Galaxies from Mid‐Infrared Emission
The Astrophysical Journal, 2005
The mid-infrared (10-20µm) luminosity of elliptical galaxies is dominated by the integrated emission from circumstellar dust in red giant stars. As a single stellar population evolves, the rate of dusty mass loss from red giant stars decreases with time, so the mid-infrared luminosity should also decline with stellar age. To seek such a correlation, we have used archival ISO observations to determine surface brightness profiles and central fluxes at 15µm in 17 early-type galaxies for which stellar ages have been determined from optical spectral indices. The radial surface brightness distributions at 15µm generally follow the stellar de Vaucouleurs profile as expected. We find that the surface brightness ratio µ 15µm /µ I−band is systematically higher in elliptical galaxies with ages < ∼ 5 Gyrs and in galaxies that exhibit evidence of recent mergers. Within the accuracy of our observations, µ 15µm /µ I−band shows no age dependence for ages > ∼ 5 Gyrs. The corresponding flux ratios F 15µm /F I−band within apertures scaled to the effective radius (R e /8) are proportional to the µ 15µm /µ I−band ratios at larger galactic radii, indicating that no 15µm emission is detected from central dust clouds visible in optical images in some of our sample galaxies. Emission at 15µm is observed in non-central massive clouds of dust and cold gas in NGC 1316, an elliptical galaxy that is thought to have had a recent merger. Recent Spitzer Space Telescope data also indicate the presence of PAH emission at 8µm. Several ellipticals have extended regions of 15µm emission that have no obvious counterparts at other frequencies.
Far-Ultraviolet Spectra of B Stars near the Ecliptic
Astrophysical Journal, 2001
Spectra of B stars in the wavelength range of 911-1100 A have been obtained with the EURD spectrograph onboard the Spanish satellite MINISAT-01 with ~5 A spectral resolution. IUE spectra of the same stars have been used to normalize Kurucz models to the distance, reddening and spectral type of the corresponding star. The comparison of 8 main-sequence stars studied in detail (alpha Vir, epsilon Tau, lambda Tau, tau Tau, alpha Leo, zeta Lib, theta Oph, and sigma Sgr) shows agreement with Kurucz models, but observed fluxes are 10-40% higher than the models in most cases. The difference in flux between observations and models is higher in the wavelength range between Lyman alpha and Lyman beta. We suggest that Kurucz models underestimate the FUV flux of main-sequence B stars between these two Lyman lines. Computation of flux distributions of line-blanketed model atmospheres including non-LTE effects suggests that this flux underestimate could be due to departures from LTE, although other causes cannot be ruled out. We found the common assumption of solar metallicity for young disk stars should be made with care, since small deviations can have a significant impact on FUV model fluxes. Two peculiar stars (rho Leo and epsilon Aqr), and two emission line stars (epsilon Cap and pi Aqr) were also studied. Of these, only epsilon Aqr has a flux in agreement with the models. The rest have strong variability in the IUE range and/or uncertain reddening, which makes the comparison with models difficult.
Comparing Galaxy Morphology at Ultraviolet and Optical Wavelengths
The Astrophysical Journal Supplement Series, 2000
We have undertaken an imaging survey of 34 nearby galaxies in far-ultraviolet (FUV, ∼ 1500Å) and optical (UBV RI) passbands to characterize galaxy morphology as a function of wavelength. This sample, which includes a range of classical Hubble types from elliptical to irregular with emphasis on spirals at low inclination angle, provides a valuable database for comparison with images of high-z galaxies whose FUV light is redshifted into the optical and near-infrared bands. Ultraviolet data are from the UIT Astro-2 mission. We present images and surface brightness profiles for each galaxy, and we discuss the wavelength-dependence of morphology for different Hubble types in the context of understanding high-z objects. In general, the dominance of young stars in the FUV produces the patchy appearance of a morphological type later than that inferred from optical images. Prominent rings and circumnuclear star formation regions are clearly evident in FUV images of spirals, while bulges, bars, and old, red stellar disks are faint to invisible at these short wavelengths. However, the magnitude of the change in apparent morphology ranges from dramatic in early-type spirals with prominent optical bulges to slight in late-type spirals and irregulars, in which young stars dominate both the UV and optical emission. Starburst galaxies with centrally concentrated, symmetric bursts display an apparent "E/S0" structure in the FUV, while starbursts associated with rings or mergers produce a peculiar morphology. We briefly discuss the inadequacy of the optically-defined Hubble sequence to describe FUV galaxy images and estimate morphological k-corrections, and we suggest some directions for future research with this dataset.
The Astrophysical Journal, 2008
We combine 2MASS data and Spitzer archival data to study the emission in mid-infrared passbands (1.2-24µm) from a sample of 18 elliptical galaxies. In general the surface brightness distributions resemble de Vaucouleurs profiles, indicating that most of the emission arises from the photospheres or circumstellar regions of red giant stars. The spectral energy distribution peaks near the H band at 1.6µm. The half-light or effective radius has a pronounced minimum near the K band (2.15µm) with a second, less consistent minimum in the 24µm passband. All sample-averaged radial color profiles λ i −λ j where λ i < λ j (and j = 24µm) have positive slopes within about twice the (K band) effective radius. Evidently this variation arises because of an increase in stellar metallicity toward the galactic cores. Color profiles K − j all have positive slopes, particularly when j = 5.8µm although no obvious absorption feature is observed in spectra of elliptical galaxies near 5.8µm. This, and the minimum in R e , suggests that the K band may be anomalously luminous in metal-rich stars in galaxy cores. Unusual radial color profiles involving the 24µm passband may suggest that some 24µm emission comes from interstellar not circumstellar dust grains.