arXiv subjects

Laura Greggio

Publications and source records attributed to Laura Greggio.

At least 37 records · Page 2Linked to original sources

arXiv2012

The IMF from Low to High Redshift

From time to time, and quite more frequently in recent years, claims appear favoring a variable Initial Mass Function (IMF), one way or another, either in time or space. In this chapter we add our two pennies of wisdom, illustrating how the IMF affects various properties of galaxies and galaxy clusters. We start by showing that even relatively small variations of the IMF slope have large effects on the demography of stellar populations, moving the bulk of the stellar mass from one end to the other of the distribution. We then point out how the slope of the IMF in different mass ranges controls specific major properties of galaxies and clusters. The slope of the IMF below ~1 solar mass controls the M/L ratio of local ellipticals, whereas the slope between ~1 and ~1.4 solar masses controls the evolution with redshift of such ratio, hence of the fundamental plane of elliptical galaxies. Similarly, the slope between ~1 and ~40 solar masses drives the ratio of the global metal mass in clusters of galaxies to their total luminosity. While we believe that it is perfectly legitimate to entertain the notion that the IMF may not be universal, our message is that when proposing IMF variations to ease a specific problem then one should not neglect to explore the full consequences of the invoked variations. This paper is integrally reproduced from Chapter 8 of the book by L. Greggio and A. Renzini: Stellar Populations. A User Guide from Low to High Redshift (2011, Wiley-VHC Verlag-GmbH & Co., ISBN 9783527409181), whose index is also appended.

astro-ph.GA↗

arXiv2010

The rates of Type Ia Supernovae. II. Diversity of events at low and high redshift

This paper investigates on the possible systematic difference of Supernovae Ia (SN Ia) properties related to the age and masses of the progenitors that could introduce a systematic bias between low and high redshift SN Ia's. The relation between the main features of the distribution of the delay times (DTD) and the masses of the progenitors is illustrated for the single (SD) and double degenerate (DD) models. Mixed models, which assume contributions from both the SD and DD channels, are also presented and tested versus the observed correlations between the SN Ia rates and the parent galaxy properties. It is shown that these correlations can be accounted for with both single channel and mixed models, and that the rate in S0 and E galaxies may effectively provide clues on the contribution of SD progenitors to late epoch explosions. A wide range of masses for the CO WD at the start of accretion is expected in almost all galaxy types; only in galaxies of the earliest types the properties of the progenitors are expected to be more uniform. For mixed models, late type galaxies should host SD and DD explosions in comparable fractions, while in early type galaxies DD explosions should largely prevail. Events hosted by star forming galaxies span a wide range of delay times; \textit{prompt} events could dominate only in the presence of a strong star-burst. It is concluded that nearby SN Ia samples should well include the young, massive and hot progenitors that necessarily dominate at high redshift.

astro-ph.CO↗

arXiv2008

A New Hubble Space Telescope Distance to NGC 1569: Starburst Properties and IC 342 Group Membership

We present deep HST ACS/WFC photometry of the dwarf irregular galaxy NGC 1569, one of the closest and strongest nearby starburst galaxies. These data allow us, for the first time, to unequivocally detect the tip of the red giant branch and thereby determine the distance to NGC 1569. We find that this galaxy is 3.36 +/- 0.20 Mpc away, considerably farther away than the typically assumed distance of 2.2 +/- 0.6 Mpc. Previously thought to be an isolated galaxy due to its shorter distance, our new distance firmly establishes NGC 1569 as a member of the IC 342 group of galaxies. The higher density environment may help explain the starburst nature of NGC 1569, since starbursts are often triggered by galaxy interactions. On the other hand, the longer distance implies that NGC 1569 is an even more extreme starburst galaxy than previously believed. Previous estimates of the rate of star formation for stars younger than ~ 1 Gyr become stronger by more than a factor of 2. Stars older than this were not constrained by previous studies. The dynamical masses of NGC 1569's three super star clusters, which are already known as some of the most massive ever discovered, increase by ~53% to 6-7x10^5 solar masses.

astro-ph↗

arXiv2008

Rates, Progenitors and Cosmic Mix of Type Ia Supernovae

Following an episode of star formation, Type Ia supernova events occur over an extended period of time, following a distribution of delay times (DDT). We critically discuss some empirically-based DDT functions that have been proposed in recent years, some favoring very early (prompt) events, other very late (tardy) ones, and therefore being mutually exclusive. We point out that in both cases the derived DDT functions are affected by dubious assumptions, and therefore there is currently no ground for claiming either a DDT strongly peaked at early times, or at late ones. Theoretical DDT functions are known to accommodate both prompt as well as late SNIa events, and can account for all available observational constraints. Recent observational evidence exists that both single degenerate and double degenerate precursors may be able of producing SNIa events. We then explore on the basis of plausible theoretical models the possible variation with cosmic time of the mix between the events produced by the two different channels, which in principle could lead to systematics effects on the SNIa properties with redshift.

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arXiv2008

Supernova progenitors and iron density evolution from SN rate evolution measurements

Using an extensive compilation of literature supernova rate data we study to which extent its evolution constrains the star formation history, the distribution of the type Ia supernova (SNIa) progenitor's lifetime, the mass range of core-collapse supernova (CCSN) progenitors, and the evolution of the iron density in the field. We find that the diagnostic power of the cosmic SNIa rate on their progenitor model is relatively weak. More promising is the use of the evolution of the SNIa rate in galaxy clusters. We find that the CCSN rate is compatible with a Salpeter IMF, with a minimum mass for their progenitors > 10 Msun. We estimate the evolution in the field of the iron density released by SNe and find that in the local universe the iron abundance should be ~ 0.1 solar. We discuss the difference between this value and the iron abundance in clusters.

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arXiv2007

The evolution of the cosmic SN rate

We briefly review the contribution of SN rate measurements to the debate on SN progenitor scenarios. We find that core collapse rates confirms the rapid evolution of the star formation rate with redshift. After accounting for the dispersion of SN Ia measurements and uncertainty of the star formation history, the standard scenarios for SN Ia progenitors appear consistent with all observational constraints.

astro-ph↗

arXiv2005

The rates of Type Ia Supernovae. I. Analytical Formulations

This paper provides a handy tool to compute the impact of Type Ia Supernova (SNIa) on the evolution of stellar systems. An effective formalism is presented to couple the SNIa rate to the star formation history, which rests upon the definition of two key properties of the progenitor's model: the realization probability of the SNIa event from a single stellar generation and the distribution function of the delay times. It is shown that the current SNIa rate in late type galaxies implies that the realization probability is on the order of 0.001. Analytical formulations for the distribution function of the delay times for Single (SD) and Double Degenerate (DD) progenitors are derived, based on stellar evolution arguments. These formulations, which agree well with the results of Monte Carlo simulations for the evolution of close binaries, have a built in parametrization of the key properties of the alternative candidates. The various models for the progenitors have different impact on the large scales. In particular, the paper examines the systematic trend of the SNIa rate per unit mass with the color of the parent galaxy, and shows that the recent observations favor the DD model. The SD scenario can reproduce the data only if the distribution of the primordial mass ratios is flat, and the accretion efficiency onto the WD is close to 100%. The timescale for the Fe release from SNIa to the interstellar medium ranges between 0.3 and 3 Gyr for a wide variety of hypothesis on the SNIa progenitors. (ABRIDGED)

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arXiv2004

The Impact of Supernovae of Type Ia on Large Scales through the Secular Evolution of their Rate

A straightforward formalism to evaluate the impact of Type Ia Supernovae on large scale astrophysical issues is presented, together with analytical formulations for the SNIa rate following an instantaneous burst of Star Formation, for both single and double degenrates SNIa progenitors' models. Some applications of the parametrized formalism are illustrated. In particular, it is shown that the current SNIa rates measured in early and late type galaxies, as well as the Fe Mass-to-Light ratio in clusters of galaxies, favour the double degenerate model.

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arXiv2002

Comparing the old Stellar Population in Globulars and Dwarf Galaxies: The Cases of Phoenix and Leo A

Due to their star formation history (SFH), the stellar population in Dwarf Galaxies (DG) is likely to have a metallicity spread which is best traced by the morphology of the Red Giant Branch (RGB). We probe here a purely empirical approach aimed at estimating average metallicity (Z) and Z spread by comparing the Color-Magnitude Diagrams (CMD) of galactic Globular Clusters (GCs) with those of two DGs: Leo A (HST data) and Phoe (VLT Fors2 data).

astro-ph↗

arXiv2002

Integrated Spectroscopy of Bulge Globular Clusters and Fields I. The Data Base and Comparison of Individual Lick Indices in Clusters and Bulge

We present a comprehensive spectroscopic study of the integrated light of metal-rich Galactic globular clusters and the stellar population in the Galactic bulge. We measure line indices which are defined by the Lick standard system and compare index strengths of the clusters and Galactic bulge. Both metal-rich globular clusters and the bulge are similar in most of the indices, except for the CN index. We find a significant enhancement in the CN/ index ratio in metal-rich globular clusters compared with the Galactic bulge. The mean iron index of the two metal-rich globular clusters NGC 6528 and NGC 6553 is comparable with the mean iron index of the bulge. Index ratios such as Mgb/ , Mg2/ , Ca4227/ , and TiO/ , are comparable in both stellar population indicating similar enhancements in individual elements which are traced by the indices. From the globular cluster data we fully empirically calibrate several metallicity-sensitive indices as a function of [Fe/H] and find tightest correlations for the Mg2 index and the composite [MgFe] index. We find that all indices show a similar behavior with galactocentric radius, except for the Balmer series, which show a large scatter at all radii. However, the scatter is entirely consistent with the cluster-to-cluster variations in the horizontal branch morphology.

astro-ph↗

arXiv2002

The Oldest Stars of the Extremely Metal-Poor Local Group Dwarf Irregular Galaxy Leo A

We present deep Hubble Space Telescope single-star photometry of Leo A in B, V, and I. Our new field of view is offset from the centrally located field observed by Tolstoy et al. (1998) in order to expose the halo population of this galaxy. We report the detection of metal-poor red horizontal branch stars, which demonstrate that Leo A is not a young galaxy. In fact, Leo A is as least as old as metal-poor Galactic Globular Clusters which exhibit red horizontal branches, and are considered to have a minimum age of about 9 Gyr. We discuss the distance to Leo A, and perform an extensive comparison of the data with stellar isochrones. For a distance modulus of 24.5, the data are better than 50% complete down to absolute magnitudes of 2 or more. We can easily identify stars with metallicities between 0.0001 and 0.0004, and ages between about 5 and 10 Gyr, in their post-main-sequence phases, but lack the detection of main-sequence turnoffs which would provide unambiguous proof of ancient (>10 Gyr) stellar generations. Blue horizontal branch stars are above the detection limits, but difficult to distinguish from young stars with similar colors and magnitudes. Synthetic color-magnitude diagrams show it is possible to populate the blue horizontal branch in the halo of Leo A. The models also suggest ~50% of the total astrated mass in our pointing to be attributed to an ancient (>10 Gyr) stellar population. We conclude that Leo A started to form stars at least about 9 Gyr ago. Leo A exhibits an extremely low oxygen abundance, of only 3% of Solar, in its ionized interstellar medium. The existence of old stars in this very oxygen-deficient galaxy illustrates that a low oxygen abundance does not preclude a history of early star formation.

astro-ph↗

arXiv2002

The Dwarf Irregular/Wolf-Rayet Galaxy NGC4214: I. A New Distance, Stellar Content, and Global Parameters

We present the results of a detailed optical and near-IR study of the nearby star-forming dwarf galaxy NGC4214. We discuss the stellar content, drawing particular attention to the intermediate-age and/or old field stars, which are used as a distance indicator. On images obtained with the Hubble Space Telescope WFPC2 and NICMOS instruments in the equivalents of the V, R, I, J and H bands, the galaxy is well resolved into stars. We achieve limiting magnitudes of F814W ~27 in the WF chips and F110W ~25 in the NIC2. The optical and near-infrared color-magnitude diagrams confirm a core-halo galaxy morphology: an inner high surface-brightness young population within ~1.5' (~1 kpc) from the center of the galaxy, where the stars are concentrated in bright complexes along a bar-like structure; and a relatively low-surface-brightness, field-star population extending out to at least 8' (7 kpc). The color-magnitude diagrams of the core region show evidence of blue and red supergiants, main-sequence stars, asymptotic giant branch stars and blue loop stars. We identify some candidate carbon stars from their extreme near-IR color. The field-star population is dominated by the "red tangle", which contains the red giant branch. We use the I-band luminosity function to determine the distance based on the tip-of-the-red-giant-branch method: 2.7\pm0.3 Mpc. This is much closer than the values usually assumed in the literature, and we provide revised distance dependent parameters such as physical size, luminosity, HI mass and star-formation rate.

astro-ph↗

arXiv2001

The Star Formation History of the Blue Compact Dwarf Galaxy UGCA 290

We present the star formation history of UGCA 290, a galaxy with properties intermediate between Blue Compact Dwarfs and Dwarf Irregulars. This galaxy is particularly interesting because its young stellar population is extremely similar to that of the well-studied type iE Blue Compact Dwarf VII Zw 403, despite its different spatial morphology and old stellar content. Our Hubble Space Telescope/Wide Field Planetary Camera 2 single-star photometry for UGCA 290 extends over nine magnitudes in I, and allows a detailed study of its star formation history. Using synthetic color-magnitude diagrams, we show that the recent "burst" which gives this galaxy its BCD status is a moderate enhancement in star formation which lasted for approximately 20 Myr, at a rate about ten times above its previous rate. The star formation history for most of the previous billion years is consistent with a constant rate, although enhancements as large as the current one are possible at times ealier than 400 Myr ago. We estimate that the total mass converted into stars in UGCA 290 more than one billion years ago is about three times the astrated mass since that time. The initial mass function is consistent with a Salpeter slope, and the stellar metallicity is bracketed by Z_sun/50 and Z_sun/5, with evidence for metallicity evolution. Similar results for the star formation history over the past 600 Myr apply to VII Zw 403. Our main result is that despite the traditional picture of BCDs, the current bursts in these two galaxies are neither remarkably intense nor short-lived, and that most of their star formation occured more than a billion years ago.

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arXiv2001

The Stellar Content of NGC 6789, A Blue Compact Dwarf Galaxy in the Local Void

We find that NGC6789 is the most nearby example of a Blue Compact Dwarf galaxy known to date. With the help of WFPC2 aboard the Hubble Space Telescope, we resolve NGC6789 into over 15,000 point sources in the V and I bands. The young stars of NGC6789 are found exclusively near the center of the galaxy. The red giant population identified at large galacticentric radii yields a distance of about 3.6 Mpc, a stellar metallicity [Fe/H] of about -2, and a minimum age of about 1 Gyr. Despite its isolated location in the Local Void,its low metallicity, and its active star formation, the properties of NGC6789 are clearly not those of a galaxy in formation.

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arXiv2001

A Near-Infrared Stellar Census of Blue Compact Dwarf Galaxies: The Wolf-Rayet Galaxy I Zw 36

We report the results of near-IR imaging in J and H, of I Zw 36 ($\approx$Z$_{\odot}$/14) with the Hubble Space Telescope. Whereas imaging with the pre-COSTAR Faint Object Camera (FOC) previously resolved hot and massive stars in the near-UV, the NICMOS data furnish a census of the cool, intermediate- and low-mass stars. There clearly was star formation in I Zw 36 prior to the activity which earned it its Blue Compact Dwarf/Wolf-Rayet galaxy classification. The detection of luminous, asymptotic giant branch stars requires that stars formed vigorously several hundred Myr ago. The well-populated red giant branch indicates stars with ages of at least 1-2 Gyr (and possibly older than 10 Gyr). We use the tip-of-the-red-giant-branch method to derive a distance of $\geq$5.8 Mpc. This is the third in a series of papers on near-IR---resolved Blue Compact Dwarf galaxies. We notice that the color-magnitude diagrams of VII Zw 403, Mrk 178, and I Zw 36 do not exhibit the gaps expected from an episodic mode of star formation. Using simulated color-magnitude diagrams we demonstrate for I Zw 36 that star formation did not stop for more than a few 10$^8$ yrs over the past 10$^9$ yrs, and discuss the implications of this result.

astro-ph↗

arXiv2000

The Cuspy LINER Nucleus of the S0/a Galaxy NGC 2681

The nucleus of the bulge-dominated, multiply-barred S0/a galaxy NGC 2681 is studied in detail, using high resolution Hubble Space Telescope FOC and NICMOS imaging and FOS spectroscopy. The ionised gas central velocity dispersion is found to increase by a factor ~2 when narrowing the aperture from R~1.5" (ground) to R~0.1" (FOS). Dynamical modeling of these velocity dispersions suggests that NGC 2681 does host a supermassive black hole (BH) for which one can estimate a firm mass upper limit M_BH < 6*10^7 Solar Masses. This upper limit is consistent with the relation between the central BH mass and velocity dispersion M_BH - sigma known for other galaxies. The emission line ratios place the nucleus of NGC 2681 among LINERs. It is likely that the emission line region comes from a rather mild, but steady, feeding of gas to the central BH in this galaxy. The inner stellar population lacks any measurable color gradient (to a radius of 0.6 kpc) from the infrared to the ultraviolet, consistently with FOC, FOS and IUE data, all indicating that this system underwent a starburst ~1 Gyr ago that encompassed its whole interior, down to its very center. The most likely source of such a widely-distributed starburst is the dumping of tidally-extruded gas from a galaxy neighbor. If so, then NGC 2681 can be considered as the older brother of M82, seen face-on as opposed to the edge-on view we have for M82.

astro-ph↗

arXiv2000

A Compact Population of Red Giants in the Blue Compact Dwarf Galaxy UGCA 290

We present HST/WFPC2 single-star photometry for the blue dwarf galaxy UGCA 290, whose morphology is intermediate between classic iE Blue Compact Dwarfs and blue dwarfs which exhibit no red background sheet of older stars. The color-magnitude diagram of this galaxy in V and I, extending over six magnitudes, is remarkably similar to that of the star-forming region in the iE Blue Compact Dwarf VII Zw 403. There is no evidence for gaps in its star-formation history over the last billion years, and the color of its red giant branch indicates a very metal-poor stellar population. From the magnitude of the tip of the red giant branch, we derive a distance of 6.7 Mpc, more than twice the distance estimated from the brightest blue supergiants.

astro-ph↗

arXiv2000

A Near-Infrared Stellar Census of Blue Compact Dwarf Galaxies: NICMOS Detection of Red Giant Stars in the Wolf-Rayet Galaxy Mrk 178

We observed the Blue Compact Dwarf/Wolf-Rayet galaxy Mrk 178 with the NICMOS camera aboard HST. The galaxy is well resolved into individual stars in the near-IR; photometry in J and H yields color-magnitude diagrams containing 791 individual point sources. We discuss the stellar content, drawing particular attention to the intermediate age and/or old stars. Mrk 178 is only the second Blue Compact Dwarf galaxy in which the red giant branch has been resolved, indicating stars with ages of at least 1-2Gyr. This allows us to derive a distance of $\geq$4.2($\pm$0.5)Mpc. The near-IR color-magnitude diagram also exhibits an abundance of luminous, asymptotic giant branch stars. We find that this requires vigorous star formation several hundred Myr ago. Some candidate carbon stars are identified via their extreme near-IR color. We argue that Mrk178 is fundamentally an old galaxy, based on the NICMOS detection of red giants underlying the blue, starburst core, and its extended, faint halo of redder color.

astro-ph↗