L. M. Buson

The far-ultraviolet spectra of early-type galaxies

New and revised ultraviolet energy distributions ar presented based on 97 IUE spectra of 31 early-type galaxies and the bulge of M31. Based on optical spectra and morphology, the sample galaxies are divided into star-forming galaxies, active galaxies, and quiescent galaxies. A well-defined nonlinear relationship between (1550-V) color and Mg2 for the 24 quiescent galaxies is obtained. The ultraviolet spectra of these galaxies can be modeled as a sum of two components: a normal, cool stellar population of main-sequence and giant branch stars that is redder at high Mg2, plus a very blue population having a steeply rising UV flux below 2000 A that increases in strength with metallicity. The UV spectra of star-forming galaxies are significantly flatter than the very blue component of quiescent galxies and are consistent with aging bursts of star formation. Two sample population models are explored for the blue stellar component of quiescent galaxies: postasymptotic giant branch stars and young stars from continuing star formation. 63 references.

The external origin of the gas in S0 galaxies

In this Letter we present three new cases of S0 galaxies, namely NGC 2768, NGC 4379, and IC 4889, out of a sample of 15, in which the gas is counterrotating or strongly kinematically decoupled with respect to the stellar body of the galaxy. Assuming gas infall with randomly oriented angular momentum, this result implies that in 40% of the galaxies in our sample, the ionized gas is of external origin. However, several pieces of evidence suggest that the acquisition of external gas in S0 galaxies can be even more general

Spitzer irs spectra of virgo early type galaxies: detection of stellar silicate emission

We present high signal-to-noise ratio Spitzer Infrared Spectrograph observations of 17 Virgo early-type galaxies. The galaxies were selected from those that define the color-magnitude relation of the cluster, with the aim of detecting the silicate emission of their dusty, mass-losing evolved stars. To flux calibrate these extended sources, we have devised a new procedure that allows us to obtain the intrinsic spectral energy distribution and to disentangle resolved and unresolved emission within the same object. We have found that 13 objects of the sample (76%) are passively evolving galaxies with a pronounced broad silicate feature that is spatially extended and likely of stellar origin, in agreement with model predictions. The other four objects (24%) are characterized by different levels of activity. In NGC 4486 (M87), the line emission and the broad silicate emission are evidently unresolved, and, given also the typical shape of the continuum, they likely originate in the nuclear torus. NGC 4636 shows emission lines superposed on extended (i.e., stellar) silicate emission, thus pushing the percentage of galaxies with silicate emission to 82%. Finally, NGC 4550 and NGC 4435 are characterized by polycyclic aromatic hydrocarbon (PAH) and line emission, arising from a central unresolved region. A more detailed analysis of our sample, with updated models, will be presented in a forthcoming paper.

The discovery of a young radio galaxy at Z = 2.390 - Probing initial star formation at Z less than approximately 3.0

We present the discovery of a weak radio galaxy from the Leiden Berkeley Deep Survey at a redshift of 2.390, the faint optical and IR counterpart of the steep-spectrum, compact radio source 53W002. Its lambda-dependent optical continuum morphology is compact with linear size approximately 10-35 kpc (H0 = 50, q0 = 0). In redshifted Ly-alpha, the galaxy is somewhat more extended (less-than-or-similar-to 67 kpc x 40 kpc). Its radio source is 7 times smaller than, and confined by, the Ly-alpha gas. It shows alignment with the Ly-alpha gas and the best seeing optical continuum images. We present nine-band photometry (Ly-alpha U BgriJHK) for the galaxy as well as surrounding objects. The source 53W002 is not variable on time scales of years, in either radio or optical. We compare its rest-frame UV continuum with IUE spectra of various nearby galaxies with relatively recent starbursts, and nearby active galactic nuclei (AGNs). The C IV/Ly-alpha and N v/Ly-alpha ratios suggest that 53W002 has a Seyfert 1-like AGN, and constrain the nonthermal component to be approximately 35% of the observed UV continuum. Several independent age estimates yield a consistent value of 0.25-0.32 Gyr: (1) its small 4000 angstrom break or UV-visual continuum amplitude compared with nearby galaxies; (2) a best model fit to the downturn of its UV spectrum below approximately 2000 angstrom; (3) limits from the lack of detected stellar absorption features; (4) its total stellar mass (from its V- and K-band luminosity) compared with its SFR [from W-lambda (Ly-alpha, z = 0)]. These parameters together suggest that at z = 2.390, 53W002 had processed only a few times 10(11) M. into stars, significantly less than most powerful radio galaxies have converted into stars at z approximately 2-3.8. Hence, star formation in 53W002 has proceeded slower, and started at a later epoch, than in the most powerful high-redshift radio galaxies. The available data are consistent with 53W002 being a genuinely young (radio) galaxy seen at z = 2.390 during its first major starburst. It likely started forming most of its current stars at redshifts 2.5-3.0 (for H0 = 50-100, q0 = 0.0-0.5). This suggests that (radio) galaxies do not form the bulk of their stars coevally, but start doing so over a wide range of cosmic time.

Optical and infrared observations of SN 2002dj: some possible common properties of fast-expanding Type Ia supernovae

As part of the European Supernova Collaboration, we obtained extensive photometry and spectroscopy of the Type Ia supernova (SN Ia) SN 2002dj covering epochs from 11 d before to nearly two years after maximum. Detailed optical and near-infrared observations show that this object belongs to the class of the high-velocity gradient events as indicated by Si, S and Ca lines. The light curve shape and velocity evolution of SN 2002dj appear to be nearly identical to SN 2002bo. The only significant difference is observed in the optical to near-infrared colours and a reduced spectral emission beyond 6500 A. For high-velocity gradient SNe Ia, we tentatively identify a faster rise to maximum, a more pronounced inflection in the V and R light curves after maximum and a brighter, slower declining late-time B light curve as common photometric properties of this class of objects. They also seem to be characterized by a different colour and colour evolution with respect to ‘normal’ SNe Ia. The usual light curve shape parameters do not distinguish these events. Stronger, more blueshifted absorption features of intermediate-mass elements and lower temperatures are the most prominent spectroscopic features of SNe Ia displaying high-velocity gradients. It appears that these events burn more intermediate-mass elements in the outer layers. Possible connections to the metallicity of the progenitor star are explored.

The mid-infrared colour–magnitude relation of early-type galaxies in the Coma cluster as measured by Spitzer-IRS

We use 16 µm, Spitzer-IRS, blue peakup photometry of 50 early-type galaxies in the Coma cluster to define the mid-infrared colour-magnitude relation. We compare with recent simple stellar population models that include the mid-infrared emission from the extended, dusty envelopes of evolved stars. The Ks-[16] colour in these models is very sensitive to the relative population of dusty Asymptotic Giant Branch (AGB) stars. We find that the passively evolving early-type galaxies define a sequence of approximately constant age (� 10 Gyr) with varying metallicity. Several galaxies that lie on the optical/near-infrared colour-magnitude relation do not lie on the midinfrared relation. This illustrates the sensitivity of the Ks-[16] colour to age. The fact that a colour-magnitude relation is seen in the mid-infrared underlines the extremely passive nature of the majority (68 %) of early-type galaxies in the Coma cluster. The corollary of this is that 32% of the early-type galaxies in our sample are not ‘passive’, insofar as they are either significantly younger than 10 Gyr or they have had some rejuvenation episode within the last few Gyr.

Nearby early-type galaxies with ionized gas: the UV emission from GALEX observations★

We present GALEX far-ultraviolet (FUV, �eff=1538 u and near-ultraviolet (NUV, �eff=2316 u surface photometry of 40 early-type galaxies (ETGs) selected from a wider sample of 65 nearby ETGs showing emission lines in their optical spectra. We derive FUV and NUV surface brightness profiles, (FUV-NUV) colour profiles and D25 integrated magnitudes. We extend the photometric study to the optical r band from SDSS imaging for 14 of these ETGs. In general, the (FUV-NUV) radial colour profiles become redder with galactocentric distance in both rejuvenated (6 4 Gyr) and old ETGs. Colour profiles of NGC 1533, NGC 2962, NGC 2974, NGC 3489, and IC 5063 show rings and/or arm-like structures, bluer than the body of the galaxy, suggesting the presence of recent star formation. Although seven of our ETGs show shell systems in their optical image, only NGC 7135 displays shells in the UV bands. We characterize the UV and optical surface brightness profiles, along the major axis, using a Sersic law. The Sersic law exponent, n, varies from 1 to 16 in the UV bands. S0 galaxies tend to have lower values of n (6 5). The Sersic law exponent n = 4 seems to be a watershed: ETGs with n > 4 tend to have [�/Fe] greater than 0.15, implying a short star-formation time scale. We find a significant correlation between the FUV NUV colour and central velocity dispersions �, with the UV colours getting bluer at larger �. This trend is likely driven by a combined effect of ‘downsizing’ and of the mass-metallicity relation.

Nearby early-type galaxies with ionized gas - VI. The Spitzer-IRS view. Basic data set analysis and empirical spectral classification

Context. A large fraction of early-type galaxies (ETGs) shows emission lines in their optical spectra, mostly with LINER characteristics. Despite the number of studies, the nature of the ioni zation mechanisms is still debated. Many ETGs also show several signs of rejuvenation episodes. Aims. We aim to investigate the ionization mechanisms and the physical processes of a sample of ETGs using mid-infrared spectra. Methods. We present here low resolution Spitzer-IRS spectra of 40 ETGs, 18 of which from our proposed Cycle 3 observations, selected from a sample of 65 ETGs showing emission lines in their optical spectra. We homogeneously extract the mid-infrared (MIR) spectra, and after the proper subtraction of a “passive” ETG template, we derive the intensity of the ionic and molecular lines and of the polycyclic aromatic hydrocarbon (PAH) emission features. We use MIR diagnostic diagrams to investigate the powering mechanisms of the ionized gas. Results. The mid-infrared spectra of early-type galaxies show a variety of spectral characteristics. We empirically sub-divid e the sample into five classes of spectra with common characterist ics. Class-0, accounting for 20% of the sample, are purely passive ETGs with neither emission lines nor PAH features. Class-1 show emission lines but no PAH features, and account for 17.5% of the sample. Class-2, in which 50% of the ETGs are found, as well as having emission lines, show PAH features with unusual ratios, e.g. 7: 7� m= 11: 3� m�2: 3. Class-3 objects have emission lines and PAH features with ratios typical of star-forming galaxies. 7.5% of objects fall in this class, likely to be objects in a starburs t/post-starburst regime. Class-4, containing only 5% of the ETGs, is dominated by a hot dust continuum. The diagnostic diagram [Ne III]15.55� m/[Ne II]12.8� m vs. [S III]33.48� m/[Si II]34.82� m, is used to investigate the different mechanisms ionizing the gas. According to the above diagram, if we exclude NGC 3258 where a starburst seems present, most of our ETGs contain gas ionized via either AGN-like or shock phenomena, or both. Conclusions. Most of the spectra in the present sample are classified as LIN ERs in the optical window. The proposed MIR spectral classes show unambiguously the manifold of the physical processes and ionization mechanisms, from star formation, low level AGN activity, to shocks (H2), present in LINER nuclei.

UNUSUAL PAH EMISSION IN NEARBY EARLY-TYPE GALAXIES: A SIGNATURE OF AN INTERMEDIATE-AGE STELLAR POPULATION?

We present the analysis of Spitzer-IRS spectra of four early-type galaxies (ETGs), NGC?1297, NGC?5044, NGC?6868, and NGC?7079, all classified as LINERs in the optical bands. Their IRS spectra present the full series of H2 rotational emission lines in the range 5-38 ?m, atomic lines, and prominent polycyclic aromatic hydrocarbon (PAH) features. We investigate the nature and origin of the PAH emission, characterized by unusually low 6-9/11.3 ?m interband ratios. After the subtraction of a passive ETG template, we find that the 7-9 ?m spectral region requires dust features not normally present in star-forming galaxies. Each spectrum is then analyzed with the aim of identifying their components and origin. In contrast to normal star-forming galaxies, where cationic PAH emission prevails, our 6-14 ?m spectra seem to be dominated by large and neutral PAH emission, responsible for the low 6-9/11.3 ?m ratios, plus two broad dust emission features peaking at 8.2 ?m and 12 ?m. These broad components, observed until now mainly in evolved carbon stars and usually attributed to pristine material, contribute approximately 30%-50% of the total PAH flux in the 6-14 ?m region. We propose that the PAH molecules in our ETGs arise from fresh carbonaceous material that is continuously released by a population of carbon stars, formed in a rejuvenation episode that occurred within the last few?Gyr. The analysis of the MIR spectra allows us to infer that, in order to maintain the peculiar size and charge distributions biased to large and neutral PAHs, this material must be shocked and excited by the weak UV interstellar radiation field of our ETGs.

The Mini-Active Galactic Nucleus at the Center of the Elliptical Galaxy NGC 4552 with Hubble Space Telescope*

The complex phenomenology shown by the UV-bright, variable spike first detected with the Hubble Space Telescope (HST) at the center of the otherwise normal galaxy NGC 4552 is further investigated with both HST imaging (FOC) and spectroscopy (FOS). HST/FOC images taken in 1991, 1993, and 1996 in the near UV have been analyzed in a homogeneous fashion, showing that the central spike has brightened by a factor ∼ 4.5 between 1991 and 1993, and has decreased its luminosity by a factor ∼ 2.0 between 1993 and 1996. FOS spectroscopy extending from the near UV to the red side of the optical spectrum reveals a strong UV continuum over the spectrum of the underlying galaxy, along with several emission lines in both the UV and the optical ranges. In spite of the low luminosity of the UV continuum of the spike (∼ 3× 10L⊙), the spike is definitely placed among AGNs by current diagnostics based on the emission line intensity ratios, being just on the borderline between Seyferts and LINERs. Line profiles are very broad, and both permitted and forbidden lines are best modelled with a combination of broad and narrow components, with FWHM of ∼ 3000 km s−1 and ∼ 700 km s−1, respectively. This evidence argues for the variable central spike being produced by a modest accretion event onto a central massive black hole (BH), with the accreted material having possibly being stripped from a a star in a close fly by with the BH. The 1996 broad Hα luminosity of this mini-AGN is ∼ 5.6 × 1037 erg s−1, about a factor of two less than that of the nucleus of NGC 4395, heretofore considered to be the faintest known AGN. Combining all observational constraints, we estimate the mass of the BH at the center of NGC 4552 to be in the range between 3 × 108 and 2 × 10L⊙. The relevance for the demography of BHs in galaxies of the high (HST) resolution imaging and spectroscopy capable of revealing an extremely low level AGN activity in normal galaxies is briefly discussed. Subject headings: galaxies: elliptical – galaxies: individual (NGC 4552) – galaxies: Seyfert I – galaxies: spectroscopy – galaxies: photometryThe complex phenomenology shown by the UV-bright, variable spike first detected with the Hubble Space Telescope (HST) at the center of the otherwise normal galaxy NGC 4552 is further investigated with both HST imaging (FOC) and spectroscopy (FOS). HST/FOC images taken in 1991, 1993, and 1996 in the near-UV have been analyzed in a homogeneous fashion, showing that the central spike has brightened by a factor ~4.5 between 1991 and 1993 and has decreased its luminosity by a factor ~2.0 between 1993 and 1996. FOS spectroscopy extending from the near-UV to the red side of the optical spectrum reveals a strong UV continuum over the spectrum of the underlying galaxy, along with several emission lines in both the UV and the optical ranges. In spite of the low luminosity of the UV continuum of the spike (~ 3 × 105 L☉), the spike is definitely placed among active galactic nuclei (AGNs) by current diagnostics based on the emission-line intensity ratios, being just on the borderline between Seyfert galaxies and LINERs. Line profiles are very broad, and both permitted and forbidden lines are best modeled with a combination of broad and narrow components, with FWHM of ~3000 km s-1 and ~700 km s-1, respectively. This evidence argues for the variable central spike being produced by a modest accretion event onto a central massive black hole (BH), with the accreted material having possibly being stripped from a star in a close flyby with the BH. The 1996 broad Hα luminosity of this mini-AGN is ~ 5.6 × 1037 ergs s-1, about a factor of 2 less than that of the nucleus of NGC 4395, heretofore considered to be the faintest known AGN. Combining all observational constraints, we estimate the mass of the BH at the center of NGC 4552 to be in the range between 3 × 108 and 2 × 109 L☉. The relevance for the demography of BHs in galaxies of the high (HST) resolution imaging and spectroscopy capable of revealing an extremely low level AGN activity in normal galaxies is briefly discussed.